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Nordic Society Oikos

Forest Edges and Habitat Selection in Birds: A Functional ApproachAuthor(s): Duncan McCollinSource: Ecography, Vol. 21, No. 3 (Jun., 1998), pp. 247-260Published by: Blackwell Publishing on behalf of Nordic Society OikosStable URL: http://www.jstor.org/stable/3682975 .

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ECOGRAPHY 1: 247-260. Copenhagen998

Forest edges and habitat selection in birds: a functionalapproachDuncan McCollin

McCollin,D. 1998. Forestedges and habitatselection n birds:a functionalap-proach.- Ecography 1: 247-260.Edgeeffectsencompassa complexpanoplyof biotic and abioticphenomena crosswoodlandborders. identify our mainexplanationswhich have beenproposed oexplainavianhabitat electionwithrespect o forestedges:1)individualisticesourceandpatchuse,2) bioticinteractions; ) microclimatemodification nd4) changes nvegetation tructure. )relatesnestsite location n woodlands elative o the edgetotheproximityof food resources. t is shownthat,all otherthingsbeing equal,birdswhicharewhollydependent n resources oundwithinwoodlandswill tend to avoidforestedges.Woodland peciesdependent ponresources ound in adjacenthabitatswill tend to be found near to edges to enable their exploitation.2) identifiescompetition,predationand broodparasitism s factorswhichhave the potential oinfluencebird habitatselectionnearedges.3) identifiesmicroclimatemodification sa potential nfluencewhichmayact directlyon nestingsuccessor indirectlyhroughits effectson food supply; ) relates he activitiesof birds,suchas nesting, eedingoruse of song posts, to vegetation tructure nd/orfloristiccompositionat the edge.Research n edgeeffectsof birds n woodlandhasprovided ewpractical ecommen-dations to conservationmanagers.Forest edge managementneeds to take intoaccountthe multiplecauseand effectswhich influencehabitatselectionat the edgeand to targetspeciesof conservation oncern.D. McCollin ([email protected]),School of EnvironmentalScience, NeneCollege, Park Campus, Northampton, U.K. NN2 7AL.

Edge effects may be defined as changes which occur atthe abrupt transition between adjacent habitats, result-ing from the juxtaposition of contrasting ecosystems oneither side of the discontinuity (Sammalisto 1957). Ithas long been recognised that edges contain increasedbiodiversity since they attract species able to exploitboth sides of the discontinuity in addition to thosespecies characteristic of either side (Leopold 1933,Odum 1971, Clapham 1973, Odum 1983). Recent con-cern over the biological consequences of habitat frag-mentation has led to a renewed interest in edge effects(Saunders et al. 1991, Bierregaard et al. 1992) yet,surprisingly, even for well studied taxa such as birds,few studies have attempted to determine what resourcefactors are supplied by edges (Luken 1990).Recent reviews of edge effect research have high-lighted two areas of particular concern: poor research

design, and the difficulties in making comparisons be-tween studies due to a lack of consistency in methodol-ogy (Paton 1994, Murcia 1995). Paton (1994)re-examined 21 avian predation and parasitismresearchpapers and found that studies which showed no evi-dence of edge effects had often failed to investigatebeyond the potential range of edge influence (50 m).Many studies show site-specific trends generating littleconsensus and apparent inconsistencies in edge effectscan often be attributed to a lack of replicates (Murcia1995).Ehrlich (1996) called for a more sophisticated knowl-edge about temperate forests to be developed, includingedge effects, for which there is a need to formulate andtest clear mechanistic hypotheses (Murcia 1995). Ac-cordingly, the primary aims here are: to distinguishbetween the main types of edge effects and to elucidate

Accepted18 September 997Copyright? ECOGRAPHY1998ISSN 0906-7590Printed n Ireland all rightsreservedECOGRAPHY 21:3 (1998) 247


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potential mechanisms which influence habitat selectionat forest edges. The review focuses on birds since theyhave been the subject of much edge effect research andrepresent a group for which habitat requirements arereasonably well known.

