The Seychelles magpie robin Copsychus sechellarum: ecology and conservation of an endangered species

Biological Conservation 1992, 61, 93-106

The Seychelles magpie robin Copsychus sechellarum: ecology and conservation

of an endangered species

Jeff Watson Scottish Natural Heritage, 9 Culduthel Road, Inverness, UK, IV2 4AG

Carol Warman The Homestead, Porth Kea, Truro, Cornwall, UK, TR3 6AL

David Todd Trezise, St Martin, Helston, Cornwall, UK, TR12 6EF

&

Vietorin Labondallon Conservation Division, Fond Boffay, Praslin, Seychelles

In 1977-78 some 40 Seychelles magpie robins Copsychus sechellarum, the entire world population, survived on Frrgate island. These lived in 12 territorial groups of up to six individuals. Their range on Frrgate was limited by the amount of feeding habitat, specifically bare earth and leaf litter which occurred under mature shady woodland and in cultivated vegetable gardens. Two attempts were made to reintroduce the species to Aride Island in 1978 and 1979. These were unsuccessful and the translocations had to be abandoned when a new threat impinged on the parent population of Frrgate in 1980. By 1981 numbers there had declined to 18, with virtually no recruitment, and an increase in the feral cat population was implicated. A successful cat eradication programme by trapping and poisoning was carried out in 1981-82. By 1983-84 the population showed a recovery with recruitment again healthy, although the abandonment of agriculture on Frrgate between 1979 and 1983 had caused a reduction in the amount of feeding habitat and in the carrying capacity of the island to around 25 individuals in eight territorial groups. A range of management options is discussed.

INTRODUCTION

For more than 40 years the Seychelles magpie robin Copsychus sechellarum Newton has occurred naturally only on the 210-ha island of Frrgate in the Seychelles archipelago (Fig. 1). This paper de- scribes research into the ecology of the bird over a 13-month period in 1977-78. It also reports on attempts to re-establish the species on the island of Aride in 1978 and 1979. The effects of an increase in the feral cat population on magpie robins on Frrgate in 1980-81 are discussed, as are the results of a cat eradication programme carried out there in

Biological Conservation 0006-3207/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain

1981-82. Finally the fate of the robin population following cat eradication on Frrgate is described.

HISTORICAL REVIEW

Pre-1950

For nearly a century after the first human settlement of Seychelles in 1770 there was no significant account of the islands' birds. An exploratory visit in 1768 provided some brief notes on birds seen (Lionnet, 1980). These were mostly imprecise but do include reference to birds which must have been magpie robins. These were reported from Ile du Sud-est, Ste Anne and Praslin at this time.

93

94 J. Watson, C Warman, D. Todd, V. Laboudallon

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Map of the granitic Seychelles. There are historical records of magpie robins from named islands although the species survived only on Fr6gate in 1977.

The species was formally described by Alfred Newton from a specimen which died in captivity on Mauritius (Newton, 1865). Shortly after this Edward Newton visited Seychelles and wrote the first comprehensive account of the islands' birds (Newton, 1867). He saw magpie robins on Praslin, La Digue and Marianne and was told that a few survived in south Mah6. Other islands which definitely held robins around 1870 were Aride (Hartlaub, 1877; Oustalet, 1878) and Fr6gate (Pike, 1872; Hartlaub, 1877). The eight islands from which there have been historical records of magpie robins are named in Fig. 1.

Newton's record from Mah6 is hearsay, but is supported by evidence in a painting by Marianne North who visited Seychelles in 1883. There are no subsequent records from Mah6 nor any of its satellite islands and the last record from Praslin was in 1878, when they were said to be difficult to locate (Blackburn, 1878). Precisely when they died out on La Digue is a mystery, although there are no records for this century. Two specimens were collected on Marianne in 1893 (Ridgeway, 1895) and the species may have survived there into the 1930s (Vesey-Fitzgerald, 1940) or even up to 1948 (Crook, 1960). A bird was collected on Aride in 1905 (Diamond & Feare, 1980) and robins survived there into the 1930s (Vesey-Fitzgerald, 1940).

Pos t -1950

The first published count of magpie robins on Fr6gate was by Crook (1960), who gave a figure of 10 pairs, and shortly after this Loustau-Lalanne (1962) reported 30 birds. Then Honneger (1966) estimated that not more than 15-20 birds remained in 1964, and the lowest recorded count was made in 1965 when only 12 birds were seen (Dawson, 1965). Apparently numbers then increased, as in 1967, 15 birds were seen and the population was estimated to be around 20 (Penny, 1968). J. Procter (pers. comm.) saw 16 birds during a short visit in 1970 when he thought there might be 25 birds in all. High (1974) counted 38 birds in 1973 and three years later Wilson and Wilson (1978) reported at least 34 individuals. Their's was the last count before the present study began in July 1977.

DESCRIPTION OF FRI~GATE ISLAND

Fr6gate is a low rocky island of 210 ha rising to a height of 125 m. There are two coastal 'plateaux' which have a combined area of about 30 ha. The hill land is mainly composed of aplite, a rock which weathers to produce deep and fertile, though boulder-ridden, soils.

Although Fr6gate experiences the same seasonal weather patterns as the rest of the granitic Sey-

The Seychelles magpie robin 95

chelles, it attracts less rain than the larger, higher islands. Limited records suggest that annual rain- fall may be about two-thirds that of coastal areas on Mah6 (Watson, 1978).

