Unsuitability of the House Finch As a Host of the Brown-Headed Cowbird’

The Condor 98:253-258 0 The Cooper Ornithological Society 1996

UNSUITABILITY OF THE HOUSE FINCH AS A HOST OF THE BROWN-HEADED COWBIRD’

DANIEL R. KOZLOVIC Department of Zoology, Universityof Toronto, Toronto, Ontario M5S 3G5, Canada

RICHARD W. KNAPTON Long Point Waterfowl and WetlandsResearch Fund, P.O. Box 160, Port Rowan, Ontario NOE IMO, Canada

JON C. BARLOW Department of Ornithology,Royal Ontario Museum, 100 Queens’ Park, Toronto, Ontario M5S 2C6, Canada and Department of Zoology, Universityof Toronto, Toronto, Ontario M5S 3G5, Canada

Abstract. Brown-headedCowbirds (Molothrus ater) parasitized99 (24.4%)of 406 House Finch (Carpodacusmexicanus) nests observed at Barrie,Guelph, Orillia, and St. Catharines, Ontario, Canada,during the periods1983-1985 and 1990-1993.Hatching success of cowbird eggswas 84.8%, but no cowbirdwas reared.Cowbird growth was severelyretarded; nestlingsrequired about twice as much time to accomplishthe sameamount of growth observedin nestsof otherhosts. Estimated final bodymass of nestlingcowbirds was about 22% lower than normal. Cowbirdnestlings survived on averageonly 3.2 days.Only one cowbirdfledged but died within one day. Lack of cowbirdsurvival in nestsof the House Finch appearsto be the resultof an inappropriatediet. We concludethat nestlingdiet may be importantin determiningcowbird choice of host. Key words: Brown-headed Cowbird; Molothrusater; House Finch; Carpodacusmexi- canus;brood parasitism; cowbirdsurvivorship; nestling diet.