The influence of edges on habitat selectionin birdsThe reasons why individuals in a woodland or forestbird population establish territories in particular loca-tions depend upon an interplay between the 'nichegestalt' of individual bird species and a number ofsite-specific factors including geographical location, al-titude, land productivity, woodland area and isolation,edge effects, stand structure and floristic composition,and social and demographic considerations (James1971, Fuller 1995). Here, I examine bird habitat selec-tion in relation to forest edges, with an emphasis onbreeding populations, distinguishing between four mainprimary causal factors which operate at the local-scale:1) species-specific differences in resource and patch use,2) biotic interactions, 3) microclimate modification and4) vegetation structure. These causal factors provide thebasis to formulate primary hypotheses forming thefoundation for erecting secondary and tertiary interac-tion hypotheses to provide a theoretical framework forinvestigating avian edge effects. Where appropriate,areas requiring further attention in addition to poten-tial confounding factors are highlighted.

Individualisticresource and patch useMany woodland birds appear to actively use or avoidwoodland edges (Galli et al. 1976, Tomialojc et al.1981, Moller 1987, Fuller and Warren 1991). However,edge species per se need to be distinguished from thosespecies which occur at the edge in response to specificaspects of the environment, e.g., vegetation (Haapenen1965, Tomialojd et al. 1981). Indeed, it is the distinctionbetween the influence of internal and external factors atthe edge which provides a useful distinction between'edge' and 'ecotone' species. Edge species may bedefined as species which occur near to edges due toinfluences external to the edge whilst ecotone speciesoccur near to the edge due to internal changes withinthe stand. Birds responding this way have also beentermed 'forest-periphery' and 'forest-margin' species,respectively, the latter being species exclusive to theedge whilst the former could occur deep in the interiorof large forest areas, but were found most often at theedge (Gromadzki 1970).Plotting incidence in small woods versus large woodsis a method that has been used to detect occupancypatterns of breeding bird species with respect to edge

(Howe 1984). Using this method, Taylor et al. (1987)identified several edge species in a British suburbanarea including the finches (Fringillidae), yellowhammer,Emberizacitrinella, and green woodpecker Picus viridis.A summary of the consistency in edge-orientated distri-bution patterns of bird species across five investigationsundertaken in northern Europe is presented in Table 1.This summary is divided into three sections: the firstcategorises species as having a preference for the edge;the second presents birds consistently regarded as 'typi-cal' (i.e., interior) forest species, and the third presentsspecies having conflicting interior and edge classifica-tions.Obligate edge speciesIn highly fragmented European agricultural landscapesspecies associated with edges adjacent to open countryare largely dependent upon food resources found infarmland (Table la). These include rook Corvus rugile-gus and carrion crow C. corone corone, which feedmainly on grain (especially Triticum,Avena, Hordeum),earthworms (Lumbricidae) and their eggs, and grass-land insects (although crow in addition takes live smallmammals and carrion meat) (Holyoak 1968). Similarly,starlings Sturnus vulgaris exploit the upper layers ofpasture soils and their diet comprises 80% leatherjack-ets (Diptera: Tipulaspp.) and 16% earthworms (Dunnet1955). Other typical edge species include finches whichdepend largely on seeds found in open farmland (New-ton 1967) and woodpigeon Columbapalumbus, whichhas its highest densities in small woods (


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Table 1. Edge-and interior-relatedroupings f birdspeciesnotedin five studiesof woodlandbirds n Europegrouped n termsof theirconsistencyof classification. ee notes below for explanation f symbols.Species Study1 2 3 4 5a. specieswith consistentedge preferencesn woodlands/forestscross studiesColumbapalumbus EFp Fg EStreptopelia turtur Ea EFp FUpupa epops EFM FLanius collurio Ec EFMSylvia curruca E Ed EMEmberiza citrinella E Ecd EFM Fg EPasser montanus E EFM ESturnus vulgaris h Ec EFP Fg ECarduelis(Chloris) chloris E EM F EC. carduelis E EFM F EPica pica Ea ECorvus monedula h EC?eC. corone cornix E E EPb. specieswithconsistent nteriordistribution atternsn woodlands/forestscross studiesDendrocoposmajor F F F LD. minor Fb F LPhoenicurusphoenicurus Fb F FTurdusphilomelos F F LParus palustris F F F LP. montanus F FP. cristatus F FP. ater F FP. caeruleus F F FPhylloscopus collybita F F LP. sibilatrix b FFicedula hypoleuca Fb FSitta europaea F F F LCerthiafamiliaris F Ff FLoxia curvirostra b FOriolus oriolus F Fc. specieswith conflictingnteriorand edgeclassificationsAthene noctua T FCuculus canorus Ea FJynx torquilla E F FAnthus trivialis b EFM FHippolais icterina Ec Ep FSylvia borin Ec EP FS. atricapilla Ec F F LS. communis Ecd EFM F LMuscicapa striata Fb EFM FParus major F Ec'e F FPhylloscopus trochilus Ed F F LTurdus merula E EFp FFringilla coelebs EFp F EPyrrhulapyrrhula F EEmberiza hortulana Ec FGarrulusglandarius Ee F F LNotes:1. Lack and Venables 1939)E= edge-species,F = hole-nesting pecies, trulywoodlandbirds',h = hole-nesting pecieswhichregularly est outsidewoodlands,T = speciesnestingn treesandfeedingoutsidewoods,a found n woods butprimarily irdsof open country,b characteristic f 'open'woods.2. Haapenen 1965)E= edge-species, main distributionn Finnish orests n cultivated reas,d also foundin openandclosedbrush or pine seedlingstands, also foundin high coniferous tands.3. Gromadzki 1970)E,_, = forest-margin pecies, occurringonly at the edge of the forest,E,_p= forest-peripherypecies,'occurring eepin the forest as well as on its periphery', = forestspecies, livingdeepin the forestin preferenceo forestperiphery, ut not avoiding he latter', refers o C. brachydactyla.4. Tomialojc t al. (1981)F = speciesbreedingn forest n BialowiezaNationalPark, = species oragingoutsideof the forest.5. Tayloret al. (1987)E= species oundpredominantlyn small woodlands