Early human history on Fr6gate is poorly docu- mented. The island was probably uninhabited until the early 19th century but by 1851 the population had reached 60. Subsequent census reports in- dicate a fluctuating population, reaching a peak of 118 in 1947 before dropping to its present total of less than 25.

I n 1787 de Malavois, the commandant of Sey- chelles, described Fr6gate as covered with timber trees of poor quality and lacking coconuts or other palms (Fauvel, 1905). Since the first perma- nent settlement all natural vegetation has been cleared to make way for a variety of crops and secondary woodland. During the early 19th century coconut palms were planted and these are now widespread. Nowadays the major crops are coconuts and to a lesser extent bananas and citrus fruit. Cattle, pigs and chickens are kept on the plateaux.

THE MAGPIE ROBIN ON FRl~GATE: 1977-78

The conclusions from a short survey in 1976 (Wil- son & Wilson, 1978) were that introduced mammalian predators had been responsible for the contraction in range of the magpie robin to a single island in the Seychelles group, and within that island (Fr6gate) numbers were stable. Guided by these conclusions the extended study in 1977-78 sought to answer the following questions:

(1) How many magpie robins are living on Fr6gate and what is their dispersion?

(2) Does the present distribution reflect a pref- erence for particular habitats or habitat features?

(3) Is the availability of food or feeding habitat limiting the present population?

(4) Is the annual production of young providing sufficient recruitment to compensate estimated adult mortality?

METHODS

Numbers and dispersion

Ten breeding adults were caught in mist-nets and marked with unique combinations of coloured leg-

. LANDWARD mOUNDARY "~ ~ OF COASTAL PLATEAUX

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Fig. 2. The locations of 13 adult male song perches used during the 12 months from July 1977 to June 1978. The break of slope at the landward edge of the two large coastal

plateaux is also marked.

rings in September/October 1977. Nine non-breeding birds were similarly marked during the course of the study and a further four birds, which were individually distinct for other reasons, were present. Finally there were features of immature plumage described elsewhere (Watson, 1978) which permitted these birds to be recognised individually for periods lasting several months. Another valuable aid to censusing was the presence of traditional song perches used by territorial males (Fig. 2). Song was delivered from near the highest point in the canopy and comprised loud and closely sequenced musical notes which were audible at least 50 m distant and often much further. Song perches of particular adult males were confined to one or a few neighbouring trees.

The entire population was censused over a two- day period each month from September 1977 until July 1978. The census method was to play tape recordings of a range of alarm or distress calls (Watson, 1978) for up to 1 h in the vicinity of traditional song perches. Birds responding to play- back recordings were counted and all individually marked robins were identified. The behaviour of individually marked birds confirmed that only members of the group associated with that particular song perch were drawn to such recordings.

96 J. Watson, C Warman, D. Todd, K Laboudallon

Records were kept on the precise locations of all sightings of marked and individually recognis- able birds throughout the study period. Similarly the locations of all territorial disputes (Watson, 1978) were recorded. Most of these data refer to the groups concentrated at the southern end of the eastern plateau and the interpretation of the nature of territoriality in the species is based on this sample.

Habitat requirements

A wide-ranging survey of the island early in the study provided data for an analysis of habitat requirements. A map (1:5000) with an overlaying 1-ha grid was used and all coastal squares containing less than 50% land area were excluded, giving a total of 203 squares in all. Each square was visited at least twice and traversed twice on each visit. If a magpie robin was encountered the square was recorded, and all squares visited by the bird in the next 60 min were also recorded. The results were used in a preliminary interpretation of habitat requirements. A detailed habitat map was prepared during this survey using the vegetation types identified by A. Robertson and D. Todd (pers. comm.).

Feeding habitat

The study of feeding habitats was confined to members of the breeding pair in six groups in which one or both members of the pair was individually marked. Once located, birds were followed for two minutes to allow them to adjust to the observer's presence and thereafter details of their behaviour were recorded at half-minute intervals. Observations continued until feeding was observed on a total of ten occasions at the time when behaviour was recorded. Because birds tended to feed in short bursts lasting for two or three minutes, interspersed with other activities such as singing or perching, there was no risk of bias introduced by the initial location of the bird in one type of habitat where they might be more conspicuous. For each feeding record the height of the bird above ground level and the substrate on which it was feeding were recorded. The method was repeated five times during the course of a day, to allow for possible changes in behaviour at different times of day, giving a total of 50 feeding observations. These 50 feeding records, obtained for a range of individuals over several months, pro-

vided the material for a descriptive and compara- tive assessment of feeding habitat requirements.

Feeding rates

The method was similar to that described above with behaviour described at 30-s intervals during continuous watches of marked individuals. Birds were followed for 30 min during each observation period, giving 60 records of behaviour per period, and observations were repeated five times during the day, giving 300 records in all. The key figure was the proportion of these records at which feeding was detected. Birds which spent more time feeding had proportionately less time available for other activities, and in particular reproduction. It was therefore assumed that 'time spent feeding' could provide a measure of differences in food potential between territories. In order to provide comparable data, observations were restricted to adult male territory holders and to individuals not feeding dependent young.

