165

VOLUME 26 NUMBER 3

Bohemian Waxwings selectively feeding on thestamens of Silver Maple

Winnie Poon

Figure 1. Location where the Bohemian Waxwingswere seen feeding; the flowering Silver Maple is nearest the picnic table. Alden Road, Markham, York,8 April 2008. Photo: Winnie Poon

IntroductionIn the winter 2007-2008, Ontario expe-rienced its largest winter finch irruptionin the last ten years. Many boreal finchesirrupted well south of their normalranges, following the largest coniferousand deciduous tree seed crop failure in adecade across the boreal forests in muchof Ontario and western Quebec. Thisevent unfolded exactly as forecasted by Ron Pittaway in his Winter Finch Fore-

cast 2007-2008 (Pittaway 2007). At thesame time, a massive crop failure of thenative Mountain Ash (Sorbus decora),also occurred across much of northernOntario. Mountain Ash berries normallyprovide an important winter food forBohemian Waxwings (Bombycilla garru-lus) across boreal regions of Canada, butthe 2007 crop failure evidently precipi-tated another irruption of this speciesacross eastern North America.

166

ONTARIO BIRDS DECEMBER 2008

This irruption turned out to be thelargest ever recorded in the GreaterToronto Area (GTA) and adjacent south-ern Ontario (Poon 2008). It lasted fromapproximately mid-October 2007 to thethird week of April 2008. It was veryextensive in terms of bird numbers, geo-graphic area covered and duration. With-in the GTA, it surpassed the previousrecord of 1999-2000, and even the influxof 1958-1959, which Gunn (1959) des -cribed as “the largest of the century”.

But the most notable feature of the2007-2008 irruption was the returningwave of waxwings in spring, that resultedin an unprecedented number of Aprilrecords for the GTA, especially withinthe City of Toronto. The number ofrecords in the database of the TorontoOrnithological Club reached 48, whereasonly eight previous years had Aprilrecords, and then only 1 to 3 each. It isacknowledged that the database may beincomplete for some earlier years. It wasduring this returning wave that theapparently rarely seen feeding behaviourof Bohemian Waxwings eating the sta-mens of a Silver Maple tree (Acer sacchar-inum) was recorded.

CircumstancesOn 3 April 2008, Siegmar Bodach postedto ONTBIRDS (the listserve of theOntario Field Ornithologists) that aflock of about 35 Bohemian Waxwingsand 9 Cedar Waxwings (Bombycillacedrorum) had been seen near the inter-section of Alden Road and Hood Road in

Markham, York Regional Municipality.The waxwings were reported to be feed-ing on “fallen shoots”, and also “newlyopened shoots” on the trees. Intrigued bythis report and being curious as to whatthe “shoots” would look like, Roy Smithand I arrived on site on 5 April for a clos-er look.

There were already six other birdersenjoying the bird activities when wearrived at 1420 h. The location was thefront lawn of a small industrial buildingon the southeast corner (Mactec Canada– 555 Alden Road) (Figure 1). Close tothe front door of the building was a 7 – 8m tall Silver Maple (Acer saccharinum) infull blossom. There were two other small-er maple trees about 10m north, but bothhad wilting blossoms. Around the north-east corner, at the back of the side yard,was a row of four very thin HighbushCranberry (Viburnum opulus) bushesthat were devoid of berries, except forsome fallen ones on the lawn beneath.Most of the winter snow had melted,leaving about 5% of patchy snow coveron the lawns, and exposing the fallenfruits as noted. Lining the sides of thelawn were 6 small, bare ash trees. Also,across Alden Road were seven other simi-lar maples in bloom.

We counted at least 38 BohemianWaxwings, but no Cedar Waxwings. Themajority were feeding actively on the oneSilver Maple at the front door, but not onthe other maples in the area. Sometimes agroup would perch on one of the smallerbare ash trees, perceivably resting after a

VOLUME 26 NUMBER 3

167

Figure 2. Nine BohemianWaxwings drinking from meltwater on manhole coverat Markham, York, 5 April2008. Photo: Winnie Poon

good meal, while several times thesebirds flew down to a manhole cover near-by and drank from melt water that hadcollected there (Figure 2). At times, a fewbirds would split off and fly to the High-bush Cranberry bushes, dropping to theground to eat the remaining fallen fruits.

All the birders were about 8-10mfrom the maple tree and waxwings whichwere pecking incessantly at what app -eared to be clusters of tiny reddish budson the branches. Using binoculars, Icould see that the birds were not pluck-ing off and consuming the buds, butinstead were just dipping their bills intothe buds. I examined closely a clump ofthese buds in my hand, and was surprisedto find that from each tiny bud (flower)were many long and thin filaments (sta-mens) with richly laden yellow pollenpods at the tips (anthers) (Figure 3).