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'foraging' points, following the method of Skalski(1987), and determining the distance travelled fromvarious nest site locations along the radius of a circularterritory using the law of cosines. A circular territory isassumed since this is the shape predicted by centralplace foraging theory to minimise the ratio of circum-ference to area (Stephens and Krebs 1986).Mean travel costs incurred increase significantlywithnest site placement from the territory centre (F = 4.4 x104, p


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on a number of factors including concealment (Moller1989a), nest density (Nilsson et al. 1985), height aboveground (Martin 1993b, Nour et al. 1993) and distancefrom the edge (Paton 1994).The reasons why predators and their prey are at-tracted to edges and the consequences of their interac-tions were summarised by Andren (1995) and Marini etal. (1995). Predation at the edge may be the result of 1)an incidental function of the preferred use of edges astravelling lines by predators (Vickery et al. 1992), 2)active searching for prey as a predator response to thehigh density of prey found at the edge, 3) higherpredator abundance in edges and 4) a more diversepredator community (Nour et al. 1993, Andr~n 1995).For prey, one consequence is an avoidance of nestingnear to the edge although this also depends on nest typesince species with partially covered are found closer towoodland edges compared to open-nesting species(Moller 1989a).If raised predation rates in edges are largely a resultof passive incidental events then predation rates shouldbe density independent. Conversely, if predation is theresult of active prey searching behaviour, then preda-tion rates are predicted to be density dependent. Al-though subject to criticism (see Moiler 1989b, Andr~n1995, Major and Kendal 1996 for discussion) the provi-sion of artificial nests has already proved an invaluabletool for investigating predation, especially when com-bined with techniques for identifying predators. There-fore, the use of artificial nests to manipulate potentialprey density provides a useful technique to distinguishbetween active and passive predation. The reader isreferred to both Paton (1994) and Andr~n (1995) formore detailed discussion of these issues.Brood parasitismConcerns for the effects of brood parasitism have beenmost acute in North America where brown-headedcowbirds Molothrusater, in particular, have been impli-cated in causing severe population declines of forestbirds including kirtland's warbler Dendroica kirtlandii(Mayfield 1978, Probst 1986), golden-cheeked warblerD. chrysopariaand bell's vireo Vireo belli (Wiens 1989,Askins 1995). Cowbirds have increased in numbersconsiderably during the period of European settlementof North America and parasitism has also been shownto be related to distance from the edge (Gates andGysel 1978, Brittingham and Temple 1983, Paton1994).Observation involving nest searches is the most directmethod of quantifying brood parasitism. This is a time-consuming procedure which necessitates a high level ofskill in finding nests and is fraught with potential biaseswith respect to nest detectability. Behavioural studiesusing mounts (Mark and Stutchberry 1994) and theintroduction of dummy eggs to nests (Paton 1994) havealready proved useful.