Annual production and survival

The basic unit for the study of breeding success (the group) was a pair of adults or a pair of adults with attendant young and/or non-breeding birds. Each group was checked for breeding at least twice a month throughout the study. Observations of marked birds quickly showed that females spent more time in breeding activities than did males. In cases where one member of the adult pair was marked the female was located and followed continuously for 30 min. Incubating females rarely spent more than 15 min off the nest and birds with nestlings normally visited the nest at least twice during a 30-min period. Conse- quently no active nest should have been missed if females were followed for this long. Nest building occurs more sporadically and it is likely that some nest-building attempts which never got beyond this stage of the breeding cycle went unrecorded during the study period. For groups where neither of the breeding pair was marked, observations were made on both members of the pair together for at least 30 min. Once an active nest was located it was checked daily until the outcome of the nesting attempt was known. The fate of fledged young was then recorded in successive monthly censuses.

Clearly there was little prospect of providing a convincing measure of adult survival during a


14-month study. However, since the study ended there have been a series of complete censuses of the population including records of colour-marked birds. In addition to this, a number of colour- ringed nestlings have entered the breeding population. An analysis of the rate of disappearance of all these individuals up to 1983 has been used to give an estimate of adult survival.

RESULTS

Numbers

The results of the monthly counts are given in Table 1. The total population varied little during the year with a minimum of 38 birds and a maximum of 41, excluding the last three months which followed the translocation of six birds to Aride. In September and October 12 groups were detected and by November one group split into two, giving 13 in all. In December two different groups amal- gamated and from then until the end of the study in July 1978 there were 12 groups. Group size ranged from one to a maximum of six birds.

Dispersion

The accumulated sightings of adult males on five territories at the southern end of the eastern plateau were plotted for successive three-month periods beginning with August-October 1977; peri-

pheral sightings were joined to form polygons and thereby an indication of home ranges. Magpie robins occupied more or less exclusive home ranges which were actively defended against neighbouring groups. Although the location of these group territories shifted during the course of the year the nature of dispersion did not and groups remained strictly territorial, in the sense of Noble (1939), throughout the year (Watson, 1978).

Distribution

The survey of the island based on the 1-ha grid revealed the extent to which the whole of Frrgate was used by magpie robins (Fig. 3). Sightings were obtained in 88 (43%) of the 203 squares. One habitat feature closely reflected the distribution of magpie robins--the presence of a ground layer of bare earth and leaf litter. In the absence of such habitat magpie robins were unlikely to be encountered (Fig. 3; Table 2).

Feeding habitat

A total of 300 feeding records were obtained from observations in six territorial groups (Table 3). For two groups records referred only to territorial males and for the remainder the records were the combined observations on territorial males and females. Over 93% of all feeding records were detected on the ground with 4% in the crowns of

Table 1. The number of magpie robins recorded in each group in each month between July 1977 and July 1978 and in April 1979

Site no. 1977

J A S O N D J

1978 1979

F M A M J J April

1 4 5 6 6 2 3 3 3 4 3 2 3 3 2 2 . . . . 4 4 4 4 4 4 3 3 2 3 3 3 3 3 4 2 3 3 3 3 3 3 4 4 4 4 4 3 3 3 2 . . . . . . . . . 5 5 5 5 5 6 6 6 6 6 5 5 5 5 9 6 4 3 3 4 4 3 3 3 3 3 3 4 4 4 7 4 4 4 4 4 4 3 3 3 3 4 4 3 2 8 2 2 2 2 2 2 2 2 1 2 2 2 2 3 9 2 2 2 2 2 2 3 3 4 4 3 3 3 4

10 2 2 2 2 2 3 3 3 2 2 1 1 1 - - 11 2 4 4 4 4 4 4 4 3 3 3 3 3 2 12 3 3 3 3 3 3 4 4 4 4 2 1 2 3 13 3 3 2 2 2 3 2 2 3 3 4 4 3 2

Monthly 38 39 39 41 39 40 40 40 40 39 35 37 35 38 totals

Note that six birds were removed for translocation to Aride in April 1978--three from site 12, two from site 10 and one from site 8. Four more birds were removed in April 1979--two from site 12 and two from site 6. Both April totals refer to the population prior to the removal experiments. The distribution of sites is shown in Fig. 2.


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Fig. 3. Records of magpie robins (O) in 1-ha squares and distribution of vegetation types with extensive bare earth/leaf litter habitat (stippled areas). This map refers to 1977/78 when vegetable production was at a peak on Frrgate. By 1984 most of the pockets of bare earth habitat on the coastal

plateaux had disappeared.

coconut palms and less than 3% on all other substrates such as trunks, branches and twigs of trees and shrubs. These proport ions were broadly consistent for birds in all six groups. Table 3 also shows the number of feeding records detected for different feeding substrates within the ground layer. Of the 280 feeding records on the ground 266 (95%) were on bare earth or leaf litter and only 14 (5%) were in herbaceous vegetation. Whilst this broad proport ion was consistent for six groups the choice of bare earth or leaf litter varied between groups and reflected the availability of each habitat within the range of a particular group.

Table 2. Number of 1-ha squares containing >50%, 5-50% and <5% bare earth or leaf litter habitat in which magpie robins were detected or not during the complete survey of Fr6gate in

late 1977

Ground habitat Robins detected Robins not detected (no. of squares) (no. of squares)

>50% earth/litter 38 0 5-50% earth/litter 42 7 <5% earth/litter 8 108

(×2=148.2; d.f. = 2; p <0-001)

Table 3. The number of feeding observations detected in different habitats in a range of magpie robin territories on F r6gate

(a = 50 for each site)

Habitat Sites a types

1 2 3 5 9 10 Total (%)

Bare earth 36 31 30 37 0 14 ~ 266 (88.7) Leaf litter 5 6 15 l0 49 33

J Grassland 1 9 3 0 0 l 14 (4-7) Coconut palm b 7 0 0 3 0 2 12 (4.0) Others l 4 2 0 1 0 8 (2.6)

a Site numbers refer to Fig. 2. b These records relate to birds feeding in the crowns of coconut palms.