Each clump was a mass of newlyopened maple flowers and I suspected

that the birds might be after the polleninstead. I investigated further using mytelescope, concentrating on one feedingwaxwing. The bird appeared to be pluck-ing off and eating only the stamens, leav-ing the small red petals intact on theflowers that it was feeding on. It requiredmuch concentration and time to see thebird swiftly plucking off the stamens,even with the help of a scope at close dis-tance. At this point, I digiscoped manyphotographs of a feeding bird, includingtwo series of continuous burst modephotos. Afterwards, by examining one ofthese series frame by frame, I was able toconfirm that the bird was eating only thestamens (filaments and anthers), whilethe outer parts of the flowers remaineduntouched from the first frame to the last(Figure 4). These photos provide materi-al evidence of the Bohemian Waxwingconsuming stamens from a Silver Mapletree in spring.

168

ONTARIO BIRDS DECEMBER 2008

DiscussionIn winter, Bohemian Waxwings are pri-marily berry and fruit-eaters, utilizing awide variety of fruits for their winter sur-vival. In Ontario, some of these fruits areMountain Ash, crab-apple (Malus sp.),buckthorn (Rhamnus sp.), juniper (Juni -perus sp.), bittersweet (Celastrus sp.),dogwood (Cornus sp.), and HighbushCranberry. When available, BohemianWaxwings may also feed on protein-richinsects (Witmer 2002) and tree buds,including American Elm (Ulmus ameri-cana) and ash (Fraxinus sp.) (Pittaway1990), as well as maple (Acer sp.) (Elder2002).

Figure 4. Bohemian Waxwing feedingon the maple tree, note that the birdhas three anthers in its bill and thatmost of the stamens have been nippedoff the flowers directly in front of it.Markham, York, 5 April 2008. Photo: Winnie Poon

VOLUME 26 NUMBER 3

169

During spring, the waxwings may alsofeed on sap drips from maple and birch(Betula sp.) trees (Bent 1950). However,for North Am er ica, there seems to be nopublished record of Bohemian Wax -wings selectively consuming flower sta-mens, despite the possibility that this

may have been observed before. Bent(1950) quoted Swarth (1922) thatBohemian Waxwings “...were seen feed-ing on insects and also on berries andother vegetable matter”. Although thisquote may be suggestive, there is no spe-cific mention of this species consuming

Figure 3. Clusters of blooming Silver Maple flower. Each flower had at least14 -15 stamens with extremely long filaments. The anthers were large andreddish-yellow in color when fresh. Markham, York, 8 April 2008. Photo: Winnie Poon

170

ONTARIO BIRDS DECEMBER 2008

flower parts in Bent's account. Witmer(2002) also quoted from Bent (1950)that the Bohemian Waxwing “often feedson flowers of trees and shrubs in spring”.It is possible that Witmer drew the abovequote from Bent’s account of the CedarWaxwing, since it is commonly acceptedthat the two North American species ofwaxwings share similar dietary and feed-ing habits. Undoubtedly, BohemianWaxwings have often been seen eatingthe same foods as Cedar Waxwings, andit seems logical to assume that the resultsof certain studies of Cedar Waxwings canbe extrapolated to Bohemian Waxwingsas well.

Bent’s account of the Cedar Waxwingcontains a number of statements withregard to flower consumption; includingthe following:� “The only other vegetable food of

importance in the diet of the Cedar-bird is flowers”

� “At New Orleans...about Feb.1, whenit arrives to feed on the fruit of hack-berry and Japan privet, and the flow-ers of the elm. It later feeds on theblossoms of the pecan...”

� There are several records of cedarbirdseating the petals of apple blossoms.

As for Cedar Waxwing specificallyconsuming flower stamens, one earlyreport (Barrows 1912), states that “Dur-ing spring and early summer the Cedar-bird appears to be very fond of blossoms,and especially of the stamens, of manytrees, particularly fruit trees. We haveseen it frequently eating the stamens of

apple, pear, cherry, oak, maple and ash,and it doubtless eats stamens of manyother varieties”. But for Bohemian Wax -wing, Barrows only noted that “...thisbird feeds mainly on the same berries,seeds and fruits as the Cedar-bird...”.

More recent studies on the CedarWaxwing found that flowers, includingstamens, comprised only 4% of theannual diet, but in May, when fruits arescarce, they could amount to 44% of thediet (Witmer 1996). Flower petals mayprovide sugars, while pollen on stamensprovides protein. Consumption of plantspecies that do not have showy petals ornectar rewards (Acer, oaks — Quercus,and poplars — Populus) indicated thatthe waxwings were partly motivated toconsume pollen. Witmer also observedthat Cedar Waxwings ate the staminatecat kins of Eastern Cottonwoods (Populusdeltoides) in combination with HighbushCranberry in spring, this observationsubsequently led to his conclusion thatthis diet-mixing behaviour was a strate-gic choice for the waxwings at that timeof year.