MicroclimatemodificationIn modern agricultural landscapes, woodland edges of-ten have abrupt boundaries with adjacent land use andclimatic conditions in woodland edges are intermediatebetween the relatively undisturbed environment of thewoodland interior and wider climatic fluctuations morecharacteristic of canopy openings (Geiger 1936 but, seeChen et al. 1993). A forest canopy significantlymodifiesthe trunk space climate resulting in diminished netradiation, reduced wind speed, precipitation and rela-tive humidity; air temperatures near to the ground arereduced during daytime but heat loss at night is slow,helping to maintain an elevated ambient vapour pres-sure and a higher relative humidity (Geiger 1966, Lee1978). In general, microclimate gradients from edgesinto forest interiors are typified by decreasing windspeed and insolation, diminished air temperature, in-creased relative humidity and vapour pressure deficit(Table 2). Such gradients show a reasonable degree ofconsistency even between studies carried out in differentclimatic regions.As a rough rule-of-thumb, microclimate modificationextends up to three times the canopy height in from theforest edge (Fritschen et al. 1971, Harris 1984, Mc-Naughton 1989) although gradients extending 10-60 minto forests have been reported (Table 2). Microclimatemodification near the edge may also be influenced by:season (Young and Mitchell 1994), structure of thevegetation (Geiger 1966, Fritschen 1985) especially interms of successional status (Matlack 1994), edge aspectin relation to direction of sunlight (Geiger 1966, Mat-lack 1993, Young and Mitchell 1994), prevailing winds(Lovejoy et al. 1984, Matlack 1994) and topography(Lee 1978, Giles 1981). Wind has a greater impactwhere it has an unimpeded fetch across open country(Peterken 1996).To my knowledge, there are no published papers onthe direct effects of edge microclimates on breedingbirds, hence discussion of its role is somewhat specula-tive. Hilden (1965) noted the potential role of shelter inbird habitat selection but suggested that this had beenlargely dismissed by previous workers on the assump-tion that birds are homeothermic and thus compara-tively independent of their physical environments.However, climatic factors have been implicated in de-termining the northern limits of wintering bird distribu-tion due to physiological limits on metabolic rates(Root 1988), hence it seems reasonable for breedingsuccess to be affected by microclimate changes in edgesand in fragmented woodlots, especially near geographicboundaries of species' distribution ranges.

Temperature has been shown to be a factor influenc-ing the timing of breeding seasons (Perrins 1965) andambient air temperature influences both the heat ap-plied by a female great tit Parus major incubating eggsat the nest (Kendeigh 1963) as well as the daily energy

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Table2. Empiricalvidence or microclimatemodification t the forestedge.Where tated, hemaximumwidthsof microclimatemodification regiven.Microclimate Temperateorest Max. width of Tropical orest Max. width ofGradient gradient (m) gradient (m)Air temperature Miller 1975) 60 Kapos(1989) 40Chen et al. (1993) Williams-Linera1990)Matlack1993 24Youngand Mitchell 1994) 30-50Photosyntheticallyctive Youngand Mitchell 1994) 10-20 Kapos(1989) 20radiationPAR)Light intensity Chenet al. (1993)Matlack 1993) 44Littermoisture Ranneyet al. (1981)Matlack 1993) 50Plantwater use Kaposet al. (1993)efficiencyRelativehumidity Miller(1975) Kapos(1989)Chen et al. (1993)Matlack 1993) 50Soil moisture Oostingand Kramer 1946) Kapos(1989)Soil temperature Chen et al. (1993)Vapourpressuredeficit Miller(1975) 60 Kapos(1989) 20Matlack 1993) 50

Youngand Mitchell 1994) 20-50Watervapourpressure Kapos(1989)Windspeed Fritschen 1985)Chen et al. (1993)

expenditure of a female great tit tending nestlings (Tin-bergen and Dietz 1994). Prevailing environmental con-ditions have also been implicated as a factor influencingthe growth of nestling great tits (Gebhardhenrich andVannoordwijk 1994). Wind stress has also been shownto affect foraging site competition between crested titsParus cristatus and willow tits P. montanus in Belgium(Lens 1996). Thus, habitat quality for certain birdspecies may be reduced at the edge due to the directeffects of temperature and other light-related micro-climate factors.

A second, indirect way in which climate potentiallyaffects forest birds is by its effects on the timing andpredictability of food resources (Saunders 1982). Birdstime their breeding to have young in the nest when foodis most abundant (Lack 1954, Perrins and Birkhead1983), hence a lack of synchronization between timingof breeding and the larval hatching of invertebrate foodsources leads to reduced fitness, and might go someway to explain the delayed natal dispersal observed incrested tit P. cristatuspopulations in small woodlots inBelgium (Lens and Dhondt 1994). This effect may bedue to asynchrony further down the food chain, e.g., acorresponding lack of synchronization between inverte-brate larval hatching and oak Quercus spp. tree budburst in small woodlots, as shown by van Dongen et al.(1994). However, although first leafing dates of oak arestrongly linked to spring temperatures,these are mainlycorrelated with those months before leaf flush (Sparksand Carey 1995). It therefore seems unlikely that asyn-chrony could be directly linked to the effects of agradient in air temperature across the forest edge.