F e e d i n g r a t e s

A total o f 15 complete sets o f 300 behavioural records were made on Frrgate on eight different adult male territory holders. Five males occupied territories on the main coastal plateau and three were on the hill. Birds on the plateau spent significantly less time foraging than did birds on the hill (Table 4; t = 4.93, d.f. = 13, p < 0-01).

A n n u a l produc t ion

A comprehensive description of the breeding cycle of the Seychelles magpie robin is given elsewhere (Watson, 1978). Over 90% of the 41 nests exam- ined were in the crowns of coconut palms and most occurred at heights between 5 and 15 m above ground. All twelve nests checked during incubation held a single egg and another eight nests first visited with young contained one chick. The incubation period was known accurately for only one egg, when it lasted for 20 days. Most chicks left the nest before they were 15 days old and were

Table 4. Amount of time spent foraging (mean % of n sets of 300 behavioural records gee text) by adult males which were not, at the time when observations were made, feeding

dependent young

Mean % time SD (n) spent foraging

10.40 2.09 (9) Frrgate (plateau)

Frrgate 16-71 2.87 (6) (hill)

Aride 6.99 2.82 (7)

Data are given separately for birds in territories on the coastal plateau and the hill on Frrgate, and for birds trans- located to Aride.


unable to fly, their wing-length at this stage being about 60% of full-grown length. Chicks were seen to fly weakly when more than 18 days and strongly when 25 days of age. Immature birds remained distinguishable from adults in the field until at least nine months, b y when they had attained most of the dark blue body feathers of adults and had lost most of the distinctive streaking on the white wing-coverts.

The 41 nesting attempts resulted in 13 young leaving the nest during the year. Of the 28 attempts which failed, nine (32.1%) occurred before egg-laying or during the first few days of incubation, 14 (50.0%) happened during incubation, and only five (17-1%) during the nestling period. Four of the five nestling failures occurred when chicks were less than five days old and therefore only 3% of failures occurred after this. Adult magpie robins showed their most aggressive nest defence behaviour towards the diurnal skinks Mabuya spp. and to a lesser extent the Indian mynah Acridotheres tristis L. (Watson, 1978). Both these animals are likely to provide a threat to eggs or small chicks and, although the precise reason for nesting failure was never witnessed, it seems prob- able that predation by skinks or mynahs was the main cause.

Although 13 territories were briefly occupied during the year, only 12 adult pairs were present in most months (Table 1). Annual production was 1.1 chick per pair, or 0.54 per breeding adult.

Recrui tment

Throughout the study period no birds in immature plumage were found breeding. However,

there were two instances of attempted breeding by birds aged 12-14 months and magpie robins are therefore assumed to be reproductively mature by this stage.

Estimates of first-year survival are derived from records of colour-ringed and individually distinct birds observed during monthly population censuses. Survival was estimated for the first six months of life from records of young fledged before February 1978 since all such birds would po- tentially still be alive by the end of the study in July. Estimates for the second six months of life were obtained from birds believed to be six months old after July 1977 but before February 1978. There were eight birds in the first category and three disappeared, presumed dead, before they were six months old, giving a survival of 62-5%. For the estimate based on the second six months ten birds were included and two died before they reached 12 months, giving a figure of 80%. Calcu- lated first-year survival was therefore 50%. With the annual production of 13 chicks recorded in this study and an estimated first-year survival of 50%, the potential recruitment into the adult population was 6.5 birds. Using the figure of 24 birds for the adult population this means that there was, during 1977-78, sufficient potential recruitment to compensate an annual adult mortality of 27%.

Adult survival

A measure of adult survival has been estimated from a series of counts of colour-ringed birds up to 1983. Throughout there was periodic replenish- ment of the pool of marked birds. Results of counts are given in Table 5. For the interval

Table 5. Counts of individually colour-ringed magpie robins on Frigate between 1977 and 1983 and estimates of annual adult survival

Date September 1977 June 1978 March 1979 October 1980 September 1981 October 1982

No. of marked 10 13 I0 9 7 10 adults present in year, Y

Date June 1978 March 1979 October 1980 September 1981 October 1982 October 1983

No. of marked adults present in year, Y, and detected in year, Y+I

9 12 6 7 6 9

Estimated annual adult survival (%) 86.9 89.9 72.4 76.0 86.7 90.0

Survival estimates are calculated using the equation given in the text. Date refers to the month and year in which successive counts were made.

100 J. Watson, C. Warman, D. Todd, V. Laboudallon

between successive counts adult survival is calculated by the equation (b/a)12~n where a is the number of marked adults present in year Y, b is the number of those adults still present in year Y + 1 and n is the number of calendar months between counts in year Y and year Y + 1. Annual survival estimates ranged from 72.4% to 90.0%, with a mean of 83.65% + 7.54.

FACTORS LIMITING THE MAGPIE ROBIN POPULATION IN 1977-78

Why are robins restricted to Fr4gate?

There is strong circumstantial evidence that introduced mammalian predators, and particularly do- mestic cats, were responsible for the extinction of magpie robins on several islands within the Sey- chelles group. Three pieces of evidence are consistent with this view.