The inter-relationships between thefruiting ecology of Highbush Cranberry,and the food requirements of CedarWaxwings, were carefully unravelled byWitmer (1994, 1996, 1998, 2001).Starting with field observations, he notedthat Highbush Cranberries tended to beignored by most bird species during thelate fall and early winter, and only eatenby Cedar Waxwings in late winter andspring, when alternative fruit resources

171

VOLUME 26 NUMBER 3

would be at minimum levels. Otherspecies virtually ignored it. One mightassume that these Highbush Cranberryfruits are generally unpalatable to birds,or perhaps contain insufficient energy orprotein to be ‘worth eating’, but Witmerfound that the aged fruit in late winterdid contain enough simple sugars tomeet the birds’ energy needs, albeit defi-cient in nitrogen content. When Witmer

presented samples of early winter fruits(preserved by freezing), and late winterones to caged Cedar Waxwings, he foundthat his experimental birds preferred theearly winter fruit over the aged fruit.By the natural process of ageing anddessication, late winter or aged fruits(Figure 5) contain higher concentrationsof secondary compounds that help pre-serve them against microbial and fungal

Figure 5. Dessicated V. opulus fruits can persistuntil late spring. Toronto, 22 May 2004. Photo: Winnie Poon

attack. Among the four classes of second-ary compounds: alkaloids, cyanogenicglucosides, terpenes, and phenolics, thelatter two especially will produce strongorganic acids if metabolized. Thus, thephysiological challenge in consumingHighbush Cranberries is that they con-tain a phenolic compound (chlorogenicacid) that renders the fruits extremelyacidic (ph 2.8 – 3.0). In addition, theosmotic load of simple sugars in dessicat-ed persistent fruits likely creates a needfor supplemental water as well (Studier etal. 1988), and waxwings often need todrink water or eat snow to meet thedemand for water.

Witmer observed that Cedar Wax -wings often ate the staminate catkins inearly spring. But during most of the year,they are one of the most obligate frugi-vores found in North America, and caneasily survive for long periods on sugaryfruits alone. So why would they needsupplemental protein at this time? Wit-mer guessed, correctly as it turned out,that the waxwings required the proteincontent from the pollen in these catkinsto balance their bodily ph during theearly spring period, when insect food is inshort supply. To counteract the acidity inmetabolizing Highbush Cranberryfruits, nitrogen (amino acids) is requiredin the physiological mechanism for acidbuffering, producing bicarbonate andammonium in the process. Eventually,bicarbonate is respired off as carbon diox-ide, and ammonium is excreted in theurine. He supported this hypothesis by a

series of experiments using caged CedarWaxwings tested with various combina-tions of aged Highbush Cranberry fruitsand the staminate catkins of P. deltoides.The results showed that the birds pre-ferred and sought out a mixed diet ofboth types. He went on to establishexperimentally that Cedar Waxwings fedon catkins alone did not obtain sufficientenergy to maintain their body mass. Sim-ilarly, birds fed on aged V. opulus fruitsalone did not thrive and started to losebody mass; in fact, results suggested thatsecondary compounds in the fruit exac-erbated nitrogen losses in the birds. Evi-dently, waxwings need supplementarynitrogen (from pollen) in order to copewith the aged fruit. Furthermore, CedarWaxwings seem to be more efficient atdigesting the protein contained in P. del-toides pollen, with a relative value of 89%quoted, versus a value of <50% for pro-tein digestion obtained in most studies ofother birds (Witmer 2001). It is beyondthe scope of this paper to discuss the sci-entific evidence in great detail here, butreaders can refer to Witmer (1994, 1996,1998, 2001, 2002) and other sourcesquoted therein.

Witmer concluded that “...The asso-ciation of waxwings and V. opulus appearsto be a result of the distinctive dietaryhabits of waxwings and the extremepersistence of these fruits”. He went onto note that “The nutritional key thatenables waxwings to feed on these ener-gy-rich, but unpalatable, fruits is the sudden appearance of a complimentary

172

ONTARIO BIRDS DECEMBER 2008

protein source with the springtimeemergence of staminate flowers.” (Wit-mer 2001).

As a result, the fruits of HighbushCranberry survive, mostly uneaten, untilearly spring, when they become availableto wandering flocks of wax wings, andthe plant achieves potential long-dis-tance dispersal of its seeds.

