Seed Handling Ability, Bill Structure, and the Cost of Specialization for Crossbills

SEED HANDLING ABILITY, BILL STRUCTURE, AND THE COST OF SPECIALIZATION FOR CROSSBILLS CRAIG W. BENKMAN • Departmentof BiologicalSciences, State University of New Yorkat Albany, 1400Washington Avenue, Albany, New York12222 USA AssT•cT.--Crossbills(Loxia) are specialized to extractand handle seeds from conifercones. I evaluatedthe ability of crossbillsto handlenonconifer seeds by comparingseed handling efficiencieswith other carduelinefinches. For all seedsizes, crossbills required seed encounter ratesor seedabundances 2-3 times greaterthan other speciesto meet their daily energy requirements.Consequently, crossbills may suffer high mortality during conifer cone failures. Crossbillsare inefficientat meetingtheir energydemands on nonconiferseeds because of their narrowmandibles, lowered horny palate,and large body size.The narrow mandibles enablecrossbills to efficientlyextract seeds from conifer cones and the loweredpalate enables them to handle small seedsrapidly. Crossbillshave evolvedlarger bills, and associated musculatureand bodymass, to providethe power necessaryto separatecone scales. Some of the increasein bodymass, however, may counterbalance the largebills to improvepredator evasion. Received27 October1987, accepted17 June1988. THE carduelinefinches (subfamily Cardueli- 1976) and Eurasia(Newton 1970, 1972),and at nae) are highly specializedseedeating birds this time many crossbillsfeed on nonconifer (Newton 1967, 1972). Most consume seeds, seed(see Newton 1970,1972). These periods of mainlyfrom dicotyledonousplants, throughout coniferseed shortage often coincide with fail- the year (Martin et al. 1951;Newton 1967,1972; ures of other boreal tree seed crops (Bock and Austin 1968). The crossbills(Loxia) eat conifer Lepthien! 976).Thus, competition for available seedsto the near exclusion of other seeds(New- seedamong cardueline finches would be inten- ton 1967, 1972; Austin 1968; Nethersole- sified. Thompson1975; Benkman 1987a). I comparedthe seed-handlingrates, on grad- Crossbillsare clearly more efficientat utiliz- ed seed sizes, of 3 noncrossbill speciesof car- ing seedsin closedconifer conesthan other duelineswith the 2 speciesof crossbillsin North cardueline finches(Newton 1967, 1972). Many America. These data are used to determine the characteristicsof the crossbill'sfeeding appa- relative seedhandling abilities of crossbillsin ratus are adaptedfor exploiting conifer seeds comparisonto other carduelinesand thosefea- (Benkman1987b). Experimental ablation of the tures that make crossbills inefficient when feed- crossedportion of the crossbill'smandibles ing on nonconiferseeds. The dataon noncross- demonstratedthat this portion was essentialfor bill carduelines are compared to those on crossbills to extract seeds from closed conifer emberizinesparrows, elsewhere (Benkman and cones(Benkman 1988). Further, no carduelines PullJam 1988). other than crossbills have been found to extract seedfrom tightly closedconifer cones(Smith METHODS and Balda 1979; pers. obs.). Crossbillsmay be limited to foraging on co- Red Crossbills (L. curvirostra),American Goldfinches nifer seedbecause they are relativelyinefficient (Carduelistristis), House Finches (Carpodacus mexicanus), and at handlingother seed types (e.g. Roberts 1936). Evening Grosbeaks(Coccothraustes vespertinus) were The reducedefficiency of crossbillsis of partic- capturedin Albany Co., New York. White-winged ular interest becauseabout every 3-4 years co- Crossbills(L. leucoptera),were caught in Laurentides nifer seed crops fail over much of the boreal Reserve,Quebec. Several days prior to and during the experiments,all specieswere fed commercialsun- forests of North America (Bock and Lepthien flower (Helianthusspp.) and niger thistle (Guizotia abyssinica)seeds, and vitamin-enriched water. For- • Presentaddress: Department of Biology,Prince- aging experimentswere conductedon the crossbills ton University,Princeton, New Jersey08544-1003 after a minimum of 5 monthsin captivity.The other USA. specieswere completedwithin 6 daysof capture.Even 715 The Auk 105: 715-719. October 1988 716 CRA•CW. BENKMAN [Auk,Vol. 105 TABLE1. Bodymasses and bill dimensionsof the 5 speciesof carduelinefinches used in the experiments. Sample size for each bill measurementis 10 birds. Bill size (ram) Bodymass (g) Depth Width Length Species • • _ SD • + SD • + SD A. Goldfinch 13.0 6.4 + 0.1 5.0 + 0.1 8.1 _+ 0.3 House Finch 21.0 8.2 + 0.2 7.2 + 0.2 8.6 + 0.4 WW Crossbill 26.4 7.8 + 0.3 5.6 + 0.3 13.8 + 0.6 Red Crossbill 35.9 9.3 _ 0.2 7.4 + 0.2 15.3 + 1.0 E. Grosbeak 55.0 14.7 + 0.5 13.8 + 0.5 15.4 + 0.7 thoughthe crossbillswere housedfor longerperiods of water.When I wasready to recordhandling times, than the other species,they appearedto remain in the lights were turned off, the door separating the excellentphysical condition. chamberswas slid open, and then the lights were I used 8 size classes of commercial sunflower and turned on. Usually the bird would hop onto the perch niger thistle seeds. Average individual seed wet in the observationchamber to feed.After feeding,the weights ranged from 2.3-117.5 mg. Sunflowerand light on the side the bird was on was turned off, and thistle were chosen becausethey are composites the bird returned to the other chamber. The door was (Compositae),and are an important natural food for closed.All seedremains were removedwith forceps many carduelines (Martin et al. 1951; Newton 1967, and weighed. 