Extinction on Aride Late last century magpie robins were evidently common on Aride. In 1868, 18-20 adults were seen and each pair 'was accompanied by two imma- tures' (Hartlaub, 1877). Then during a visit in the 1870s no less than 24 specimens were collected (Oustalet, 1878). Despite this lamentable excess the population apparently recovered and survived into the 1930s (Vesey-Fitzgerald, 1940). Cats were deliberately introduced to Aride in 1918 and tem- porarily overran the island in the 1920s (Ridley & Percy, 1958). Todd (1982) estimates that the cat population might have risen to over 200 by 1925 when boys and dogs were employed to eliminate them. Ridley and Percy (1958) report that this was successful in about three years although P. Medor (pers. comm.), who was a boy on the island at that time, recalls that hunting continued up to 1932. He also states that magpie robins survived after the hunting of cats ceased, although the birds died out some five years later. It is possible that the release of two neutered cats on the island in 1937 may have delivered the final blow to the already depleted robin populations (Todd, 1982).

Extinction on Alphonse Prior to 1892 magpie robins were successfully introduced to Alphonse (Ridgeway, 1895), a coralline island some 400 km southwest of the granitic Seychelles. Vesey-Fitzgerald (1940) re-

ported a thriving population there in the 1930s. J. F. G. Lionnet (pers. comm.) informs us that cats were released on Alphonse in the 1950s. By 1959 the population had dwindled to one bird (Loustau-Lalanne, 1961) and visits in 1965 (Gaymer et al., 1969) and 1974 (Collar & Stuart, 1985) could only confirm the species' extinction.

Decline on Frigate in the early 1960s The fluctuations in robin numbers on Fr6gate in the 1960s have been described earlier in this paper. The 20 or so birds in 1960 declined to a dozen in 1965 before recovering to more than 30 in the early 1970s. Crook (1960) reported that cats had recently become established on Fr6gate and, following his warnings of the potential consequences for the robin population, a campaign to eliminate them resulted in 86 being killed in March 1960 (Department of Agriculture, 1961). Numbers of cats were significantly reduced, although they were not entirely eliminated (see below).

Magpie robins are particularly vulnerable to cat predation because young birds leave their nest sites up to 10 days before they are capable of strong flight. We therefore believe that feral cats have been principally responsible for the contraction in range of the Seychelles magpie robin from eight to a single island in the group.

What factors limit the numbers on Fr4gate?

The assessments of annual production and potential recruitment showed that there was no short- age of young reared in 1977-78 to compensate annual adult mortality measured over the six-year period 1977-83. The figure for production of young in a later study (Todd, 1982) when eight young were fledged by nine pairs over nine months (1.08 young/pair/year) was almost identi- cal to that for 1977-78.

In this study the distribution of magpie robins was closely linked to that of a bare earth or leaf litter ground layer. Communities with lush herbaceous vegetation were avoided. This distribution reflected the importance of bare earth and leaf litter for feeding. Most of the food taken by robins comprised items taken mainly from bare earth or leaf litter substrates (Collar & Stuart, 1985). We therefore conclude that the range of the magpie robin on Fr6gate is limited by the availability of suitable feeding habitat.

Within their range robins lived in territorial groups of up to six birds which invariably


contained a breeding pair, sometimes dependent young and up to three or four 'surplus' birds in adult, subadult or immature plumage. 'Surplus' birds took no active part in breeding (Watson, 1978). The existence of these birds, up to 12 in the population at any one time, is consistent with the view that the territorial behaviour of breeding adults, advertised by regular and conspicuous vocal behaviour of the territorial male and sus- tained through aggressive defence of territorial boundaries, operates as a proximate limiting factor within the population. Though ultimately limited by food or feeding habitat, we believe that, at least during 1977-78, territorial behaviour served as a proximate limiting factor preventing some birds physiologically capable of breeding from doing so (Watson & Moss, 1970). An opportunity to test this hypothesis arose when the translocation experiment was done.

ATTEMPTS TO RE-ESTABLISH THE MAGPIE ROBIN ON ARIDE IN 1978 AND 1979

Reasons for the transiocation

It was recognised (Watson, 1978) that habitat management on Frrgate would never be likely to bring about an increase of more than three or four pairs or territorial groups. By contrast a translocation to a second island, if successful, would provide the opportunity for a relatively rapid increase, and would give the species the added security of two self-sustaining populations. For these reasons the principle of a translocation was accepted in early 1978.

The resilience of the Frrgate population in 1978 was believed to be relatively high for two reasons. Recruitment was high in relation to tentative estimates of adult mortality. Secondly, the existence of up to a dozen 'surplus' non-breeding birds indi- cated that there was the potential for replacement of adult breeders if some of these were removed.

Of the eight previously occupied islands only Aride definitely held no cats in 1978. A detailed description of Aride is given in Warman and Todd (1984). Whilst there are some key differences from Frrgate, notably the presence of a million nesting seabirds at the height of the season in July-August, there are substantial parts of the island which appear to hold the critical feeding habitat for robins of bare earth/leaf litter. The 72 ha of Aride comprise some 67 ha of hill land

and a single coastal plateau of 5 ha, which has patches of mature trees, banana and coconut plantations and vegetable growing areas akin to the coastal plateaux on Frrgate. The hill holds at least 20 ha of Pisonia grandis R. Br. woodland. The dense shade cast by these trees gives rise to typically sparse ground cover similar to that found under mixed woodland on Frrgate in places where the canopy is more or less continuous. Pisonia woodland has always been part of the natural vegetation cover of Aride and must, given the numbers of robins recorded historically from the island (Hartlaub, 1877; Oustalet, 1878), have once supported considerable numbers of the species.