INTRODUCTION (Woods 1968). Like other members of the Car- The Brown-headed Cowbird (Molothrus ater) is duelinae, House Finches are unusual among an obligate brood parasite that lays its eggsin cowbird hosts in that they feed their young pri- the nests of many host species, which provide marily plant material. The food is given to nest- parental care (Friedmann and Kiff 1985). For lings by regurgitation; it is neither partially di- parasitism to be successful,hosts must not only gested nor does it contain nutritive secretions accept and incubate cowbird eggs, (Rothstein from the adult (Newton 1972). Similar diets fed 1975) but they also must provide the nestling to cowbirds by other species are insufficient, parasite with adequate nourishment for proper which implies that the House Finch, too, would development. The food of cowbird hosts varies be unable to rear the parasite. The purpose of widely from animal to plant material (Martin et the present study was to determine the frequency al. 195 l), but almost all feed their young pri- of successfulparasitism on House Finch nesting, marily animals. Some taxa, however, feed their and hence the suitability of this speciesas a host nestlings plant material and failure of cowbird of the Brown-headed Cowbird. parasitism in the nestsof thesespecies is believed to be the result of inadequate diet (Eastzer et al. METHODS 1980, Middleton 199 1). Data were collected at sites in southern Ontario The House Finch (Carpodacus mexicanus) is in the towns of Barrie, Guelph, Orillia, and St. an occasionalhost of the Brown-headed Cowbird Catharines, from May to August 1983-l 985 and (Friedmann 1966, Friedmann et al. 1977). This 1990-1993. House Finch nests were found by finch feeds on a variety of plant materials, but systematically searching through residential most of the diet consistsof weed seeds.The few neighborhoodsfor singing territorial males or ev- animals taken are mainly aphids and caterpillars idence of nest construction. Nests were com- monly placed in ornamental conifers near dwell- ings and were positioned 0.9 to 6.0 m (X = 2.44, 1Received 20 July 1995.Accepted 10 January 1996. SD = 0.729, n = 373) above ground. Most nests 254 DANIEL R. KOZLOVIC ET AL. were easily reached using a six-foot (2 m) step- digested residue. Nonetheless, food became in- ladder. The contents of higher nests were ob- creasingly difficult to identify with age of nest- served with the aid of a small mirror positioned lings becausesamples from older young contained on the end of a telescopicpole. Parasitized nests relatively more digested food. contained cowbird eggs, cowbird nestlings or Growth coefficients were calculated for in- both. These nests were monitored daily at ap- crease in body mass based on a logistic model proximately the same time except in 1992 and of growth (Ricklefs 1984). The relative growth 1993 when nests were visited twice per week. rate, K, asymptotic body mass,and time required The fate of most cowbird nestlings was deter- to complete 10 to 90% of the asymptote, tlsgO, mined but data on their growth and survivorship were determined using nonlinear least-squares were taken only from individuals that were ob- regression(Gauss-Newton method, NLIN of SAS served daily from hatching (day 0). Cowbird Institute 1985). Because sample sizes varied nestlings in a nest were marked uniquely by toe- greatly among age groups of cowbirds, the data nail-clipping (St. Louis et al. 1989). Nestling body were weighted according to sample size. Thus, mass was measured using lo- and 50-g Pesola@ body mass values for each age group were ac- spring scalesaccurate to 0.1 and 0.25 g, respec- curately represented in the calculation of growth tively. Wing chord and length of ninth primary parameters. Cowbird growth data from Scott (tip of feather to the point of emergencefrom the (1979) were similarly analyzed. The logisticmodel skin) were taken to the nearest 0.05 mm using provided a suitable description of growth (9 ap- dial calipers. Nestlings that disappearedwere as- proximation 2 0.8433). sumed to have died in the nest, the corpsehaving been removed by the foster parents (Welty and Baptista 1988). Predation of cowbirds was ruled RESULTS out if the nest continued to hold House Finch The Brown-headed Cowbird parasitized 99 eggsand/or young. (24.4%) of 406 House Finch nests. Parasitized A nestling cardueline stores food temporarily nests contained a total of 127 cowbird eggs.Of in its distensible gullet before digestion (Newton these, 79 survived through the incubation period 1972). In House Finch nestlings the full gullet and produced 67 (84.8%) nestlings. In addition, appears as a large bulge on the right side of the 11 cowbirds of varying age were discovered after neck, the contents of which can be easily ob- they had hatched. No cowbird was successfully served through the thin, translucent skin. Initial reared in a House Finch nest. Nestlings that per- observations of House Finch diet were made by ished in the nest were either found dead there external examination of the gullet. All young ap- (35.6%), or removed (64.4%) by the foster par- peared to be receiving plant material mostly in ents. Two discarded corpseswere found on the the form of seeds.Finch and cowbird diets were ground near their respectivenests. Only one cow- studied more thoroughly by examination of nest- bird fledged. It left the nest at age 14 d but was ling feces.At St. Catharines, fresh feceswere col- found dead the following day. Proportional sur- lected from young during nest visits throughout vivorship of cowbird nestlings is shown in Figure the 199 1 breeding season.Each sample was sealed 1. The average survival time was only 3.2 d (SD in a separatevial and later stored at -20°C. Sam- = 2.87, 12= 25). Two birds did not survive be- ples were taken when House Finches and cow- yond their day of hatching, whereas only one birds were O-8 and 2-5 days old, respectively. individual survived to 14 d. One-hundred and thirteen fecal samples were Cowbird hatchlings had a mean body mass of collected from 67 House Finches at 23 nestsand 2.79 g (SD = 0.273, n = 23) and body mass four samples were taken from two cowbirds at increased in a largely linear fashion over the en- two nests. Upon examination, the samples were tire growth period (Fig. 2). Two nestlings failed moistened with 70% ethanol, teased apart with to gain mass beyond two and five days of age dissecting needles, and the constituents identi- and one individual lost mass after two days of fied under a binocular dissecting microscope. A age. The maximum nestling mass recorded was drawback of using fecal samples for the analysis 22 g. Specific growth parameters for cowbirds of diet is the fragmented nature of the food (Ro- reared by House Finches and other speciesare senbergand Cooper 1990). This is less of a con- given in Table 1. Cowbird growth was severely cern with nestlings as their feces retain an un- retardedin House Finch nests.Cowbirds achieved COWBIRD PARASITISM OF FINCH NESTING 255

0.8 I.’ .-CL ..‘ AI r 0.6 ,. 3 '2 0.4 cz

0.2

0 ~ ~ 0 1 2 3 4 5 6 7 14 Age (days) FIGURE 1. Proportionalsurvivorship of Brown-headedCowbirds (original n = 25) from hatching(0) to 14 d in nestsof the HouseFinch. an estimated asymptotic body mass that was plant material including whole seeds,cotyledons 22.4% smaller than in nests of other hosts. The (primary embryonic leaves) and the seed coats relatively smaller growth rate, K, approximately that cover them, plant fragments and pulp. Most doubled the time required for growth. seedswere small and ranged in length from 1 to Nestling cowbirds developed teleoptiles, but 4 mm. Animals were identified in only eight barbs did not emerge from the sheaths of most (7.1%) samples and included eight mites (Acari), individuals. The single cowbird that fledged had three springtails (Collembola) and three aphids well developed plumage when it left the nest. It (Aphididae). Grit first appeared in the meals of showed substantial growth of barbs of all feather one-day-old House Finches. Samples of cowbird tracts and attained wing chord and ninth primary diet contained only plant material consisting lengths of 63.00 mm and 33.15 mm, respective- mostly of whole seeds and seed parts and ap- ly. peared to be largely undigested with very little House Finch diet consisted almost entirely of indistinguishable plant material present.