ConclusionIn view of the above information, itbecame clear that the feeding behaviourof the Bohemian Waxwings observed on5 April 2008 was exactly analogous tothe mutualistic relationship betweenHighbush Cranberry (V. opulus) andCedar Waxwings proposed by Witmer.It seems that Bohemian Waxwings arealmost certainly subjected to the samephysiological stresses and requirementsas Cedar Waxwings when consuming V. opulus fruits in spring. The rareoppor tunity to observe this feedingbehaviour of Bohemian Waxwing wasprobably ena bled by four factors. Firstly,the 2007-2008 irruption lasted longerthan in most previous years, allowing thewaxwings to linger well into April. Sec-ondly, because they stayed later than‘normal’, the presence of some birdscoincided with the flowering of SilverMaple trees. Thirdly, site conditions pro-vided three key components — availableHighbush Cranberries, freshly emergedstamens, and water from the meltingsnow, in close proximity. Lastly, therecent development of digiscoping as a

useful tool in birding made it easier toobtain good photographs.

There must be a relatively narrowwindow of opportunity in April andearly May in northeast North America,when newly opening flowers of varioustree species provide the supplementalprotein that wandering flocks ofwaxwings require in order to process thepersistent dessicated, but well preservedand chemically laden fruits of V. opulus.Presumably, other frugivores such asAmerican Robin (Turdus migratorius) areunable to handle this and so seldom eatthese fruits. Witmer may not have hadan opportunity to study BohemianWaxwings under the same circum-stances, but the observation recorded inMarkham on 5 April 2008 suggests thatgiven the right conditions, their feedingbehaviour may be exactly analogous.Cedar Waxwings are great wanderers,and Bohemian Waxwings even more soduring their irregular irruptions. Hence,from the plant’s perspective, to be des -cended upon by a flock of BohemianWaxwings would be the equivalent ofhitting the seed dispersal jackpot!

AcknowledgementsFirstly, my sincere thanks to all the observerswho reported their Bohemian Waxwing sight-ings on ONTBIRDS and elsewhere, whichraised awareness and a great deal of interest inthe last irruption. Thanks to Roy Smith for hisvaluable inputs and edits of earlier drafts. Myspecial appreciation goes to the editors ofOntario Birds for a final review of this article.

173

VOLUME 26 NUMBER 3

Literature CitedBarrows, W.B. 1912. Michigan Bird Life.Michigan Agricultural College Special Bulletin. East Lansing, Michigan.

Bent, A.C. 1950. Bohemian Waxwing,pp.62-79 and Cedar Waxwing, pp. 79-102.Life Histories of North American Wagtails,Shrikes, Vireos, and their Allies. Dover Publications, Inc., New York

Elder, D.H. 2002. Feeding Behaviour ofBohemian Waxwings. Ontario Birds 20:19-20.

Gunn, W.W.H. 1959. Ontario-western New York Region. Audubon Field Notes13:289-292.

Pittaway, R.J. 1990. Bohemian Waxwingseating tree buds. Ontario Birds 8:75-76.

Pittaway, R. J. 2007. Online. Winter FinchForecast 2007-2008. Accessed 15 September2008. < http://www.ofo.ca/reportsandarti-cles/winterfinches.php>.

Poon, W. 2008. The Bohemian WaxwingIrruption 2007-2008. Toronto Birds 2:72-78.

Studier, E.H., E.J. Szuch, T.M. Tompkins,and V.W. Cope. 1988. Nutritional budgets infree flying birds: Cedar Waxwings (Bombycillacedrorum) feeding on Washington hawthornfruit (Crataegus phaenopyrum). ComparativeBiochemistry and Physiology 89A:471-474.

Swarth, H.S. 1922. Birds and mammals ofthe Stikine River region of northern BritishColumbia and southeastern Alaska. Universi-ty of California Publications in Zoology24:125-314.

Witmer, M.C. 1994. Contrasting digestivestrategies of fruit-eating birds. Ph.D. disserta-tion, Cornell University, Ithaca, New York.

Witmer, M.C. 1996. Annual diet of CedarWaxwing based on U.S. Biological Surveyrecords (1885-1950) compared to diet ofAmerican Robin: contrasts in dietary patternsand natural history. Auk 113:414-430.

Witmer, M.C. 1998. Ecological and evolu-tionary implications of energy and proteinrequirements of avian frugivores eating sugarydiets. Physiological and Biochemical Zoology71:599-610.

Witmer, M.C. 2001. Nutritional interactionsand fruit removal: Cedar Waxwing consump-tion of Viburnum opulus fruits in spring. Ecology 82:3120-3130.

Witmer, M. C. 2002. Bohemian Waxwing(Bombycilla garrulus). In The Birds of NorthAmerica, No. 714 (A. Poole and F. Gill, Eds.).The Birds of North America, Inc., Philadelphia, Pennsylvania.

Winnie Poon, 1002 – 20 Harding BoulevardWest, Richmond Hill, Ontario L4C 9S4

174

ONTARIO BIRDS DECEMBER 2008