It was therefore decided to attempt to re- establish the bird on Aride.

The translocation in 1978

We decided to select six birds, three males and three females--if possible established mated pairs-- and not to catch any birds from the central core of territories on the large eastern plateau. The six robins were caught in mist-nets between 17 and 19 April 1978 and were individually marked with coloured plastic leg-rings. Mated pairs were caught at sites 10 and 12 and a territorial male was trapped at site 8 (Fig. 2). The sixth bird was a subadult female caught at site 12. Birds were kept in pairs in wooden cages measuring 50 × 50 x 50 cm with chicken-wire fronts. Perches were provided, drinking water was changed twice daily and birds were given freshly killed cockroaches three times a day, on each occasion until they stopped feeding.

The transfer took place in the morning of 20 April and this allowed all birds at least 24 h to re- cover from the effects of capture. The flight to Praslin took 28 min and the birds showed no signs of distress on arrival. They were then transported from Praslin to Aride in a fast motor boat and landed on Aride in the island pirogue. Following arrival they were allowed to settle for half an hour before being given a final meal of cockroaches. At midday the six birds were released at three different points on the coastal plateau of Aride. Throughout the period of captivity the birds were handled as little as possible.

Observations on Aride post-translocation

One of us (C.W.) followed the fate of the robins on Aride from April until November 1978 (Hellawell, 1979). Five of the birds--two females


and three males--were seen again after they were released; the subadult female was never relocated. All five birds were definitely alive until June, when a male disappeared. The four surviving birds were then seen regularly until September, when a second male disappeared. The three remaining birds were alive when observations ceased in November.

During the first month after release birds were rarely observed together and pair bonds which had existed on Fr6gate certainly broke down. By the end of May two pairs had established territories on the coastal plateau; territories included small areas of the fringing Pisonia woodland, parts of the vegetable gardens and the housing area. The third male was eventually and briefly located in Pisonia woodland in a remote part of the island. When one of the plateau males disappeared in June he was replaced by this male. Be- tween mid-August and late October there were four breeding attempts although none was ultimately successful.

The feeding behaviour of robins on Aride was similar to that recorded on Fr6gate. Birds were also recorded feeding on the ground beneath Piso- nia woodland, where on several occasions they were seen eating eggs of lesser noddy terns Anous tenuirostris (Temminck) and scavenging fish dropped by seabirds. Observations on feeding rates were made on Aride between May and September. Whereas birds on hill sites on Fr6gate spent more than 16% of their time foraging and birds on plateau sites spent 10% of time foraging, those on Aride, where observations were mainly on the plateau, spent on average 7% of their time foraging (Table 4).

Between November 1978 and March 1979 at least one magpie robin was fledged, although it did not survive longer than a month. In March 1979 all three adults recorded by C.W. in Novem- ber 1978 were still alive. One of the authors (J.W.) visited Aride twice in March/April 1979 and saw two of the original adults, a male and female, but not the second female.

Observations on Frigate post-translocation

The population on Fr6gate was censused on three occasions (May, June, July) immediately following the transfer and again in April 1979 (Table 1). It dropped to a low of 35 birds in May 1978 but had recovered to 38 by April 1979. By mid-May three of the vacancies created after the transfer had been filled. A 'surplus' female, previously colour-

ringed, became resident at site 12. The adult female at site 8 was joined by another 'surplus' bird, this time a previously colour-ringed male, and a subadult bird was in residence at site 10. By mid- July one more vacancy was filled, the female at site 12 being joined by an unringed male from the pool of 'surplus' birds. So, within three months of the translocation four of the five adult vacancies had been filled. By April 1979 sites 8 and 12 were each occupied by three birds, an adult territorial pair and a single subadult bird in both cases. There were no birds at site 10.

The translocation in 1979

One of the authors (J.W.) spent four weeks in Seychelles in March/April 1979 in order to monitor the progress of the 1978 translocation and to consider the case for a second transfer (Watson & Trowbridge, 1979).

Evidence from Aride was equivocal. Adult survival had been poorer than expected, with three birds disappearing in the first six months but only one, or possibly none at all, during the second period. The number of breeding attempts was en- couraging. Proof of breeding, combined with the feeding observations which showed that robins spent less time foraging on Aride than in equivalent habitats on Fr6gate, convinced us that there was a case for a second introduction if the Fr6gate population had recovered. The census on Fr6gate revealed a substantial recovery there, with 38 birds in April 1979 compared with 39.5 + 0.85 (n = 10) during 1977-78. The proportion of first-year birds had also returned to a level close to that pre- ceding the translocation. There was, however, one territory which had not been filled a full year after the birds were removed. It was therefore decided that, whilst a second translocation could be sus- tained by the parent population, it would be wise to remove only four birds in 1979.

As in 1978, the robins chosen for the transfer were mated pairs and territory holders and the pairs were taken from sites 6 and 12 (Fig. 2). The methods used were similar to those used in 1978 except that it took nearly 24 h to complete the transfer because exceptional rainstorms forced our boat to turn back from Aride on 12 April. Unlike 1978 it was not possible to monitor the birds following the translocation and we have no detailed information on either the Aride or Fr6gate populations until one of us (J.W.) visited the islands in October 1980.