1972;Austin 1968). All 5 speciesreadily ate these Initially seed hulls and kernel remains were seedsand visitedfeeders, though crossbills do soonly weighed separately,but it soon becameevident that infrequently.The shapeof thistleand sunflowerseeds seed hulls were not consumed in any measurable differsfrom that of conifers,but the relationshipsbe- amount. I subtracted total seed remains from initial tween massof seedcovering (y; rag) and kernel mass weight to obtain the averageweight of kernel con- (x; mg) were nearly identical for the seedsused in sumedper seed.Wet weightswere usedbecause dry the experiments(y = 0.46x•-%r 2 = 0.94, n = 8, P < weights were over 97% of wet weight. Variation in 0.01)and coniferseeds in the Northeast(y = 0.47x•.% seedsize classesbetween finch speciesresulted from r 2 = 0.93, n = 10, P < 0.01). Seed size classes were seedsselectively consumed. This was especiallytrue initially sortedwith a sieve. Then seedsmost similar for seedsthat were relatively difficult to handle. in size and shapewithin size classesand most similar Handling time beganwhen a seedwas picked up in shape among size classeswere individually se- and continueduntil no part of the seedwas being lected and examined for cracks in the seed hull. I mandibulated.Time spent handling seedsthat were usedonly seedswith undamagedand uncrackedhulls. droppedwhole wasexcluded from analyses.I watched Seedswere weighed in groups of ten to the nearest birds through a one-way window and recordedhan- 10mg for sunflowerseeds and 0.1 mg for thistleseeds, dling times on an Apple II computer,programmed and reexamined for consistencyin size and shape. to time and recordevents to 0.1 s. Data were gathered Variance in seed shapeand quality was minimized on 3-5 individuals of each species.Each datum pre- so that seed handling differenceswould reflect seed sentedrepresents the mean for onebird handling on size differences. Two additional size classes of sun- average 15.1 seeds(SD = 5.7, range = 3-31). flower seedswere given to the goldfinchesand White- I estimated the rate at which seeds must be en- winged Crossbillsto provide finer testing of their countered (k), where k = 1/T and seedhandling abilities. 3.6 x 104s(S) During the tests,10 preweighedseeds of the same T- h. DEE size were placed in a partially covered plastic tray attachedto a perch 40 cm abovethe floor of a (60 cm)3 I assumed 3.6 x 104 s (10 h) to be the maximum time wooden chamber.Because carduelines normally for- availableto forageduring a midwinter day. Midwin- agewithin vegetationabove the ground(Newton 1967, ter was chosen because this is when diets consist most- 1972), the birds were required to handle seedswhile ly of seeds(Martin et al. 1951) and winter is a period perched.The food tray was designedso any kernel when food is mostlimiting for carduelines(Newton (embryo and endosperm)dropped by the birds fell 1967,Benkman 1987a). S is the productof mass(rag) to the floorwhere recoverywas prevented by a screen. of kernel consumedper seed,specific energy value A secondchamber was connectedby a sliding door (J/rag) of the kernel, and assimilationefficiency. I at the level of the perch. Birdswere held overnight measured mass of kernel consumed, and I assumed without food for >16 h prior to the seed-handling seedkernels commonly consumed by fincheshave 23 experiments.Each chamber had one perch and a dish J/rag (Grodzinskiand Sawicka-Kapusta1970) and as- October1988] Costof Specialization 717 •o• 80. , W-WOROSSaLL•DCmSSaLL ---•1It ß W-WCROSSBILLRED CROSSBILL n- .o *.c-ou•.c. • to•[O &= A.HOUSEFNCH GOLDFINCH z • • o ß Z .1 . ;0. '; -I A' O• ß byFig.5species 1.The ofcarduelineamount offinches.seed kernelData consumed/seedforeach species •) .01 •' •,•3 n_ •ß ß • ßß areindicated byunique symbols thatare connected • • a3n(• o point is usually the mean for 20 seeds.For each seed size the different points representdifferent individuals,but data for the different seed sizes are from the .001 bysamesecond individuals.orthird Variancesorder polynomial are not givencurves. because Each UJ•• 0 20 40 60 80 100 120 seedremains were weighed in groups,usually

Load more