0 ’ I I I I I I I I If I 0 1 2 3 4 5 6 7 14 Age (days) FIGURE 2. Increaseof body massof Brown-headedCowbirds from hatching(0) to 14 d in nestsof the House Finch. Horizontalbars indicate sample means, vertical bars the range,and the rectanglesenclose + 1 standard deviation.Sample size is shownabove plotted values. 256 DANIEL R. KOZLOVIC ET AL.

TABLE 1. Specificgrowth parameters of nestling Brown-headed Cowbirds.

Adult A;,w;P- body Ratio Growth rate’ Host n (B) “(T” (W (K) (t1wo) Source Ovenbird, Wood Thrush 2 30.0 43.5 0.69 0.576 7.6 Ricklefs 196P Red-winged Blackbird, Song Sparrow, Yellow Warbler 8 28.8 43.5 0.66 0.597 7.4 Scott 1979 House Finch 23 22.8 43.5 0.52 0.318 13.8 present study

’ Estimatedfinal hod massof nestlinggrowth. b From Ricklefs (196I ). cLogistic growth rate constant,K, and time requiredto complete10 to 90 percentof the asymptote,tllYa. dData from Norris (I 947).

DISCUSSION fledgling cowbird (M. a. obscurus)was seenbeing fed by a House Finch. The rearing of cowbirds Results indicate that the House Finch is an un- by this host appears to be very rare. suitable host of the Brown-headed Cowbird. All The type of diet that parents feed to their off- attempts at parasitism failed with most cowbird spring is important in determining host suit- nestlings perishing in the nest. There are no re- ability. Most passerines feed their young with ports in the literature of cowbird nestlings in arthropods and nestling diets show considerable House Finch nests. Most of the existing records overlap among sympatric species (Orians and of parasitism mention the presence of cowbird Horn 1969, Maher 1979). Thus, there appears eggs in House Finch nests (Friedmann 1963, to be little restriction among hosts concerning Friedmann et al. 1977, Friedmann and Kiff 1985 the food of nestlings; a variety of speciesappear and referencestherein), but provide no account to provide equally adequate diets to cowbird of cowbird hatching successor nestling survi- young (see Norris 1947, Scott 1979). However, vorship. Some data on cowbird nestling life in cowbirds fail to survive in nests of hosts that eastern North America are provided by the feed their nestlings regurgitated seeds, fruit or American Nest Records Card Program and in at other plant material. Hatchling Brown-headed least two cases corroborate the results of this Cowbirds died, most within six days, after being study. Three recordsindicate cowbirds surviving placed in nests of the House Sparrow (Passer from at least 2 to 10 d before predation of nest domesticus,Eastzer et al. 1980) which may feed contents, or observations ended. Another nest their nestlings large quantities of plant material contained one dead cowbird nestling of unknown (Bent 1958). Cowbirds survived an average of age with two unhatched finch eggs.One observer only two days in American Goldfinch (Carduelis reported the disappearance of a two-to-six day- tristis) nests. Most cowbirds died by the fourth old cowbird from a nest that also held two finch day and only one survived 12 days (Middleton nestlings that later fledged successfully.The dis- 199 1). Cowbirds may fledge from Cedar Wax- appearance of cowbird nestlings is implied in wing (Bombycilla cedrorum) nests but occasion- four other instances, including one record from ally die after three days when parents start to Oregon. Presumably, cowbird nestlings that dis- feed their young fruit rather than insects (Roth- appeared from thesenests had perishedthere and stein 1976). House Finches gave cowbird nest- their bodies were removed by the foster parents. lings a specific diet consisting mainly of seeds. Given these findings, the probability of survival The failure of cowbirds to thrive on this food is of cowbirds in House Finch nests appears to be additional evidence that this speciesis unlikely exceedingly small. to survive on the special nestling diets of some Despite this high level of mortality, cowbird granivorous or frugivorous species. nestlings occasionally fledge from House Finch Most altricial young grow rapidly (Ricklefs nests. In the present study, only one cowbird 1968) which requires a protein-rich diet (O’Con- survived to leave its nest but either died of mal- nor 1984). Food low in protein content may ar- nutrition (see below) or received no foster-pa- rest growth severely (Johnson 197 1, Roudybush rental care thereafter. Wauer (1964) reported an and Grau 1986, Boag 1987). The poor devel- exception observed in California in which a opment of cowbirds witnessed in House Finch COWBIRD PARASITISM OF FINCH NESTING 257