The failure of the translocation experiment

During a week spent on Aride in October 1980 only one magpie robin was located, and he was the surviving male from the 1978 introduction. Clearly the experiment had failed. In the absence of follow-up observations on Aride it was imposs- ible to be certain of the causes of the failure. In retrospect several factors were identified which might have contributed. Hellawell (1979) sus- pected that chemicals were being used for pest control in houses and vegetable gardens in 1978 and this may have continued into 1979 with associated risks to magpie robins. Again, in 1978, observations revealed that several robins encountered the sticky fruits of Pisonia grandis on Aride. Many nesting seabirds die after their feathers become entangled in Pisonia fruits (Warman & Todd, 1984) and, although it is a native plant in Seychelles, it is now virtually absent from Fr6gate and robins may need to re-learn avoidance behaviour and methods of ridding themselves of this threat. Finally, the 24-h delay encountered during the 1979 transfer may have increased stress amongst the birds and contributed to mortality during the immediate post-translocation period.

Circumstantial evidence pointed to a growth in the island's feral cat population as the main reason for these losses. The best evidence for an increase in the number of cats came from the people of Fr6gate. They had killed 17 cats during the five months prior to February 1981, compared with only five during the previous two years. Further- more, during the four-day survey of the island in February two cats were seen and this was as many as seen during a six-month period in 1977-78.

Another change since 1977-78 had been the reduction in suitable feeding habitat in at least three territories as the management of the island turned away from vegetable production and towards the more lucrative tourist industry. This was probably the most significant factor involved in the deser- tion of territories 1 and 13, where outlying houses were abandoned and their associated vegetable plots allowed to revert to scrub.

CATS AND MAGPIE ROBINS ON FRI~GATE DURING 1981-82

Cat eradication

DECLINE OF THE FRI~GATE P O P U L A T I O N IN 1980-81

In October 1980 one of us (J.W.) revisited Fr6gate and could only locate 27 magpie robins dis- tributed amongst nine territories (Table 6). Al- though there was one more adult than had been left on the island in April 1979 there had been a dramatic decline in the number of first-year birds from eleven to just three. A subsequent census (co-ordinated by D.T.) in February 1981 revealed a further decline. Only 22 adults and two first-year birds were located.

Table 6. Sample counts of magpie robins on Frigate 1978-84

Date Author No. of No. of first- Total of count of count adults year birds

March 1978 JW 29 11 40 April 1979 JW 27 11 38 October 1980 JW 24 3 27 February 1981 DT 22 2 24 August 1981 DT 16 2 18 March 1982 DT 16 8 24 October 1982 JW 20 4 24 October 1983 JW 19 6 25 July 1984 VL 22 3 25

In July 1981 a team under the initial direction of Richard Veitch of the New Zealand Wildlife Service started a cat trapping and poisoning programme. The programme was continued by two of us (D.T. and V.L.) with the support of the Conservation Division in Seychelles until March 1982 and thereafter periodic visits were made to the island by one of us (V.L.) to ensure that no cats survived.

Methods The traps were set along paths to give a near regular distribution over the whole island. In order to overcome the problem of cats habituating to the traps, trapping was limited to periods of two to four weeks interspersed with periods when no traps were set. For the same reason traps were moved at irregular intervals and a variety of baits were tried. Some traps were left unbaited while others were set across narrow sections of paths where cats could be guided into them. All traps were visited twice a day, as soon after dawn as possible and again at dusk. In magpie robin territories and near habitation where there were free-range chickens, piglets and dogs, the traps were covered during the day.

104 J. Watson, C. Warman, D. Todd, II. Laboudallon

Poison baits were injected into small cubes of fresh fish. These were laid at approximately 20-m intervals along the middle of paths at dusk and uneaten baits were collected early the next morning.

probably caused by, this shift away from vegetable production. Under the conditions prevailing in 1983-84 it seemed that Frrgate could sustain only eight territories or magpie robin groups.

Results During the periods of concerted trapping up to March 1982, 46 cats were caught. A further three cats were caught in traps lent to the island staff and set when project members were not present on the island. Since March 1982 one of us (V.L.) made a further four visits to Frrgate and set traps but no more cats were caught and no cat signs were detected. It is not possible to give a precise figure for the number of cats poisoned although we believe that a minimum of four may have been killed in this way.

A total of 56 cats were accounted for between July 1981 and March 1982. It was difficult to be certain at the end of March 1982 that there were no cats left on the island since they leave few signs of their presence, due mainly to the nature of their diet. However, the complete absence of signs during four subsequent visits and the continued failure to trap any more animals makes it extremely unlikely that any cats now survive on Frrgate.

Changes in the robin population on Frigate in 1981--84

Counts of robins on Frrgate in October 1980 and February 1981 had revealed totals of 27 and 24 birds respectively (Table 6). By the start of the cat eradication programme in July 1981 there had been a further decline to 21. This was followed by three deaths in August, giving a population of 18 including just two first-year birds. Clearly the population was in a precarious state. Once the trapping had started, however, the number of fledged first-year birds increased quickly, reaching eight in just six months. Three more counts were made between October 1982 and June 1984 and whilst these confirmed that the recovery to around 25 birds was being maintained, there was no evidence of a return to the 1977-78 numbers.