In the temperate zone, the effects of microclimatemodification on birds may be revealed by contrastingnorth- and south-facing edges. The use of single species,even-aged stands would effectively control for the po-tentially confounding effects of vegetation structure.The structural contrast between forest and adjacentland use may be an important influence on micro-climate at the forest edge. Further work is needed toinvestigate this. If microclimate modification actsthrough disruption of phenology, studies need to beundertaken to compare timing of biological events insmall versus large woods.

Vegetation modificationIncreased exposure at the edge leads to elevated soiltemperatures (Miller 1975, Chen et al. 1993), drying ofsoil (Oosting and Kramer 1946, Kapos 1989) and leaflitter (Ranney et al. 1981, Matlack 1993) and changesto leaf relative water content (Kapos 1989) and plantwater use efficiency (Kapos et al. 1993). Vegetationmodification due to habitat fragmentation, successionand management may change the availability of nestsites or affect food availability for birds at the edge.Few studies have tested either of these factors but anunderstanding of the successional dynamics in edges isbeginning to emerge.Clearance renders newly-created forest edges open toclimatic extremes with elevated tree mortality beingreported largely due to uprooting by severe winds(Williams-Linera 1990, Esseen 1994). In the tropics,

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canopy and sub-canopy damage has been reportedwithin 150 m of forest edges with increased leaf-fall andreproductive asynchrony (Lovejoy et al. 1983, 1984,Laurance 1991) also being reported. Increased insola-tion favours germination of pioneer species from theseed bank (Ng 1983) and, within a few years, promotesthe development of adventitious limbs on trees at theedge, a high density of saplings and greater shrubcover.Within five years of clearance of tropical forest adense wall of vegetation develops at the edge effectivelyinsulating the forest interior from the extremes of edgeclimates although in temperate forests the rate of sidecanopy closure is slower and may not be fully completeeven within half a century (Williams-Linera 1990, Mat-lack 1994). Edges opened by clearance set in motionsuccessional dynamics similar to gap-phase regenera-tion in natural canopy gaps (see Brokaw 1985 andHubbell and Foster 1986 for reviews) in which thedegree of microclimate modification is related to gapsize. Most canopy gaps tend to be small (Foster andBoose 1992), although edges open to climatic extremeswill be generated by large magnitude, albeit infrequent,disturbance events such as hurricanes. Within a fewyears edges are soon enveloped except where the pro-cess is halted or modified, for example, by land usechange and/or by the invasion by domesticated ani-mals, capable of removing the forest understorey(Levenson 1981, Howe et al. 1981, Whitney and Somer-lot 1985), affecting seed predation (Burkey 1993) andthe dynamics of recolonisation (Janzen 1983).Vegetation effects on bird habitat selection at the edgeVegetation potentially influences the distribution ofbirds at the woodland edge in the provision of nestsites, food, and song-posts. The effect of vegetationstructure on bird species in woodlands is lucidlydemonstrated by studies of coppice management, inwhich periodic felling of trees promotes the develop-ment of even-aged stands, each having distinctive struc-tural characteristics.Bird species richness and density incoppice reach peaks during the thicket stage immedi-ately before canopy closure (Fuller and Henderson1992) and the densities of bird species associated withparticular structural phases show clear trends whichparallel maturity-related changes in high forest stands(Ferry and Frochot 1970, Moskit and Waliczky 1992).In Britain, early stages after coppicing are character-ised by the presence of tree pipit Anthus trivialis andwhitethroat Sylvia communis;thicket stages are charac-terised by the presence of migrants such as willowwarbler, chiffchaff Phylloscopus collybita, blackcap,garden warbler Sylvia borin and nightingale Lusciniamegarhynchos,whilst residents, such as robin Erithacusrubecula, are more abundant after canopy closure andmaturation (Fuller 1992). Depending on location, thesemay be joined by other species such as dunnock