nests appearsto be the result of protein deficien- Society of CanadianOrnithologists to DRK, aswell as cy. Nestling body mass did not follow a sigmoi- operatinggrants U0177 and A3472 to RWK and JCB, respectively, from the Natural Sciencesand Engineer- da1 pattern typical of normal growth (Ricklefs ing ResearchCouncil of Canada. 1968) and never attained the asymptotic level or fledgingmass of cowbirdsreared by suitable hosts. The body mass of two cowbirds did not increase, LITERATURE CITED whereas another individual experienced weight BENT,A. C. 1958. Life histories of North American recession. Feather growth was delayed and re- blackbirds, orioles, tanagers,and allies. U.S. Nat. quired additional time (about four days for the Mus. Bull. 211. only individual that fledged)to achieve sizesob- BOAG,P. T. 1987. Effects of nestling diet on growth and adult size of Zebra Finches (Poephilug&tutu). served for cowbirds ten days old (Scott 1979). Auk 104:155-166. Unlike suitable hosts, House Finches feed their EASTZER,D., P. R. CHU, AND A. P. KING. 1980. The young a diet that is limited in protein. Seedsand young cowbird: averageor optimal nestling?Con- particularly fruits are generally low in protein dor 82:417425. FRIEDMANN,H. 1963. Host relations of the parasitic (Newton 1972: 179, Morton 1973, O’Connor cowbirds. U.S. Nat. Mus. Bull. 233: l-276. 1984, Johnson et al. 1985). Furthermore, plant FRIEDMANN,H. 1966. Additional data on the host proteins often lack one or several of the essential relations of the parasitic cowbirds. Smithsonian amino acids that cannot be synthesized by the Misc. Coll. 149:1-12. animal itself (Needham 1964, Parrish and Mar- FRIEDMANN,H., AND L. F. KIFF. 1985. The parasitic cowbirdsand their hosts.Proc. West. Found. Vert. tin 1977, Sedinger 1990). Cowbirds in finch nests Zool. 2~225-304. received a minuscule quantity of animal food, FRIEDMANN,H.,L.F.KIFF,ANDS.I.ROTHSTEIN.1977.A probably inadvertently consumed by foraging further contribution to knowledge of the host re- parents, that was grosslyinsufficient to meet their lations of the parasitic cowbirds. Smithsonian Contr. Zool. 235: l-75. protein requirements. JOHNSON,N. F. 1971. Effectsof levels of dietary pro- The failure of cowbird parasitism on House tein on Wood Duck growth. J. Wildl. Manage. 35: Finch nesting may lend insight in the evaluation 798-802. of host suitability by cowbirds. Becausethere is JOHNSON,R. A., M. F. WILISON,J. N. THOMPSON,AND no reproductive successin parasitism of House R. I. BERTIN. 1985. Nutritional values of wild fruits and consumption by migrant frugivorous Finches, they should be avoided by cowbirds. birds. Ecology 66:8 19-827. The frequencyof parasitismobserved in this study KOZLOVIC,D. R. 1994. 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Field Omithol. 62:200-203. MORTON.E. S. 1973. On the evolutionarv advantages We thank the many peopleof Barrie,Guelph, Orillia, and hisadvantagesof fruit eating in tropical bir&. and St. Catharines,Ontario who kindly allowedus ac- Am. Nat. 107:8-22. cessto their property.This studywould not havebeen NEEDHAM,A. E. 1964. The growth processin ani- possiblewithout their cooperationand interest.We mals. Pitman and Sons Ltd.,. London. alsothank G. Melvin for assistancein thefield. J. Lowe NEWTON,I. 1972. Finches. William Collins Sonsand providedrecords ofcowbird parasitism of HouseFinch Co. Ltd., Glasgow. nestingfrom the North AmericanNest RecordsCard NORRIS,R. T. 1947. The cowbirds of Preston Frith. Programhoused at the Cornell Laboratoryof Omi- Wilson Bull. 59:83-103. thology.Additional information of parasitismwas sup- O’CONNOR,R. J. 1984. The growth and development plied by E. Potter.Constructive comments on a draft of birds. John Wiley and Sons Ltd., Chichester. of this-manuscript were provided by T. Lang, J.D. ORIANS,G. H., AND H.-S. HORN. 1969. Overlap in Rising and S.G. Sealy. This study has been supported foods and foragingof four soeciesof blackbirds in by a Paul A. Stewart grant from the Wilson Omitho- the potholes 07 central Washington. Ecology 50: logical Society and a P.A. Taverner grant from the 930-938. 258 DANIEL R. KOZLOVIC ET AL.

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