Evidence of a gradual decline in the amount of land used for vegetable production was first noted in October 1980 and this contraction continued until, by late 1983, there was virtually no agriculture on the island save the collection of the coconut crop. The abandonment of four territories by magpie robins after 1979 paralleled, and was

DISCUSSION

Endemic birds on islands are more prone to extinction than any other group of avian species. King (1980) showed that over 90% extinctions over the past 300 years were island endemics and today some 240 species are classed as endangered (King, 1981), of which 130 (54%) are island- dwellers (Temple, 1986). A variety of explanations have been put forward to explain extinctions; however, attempts to isolate a single factor as the 'cause' of a particular extinction are probably unrealistic (Ralph & van Riper, 1985). Temple (1985) recognised that island species were particularly vulnerable because of their small and closed populations, and the evolutionary changes which are a frequent consequence of island colonisation. The latter include lowered intrinsic growth rates, adaptive response to the more limited range of ecological conditions on islands compared with equivalent continental areas, and reduced predator avoidance behaviour and resistance to disease or parasites.

Why is the Seychelles magpie robin endangered?

The magpie robin conforms to several of Temple's prerequisites for an endangered species. It has a low reproductive rate and therefore a low intrinsic growth rate. Its resource requirements in terms of food are relatively restricted, with a clear depen- dence on a narrowly defined feeding habitat. Al- though it does depend almost exclusively on bare earth/leaf litter, the range of circumstances under which such habitat is suitable varied widely and birds were able to exploit it whether in deep shade, secondary woodland, or in high light areas where herbaceous growth was continually removed by agricultural activity. The species' vul- nerability to cat predation clearly reflects reduced predator avoidance behaviour. We therefore conclude that the magpie robin is endangered principally because large parts of its range are no longer suitable following the introduction of feral cats, and because the single surviving population is exceptionally vulnerable to chance events in its biotic and physical environment and to demo-


graphic accident such as the development of a badly skewed sex-ratio (Temple, 1985).

What management techniques should be adopted?

The primary aim of management should be to reduce the likelihood of extinction caused by an ecological catastrophe or demographic accident. The range of management options for endangered species was explored in a review volume (Temple, 1978) and includes habitat management, control of predators and the translocation of birds to un- occupied habitat. The last two have both been tried for magpie robins and the successful eradication of cats from Frrgate island ensures that one major threat has been removed. In the light of the decline in small-scale agriculture on Frrgate, and the associated decline in the carrying capacity of the island, habitat management is clearly needed. Todd (1983) proposed the systematic re-establish- ment of mature woodland throughout the coastal plateaux and we strongly advocate this as the principal management option for Frrgate. Once achieved this would be likely to lead to a recovery at least to numbers recorded in 1977-78 and quite possibly to a doubling of the present population. If successful this would reduce the risk of extinction caused by demographic problems but would not significantly alleviate the risk of extinction caused by an ecological disaster such as a fire. The only insurance against such a possibility would be the successful translocation to a second island.

Fyfe (1978) reviewed the problems associated with translocation experiments. He listed five requirements which should be fulfilled in order to improve the prospects for a successful reintroduc- tion. These are:

(1) Prior identification of the species' requirements. (2) Concentration of releases to promote mating. (3) Reduction of mortality in released birds. (4) Release of genetically and behaviourally fit

individuals. (5) Adequate monitoring of the released birds.

In the Aride experiment we believe that (1), (2) and (5) were adequately covered, although there was inadequate monitoring following the 1979 releases. Requirement 3 was probably not fulfilled and the behavioural element of requirement 4 may also have been unsatisfactory, particularly when released birds encountered the novel experience of the sticky Pisonia fruits on Aride. Any future rein- troduction must therefore include an adequate

monitoring programme, seek ways of eliminating post-introduction mortality and devise ways by which released birds can learn to cope with the dangers posed by Pisonia fruits. If the above can be achieved, and when the Frrgate population has again recovered to levels similar to those of 1977-78, we would advocate a renewed attempt to re-establish magpie robins on Aride.

ACKNOWLEDGEMENTS

We are grateful to Warren King who instigated this project and to World Wide Fund for Nature (Project 1590) for financial support during 1977-78 and in 1979. Professor Geigy, IUCN, World Wild Fund for Nature and the Royal Soci- ety for Nature Conservation funded the emergency cat eradication project in 1981-82. The New Zealand Wildlife Service seconded Richard Veitch to advise on the cat eradication programme and his expertise was invaluable. We are indebted to ICBP, initially through the late Phyllis Barclay- Smith and subsequently Robin Chancellor and Nigel Collar, for administrative and much practi- cal support throughout. In Seychelles we owe a special debt to members of the Conservation Divi- sion, and particularly Lindsay Chong-Seng. Christopher Cadbury has unflaggingly supported our work on Aride. We are grateful to Dr Otto H~ippel for permission to work on Frrgate and to Yvon Savy, Paule Hoarau and Gilbert Irene for local support. Many others have helped the project over the years, and of these we would especially mention Boris Adam, Mike Brooke, Heather and Les Bunce, Professor George Dunnet, Martin and Annie Garnett, Vanessa Halhead, Jenny Hendry, Sharon Jones, Maurice Lalanne, Guy Lionnet, Chris and Cecile Lomer, Paul Medor, Martin Nicoll, Ron Nussbaum, Debbie Powell, President Albert Renr, Iain and Anne Robertson, Jim and Mary Romanos, Serge Savy, the late Sir Peter Scott, Kantilal Jivan Shah, Adrian and Judith Skerrett, Beverley Trowbridge, Don Turner, Stephen Warman and Roger Wilson.

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The Seychelles magpie robin Copsychus sechellarum: ecology and conservation of an endangered species

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