Prunella modularis and yellowhammer in the establish-ment phase and blackbird Turdus merula, chaffinchFringilla coelebs, blue tit Parus caeruleus and great titP. major, after canopy closure (Fuller and Moreton1987). Species associated with thicket stage coppice arethose most often found at edges close to open farmland(Fuller and Warren 1991). This could be attributed tothe structure of the vegetation at the edge (Fuller andWhittington 1987) and the dependency of such specieson vegetation structure, rather than edge per se, possi-bly explains the inconsistencies in edge classification inTable Ic. Variations in vegetation structure betweenstudies leads to a highly variable classification of eco-tone species.Whilst the dominant tree species in woodlands mayinfluence bird community composition (Opdam et al.1985, Fuller 1992, 1995), few attempts have been madeto resolve the degree to which birds in temperate forestsare influenced by tree specificity or by the overallstructure of woody vegetation (but, see Bersier andMeyer 1994). Specificelements of the vegetation may beimportant for some bird species (Blake and Karr 1987,Fuller 1995), hence associations between particulartrees and shrubs with edges deserve further consider-ation. However, whilst understorey plant species havebeen shown to have edge distribution patterns whichcorrelate closely with light-related environmental gradi-ents, little evidence has emerged so far that trees showsimilar patterns except, perhaps, for sapling densities(Brothers 1993, Fraver 1994, Matlack 1994, Young andMitchell 1994).

Vegetation structure can be manipulated experimen-tally by the removal of vegetation layers (e.g., Slagsvold1977, Howlett and Stutchbury 1996). Managed coppicewoodlands comprise ideal stands for such experimentalmanipulation and the use of single species even-agedcoppice could control effectively for floristic differencesin vegetation. A direct effect of vegetation structure onbird densities predicts that a removal of vegetationwould result in a decrease in bird density. If vegetationstructure is the main determinant controlling distribu-tions of particular bird species, vegetation in the wood-land interior should contain similar species/densities asthat at the edge and vegetation removal should producesimilar trends in both the woodland interior and edge.

DiscussionThe boundaries between the four causal factors areartificial in the sense that they may be difficult todisentangle, for example, the degree to which vegeta-tion dynamics can be considered in isolation frommicroclimate modification (see later). In attempting totest the primary hypotheses, difficulties may arisethrough the inability to control for the various interac-

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Table3. Primary ndsecondarynteraction ypotheses.The table s arrangedwith the causative actorsalongthe horizontal ndfactorswhichareactedupon alongthe vertical.Theitemsalongthediagonal ormtheprimary ypotheseswhilst he remainingitems are the secondarynteractionhypotheses.The predictedbird densitiesnearthe edgesare indicated or each hypothesis:negative -ve); positive + ve). See text for further xplanation.Primary actors

Patch use in Interspecific Microclimate Vegetationrelation o location interactions modification structureof resourcesPatch use HYPOTHESIS Edgeavoidancein relation Interior - ve) and due to threatofto location edge species + ve) predation- ve)of resources

Secondary Interspecific - HYPOTHESIS II - Ecologicalfactors interactions (- ve) trap hypothesis(+ ve)Microclimate - - HYPOTHESISII Effects of vegetationmodification i) directeffects(+ ve) structure n micro-ii) indirect ffects(-ve) climateVegetation - - Effects of micro- HYPOTHESIS IVStructure climatemodification Availability f:on vegetation tructure ii) song posts (+ ve)iii) nest sites(+v e)iii) food (+ve)

tions between causal factors. In addition, edge effectsmay be additive such that a point in a small wood-land might be subject to a cumulatively larger edgeeffect than a point at the same distance from theedge in a larger woodland (Malcolm 1994). Notwith-standing these reservations, the ways in which theseprimary factors potentially interact may provide in-sight into the complexities of edge phenomena.

Secondary interactionsEach of the primary causal factors potentially interactto generate a number of secondary interaction hy-potheses. Each of the major hypotheses may be re-garded as primary or secondary factors. Each of thefour primary factors are arranged along the sides ofTable 3. For each possible interaction the primaryfactor is along the top whilst the factor which isacted upon is along the side. In the table itself, theprimary hypotheses are arranged along the diagonal,e.g., vegetation structure is a primary factor affectingbird species through the provision of nest sites, food,cover and song-posts. In each case the predicted out-come in terms of bird densities at the edge is given.An example of a secondary interaction hypothesisis the influence of habitat structure at the edge on themagnitude of predation due to its role in nest con-cealment (Redmond et al. 1982, Yahner and Wright1985, Yahner and Cypher 1987, Ratti and Reese1988), in providing perch sites for nest predators(Yahner and Wright 1985, Ratti and Reese 1988,Moiler 1989a) and through its influence on food re-

sources - attracting a higher density of potentialprey. Birds might thus be attracted to the edge butsuffer diminished breeding success as a result of in-creased predation. Gates and Gysel (1978) termed thisan 'ecological trap'. This hypothesis predicts that dif-ferences in the composition and density of the avi-fauna in edge versus interior should reflect differencesin food supply. Manipulation of invertebrate abun-dance by experimental defaunation could prove to beinformative here.

Increased cover at the edge is predicted to affectonly visual predators; it should not inhibit olfactoryor auditory predators (such as rodents or reptiles).Support for this prediction is provided in a review byClark and Nudds (1991), who considered the impor-tance of concealment on nesting success. Concealmentwas by far the most important determinant of nestingsuccess in cases where birds were the major predators,but nesting success was considerably diminished incases where mammalian predators were present.Nest predation is the main cause of reproductiveloss for many bird species and whilst there may bephylogenetic constraints, birds may attempt to reducepredation through behavioural mechanisms orthrough nest site choice. Gbtmark et al. (1995) sug-gested that nest site selection may represent a trade-off between concealment and the need for visibility.They found that song thrushes Turdusphilomelos didnot maximize concealment within trees as expectedbut selected intermediate levels of cover. Risk of pre-dation for artificial nests was found to be inverselyrelated to cover although they were unable to detectsimilar trends predation rates for natural nests.

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Experimental manipulation of vegetation structureneeds to be combined with nest predation studies suchas the use of artificial nests. Assuming a relationshipbetween bird density and vegetation structure, experi-mental manipulation of vegetation structure at the edgemay be used to manipulate prey density to test theeffects on predation rates. Again, coppice woodlandswould appear to be ideally suited to this task. Thicketstage coppice may be thinned to reduce the density ofvegetation in order to compare the effects of vegetationdensity, and hence concealment, on predation rates.

Comparisons of successional pathways, recruitmentand vegetative development in stands contrasting inaspect would be useful to test for the influence ofmicroclimate gradients across the edge. Microclimatemodification as a result of coppicing is well established(Ash and Barkham 1976, Mitchell 1992) although com-parative studies of microclimate in coppice panels at theedge compared to those in the interior would be infor-mative. Vegetation development at the edge effectivelyseals the trunk space from external influences and di-minishes the edge effect. Hence, microclimates need tobe monitored over the complete cycle of a coppicerotation in order to establish the feedback betweenvegetation and microclimate. Again, contrasting studiesbetween north- and south-facing edges may be instruc-tive.

Higher-orderinteractionsA tertiary hypothesis, involving the interaction of threeprimary factors was suggested by Fuller (1990) to ex-plain edge-orientated distribution patterns of birds incoppice woodlands. The high density of the shrub layerat the external edge may result in increased availabilityof food resources for birds since woodland edges havebeen shown to contain increased diversity of arthro-pods (Helle and Muona 1985, Bedford and Usher1994). In turn, both of these factors may be related toa third factor, the increased insolation at the edge. Thedensity of birds in upland spruce plantations wasshown to be higher within 10 m of edges compared tothe interior. This pattern was indirectly linked to higherproductivity at the edge, as measured by counts offallen cones (Patterson et al. 1995).

ConfoundingfactorsSocial and demographicconsiderationsThe close proximity of thicket stage vegetation wasimplicated in an edge-related gradient in migrant song-bird density in old coppice by Fuller et al. (1989).Similarly, Patterson et al. (1995) found that densities ofmeadow pipits Anthus pratensis and willow warblerswere influenced by the presence of trees in the 0-8 yr

age-class outside plots. This edge effect, like the onedescribed below for birds in coniferous plantations inWales, could arise due to demographic considerations;in years when population levels are high, birds mayoccupy a wider range of habitats, including sub-optimalhabitats with individuals being forced out of the pre-ferred habitat into adjacent less-preferred areas(O'Connor and Fuller 1985).

Where woodland edges are contiguous with struc-turally similar stands, species may nest near to the edgesimply due to a spillover of individuals from nearbypreferred habitat. Individuals in such populations maythus represent sub-dominant members of a larger popu-lation and show body size characteristics consistentwith this such as those detected in work on blackbirdpopulations in small woodlots (Moller 1991, 1995).Density and spatial configuration of nests are alsoattributes of populations dynamics and both have beenshown to have an influence on predation rates at theedge (e.g., Hogstad 1995). At a broader scale, popula-tion models may need to incorporate terms for habitatquality (Pulliam 1988, Harrison 1991, Dias 1996).Landscape contextThe structure, constitution and geographical position ofa landscape influences the composition of its species-pool and thus the potential interactions between itsconstituent bird species. The presence of particular birdspecies in a regional species pool is dependent not onlyon landscape and habitat structure but also on thehistory of interactions between species according to thetrajectory of the fragmentation process. In this sense,each landscape is unique rendering it difficult to devisegeneral laws.

Assuming an edge-zone of set width, a simple modelwhich illustrates the change in the proportion of edgeto interior in relation to the size of individual woodedpatches was described by Levenson (1981). Smallpatches may be viewed as 'edge' habitat whereas largerpatches contain a disproportionate amount of 'interior'.Thus, as mean patch size decreases due to habitatfragmentation the balance is tipped over in favour ofedge habitat. Within a range of edge widths between 10m and 50 m, woodlands > 100 ha in size are predomi-nantly interior (Fig. 2). In England, woodlands ofconservation interest over 100 ha in size comprise only1.9% (by number) of sites in England and Wales butaccount for 23.6% of the area. Many remaining frag-ments lie in the size range 1-5 ha (44.0% by number,9.6% by area) and therefore constitute 'edge' habitat(Spencer and Kirby 1992).The total forest edge habitat in a landscape dependsnot only on cover but also on the grain of the landscape(Gardner et al. 1987, Lavorel et al. 1993). Fragmenta-tion results in an increase in the amount of edge in alandscape with a maximum occurring when half thelandscape is occupied by forest (Franklin and Forman

ECOGRAPHY 21:3 (1998) 255


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1987). However, when two landscapes with the samecoverage of forest are compared, a coarse-grained land-scape with a few large forest blocks will have consider-ably less edge habitat than a fine-grained landscapewhere there are many small forest patches. Thus, differ-ences in the grain of a landscape result in differentproportions of edge to interior for the same amounts ofcover (Gustafson and Parker 1992).The presence of particular predators in forest birdcommunities is linked to the amount of forest coverand farmland in a landscape (Andr~n 1995). Edge-re-lated increases in predation are more common in forest-dominated landscapes and changes in landscapecomposition due to fragmentation results in a shift inthe balance from specialist nest predators of forest (e.g.,jay Garrulusglandarius and raven Corvus corax) togeneralist nest predators associated with agriculturalland (e.g., hooded crow C. c. cornix, jackdaw C.monedula and magpie Pica pica) (Andren et al. 1985,Angelstam 1986, Andren 1992, Santos and Telleria1992).Edge contextFew investigations have considered the influence ofadjacent land uses on bird densities in woodland. InWales, Bibby et al. (1985) analysed the effects of adja-cent land use on densities of birds at the edge (mea-sured at 30-40 m) compared to densities in the interior(>40 m) of coniferous plantations. No significant dif-ferences in bird densities were found where plots abut-ted broadleaf woodland, grass, heather or clear-felledforest, although bird densities were significantly lowerat the edge than in the interior where plots wereadjacent thicket stage conifers. The authors recognised,however, that with no plots


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From a conservation perspective the primary goalmust be: first, to identify which extant species are ofconservation interest; then to undertake managementpractices to the benefit (or, at least, not to the disadvan-tage) of the target species. The framework presentedhere provides a conceptual model which highlights po-tential secondary and tertiary knock-on effects of habi-tat or landscape manipulation. Forest and landscapemanagement prescriptions must not overlook or over-simplify the potential repercussions arising from thecomplex array of biotic and abiotic phenomena encom-passed by edge effects.Where management of edges is bird-orientated itshould operate with clear objectives since managementfor certain 'interest groups' may be directly at variancewith others, e.g., predators and their prey. In Britain,where only a few woodland bird species attract conser-vation concern, species associated with other habitats,e.g., merlin Falco columbarius of high moorland, havebenefitted from the introduction of large-scale forestryin previously non-wooded areas due to the increasedopportunities for nesting in addition to availability ofprey along forest edges (Little et al. 1995). Such anincrease would be unfortunate if their prey were also ofhigh conservation interest (Lavers and Haines-Young1997).Acknowledgements I thankShelleyHinsley,HenrikAndr6nandJeff Ollerton or their constructive omments.

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