The Young Cowbird: Average Or Optimal Nestling?

Condor, 82:417425 0 The Cooper Ornithological Society 1980

THE YOUNG COWBIRD: AVERAGE OR OPTIMAL NESTLING?

DAVID EASTZER PENN RICHARD CHU AND ANDREW P. KING

ABSTRACT.-We studied whether the young of the Brown-headed Cowbird are more successful than those of nonparasitic passerines when raised by other species. Eggs and nestlings of seven species were placed in Barn Swal- low and House Sparrow nests, and the hatching success and nestling survival rates compared. The hatching success was high for all species in both host species ’ nests. In the Barn Swallow nest, the nestlings of four of the five nonparasitic species did as well as the cowbird nestlings. On the other hand, the three foreign species placed in the House Sparrow nest, including the cowbird, all failed to survive through the nestling stage. In addition, a Red- winged Blackbird, a catbird, and a cowbird were observed immediately after departing from their Barn Swallow host nests; only the cowbird elicited feed- ings from its host parents. Thus the young cowbird fared no better than the other species as a nestling. Although cowbirds can be reared by their most frequent host species, they may not be particularly successful with some of their less common hosts. They may be especially successful as a fledgling, a hypothesis that awaits further investigation.

The young of many avian brood parasites deed, Nice (1939) stated that “there appears possess striking adaptations to their unusual to be no conclusive evidence that the young rearing situations. The eggs, nestling ap- cowbird shows any such [parasitic] adapta- pearance and mouth markings of several tion.” If this is the case, the requirements parasitic widowbirds (Vidua spp.), for ex- of any developing passerine may be so gen- ample, are almost indistinguishable from eral that virtually any species can be reared each species ’ respective host (Nicolai 1974). by any other. This proposition seems un- In addition, the begging postures and vocal- likely considering the complex and dynam- izations of each widowbird species match ic nature of avian parent-offspring relation- those of its host species. Newborn cuckoos ships (e.g., Muller and Smith 1978) but it (Cuculus spp.) of parasitic species eject the remains to be tested thoroughly. eggs or other nestlings from the nest by ma- The chief purpose of our study was to de- neuvering them onto their slightly concave termine if young cowbirds survive at a back and pushing them out. A newly higher rate than nonparasitic passerine hatched Lesser Honeyguide (Indicator mi- nestlings that are reared by parents of nor) has a mandibular hook with which it another species. Our method was to cross- attacks and kills its nestmates (Skutch 1976). foster the eggs (and occasionally nestlings) The Brown-headed Cowbird (Molothrus of several species into other species ’ nests ater) is known to use more host species than and to compare egg and nestling survival any other brood parasite: its eggs have been rates. We predicted that if the young cow- found in the nests of 217 species and young birds were more versatile than the non- cowbirds have been reared by 139 species parasitic species ’ young in their ability to (Friedmann et al. 1977). Do developing be reared by foster parents, they would sur- cowbirds possess some special adaptations vive at higher rates than the nonparasitic that allow them to be raised by such a di- young. We also observed several fledglings versity of species? Friedmann (1929) sug- immediately after leaving the nest in order gested that the short incubation period and to ascertain their ability to induce their rapid nestling growth of cowbirds are ad- foster parents to feed them. aptations to the parasitic habit, but subsequent investigations indicate that these METHODS characteristics are in line with those found LOCATION in many nonparasitic avian species (Nice The study was carried out within a 25-km radius of 1937, 1939, Norris 1947, Ricklefs 1968). In- Cornell University campus in Ithaca, New York, in

[4I71 418 D. EASTZER, P. R. CHU AND A. P. KING

TABLE 1. Incubation period,” nestling period,” and tents, and approximate stage of incubation of all nests criterion age for nestling survival of each experimental were recorded parasite species. A total of 180 eggs were cross-fostered into Barn Swallow and House Sparrow nests in 1974 and 1977. Nestling Criterion permd age Five cowbird eggs placed in House Sparrow nests dur- Species (days) (days) ’ ing the summer of 1973 were also included, making a total of 185 eggs switched. In addition, 34 nestlings Barn Swallow 14-16 (4) 17-24 10 House Sparrow 11-14 (3) 15 8 less than three days old and three four-day old nest- Brown-headed lings were placed in host nests, for a total of 222 cross- Cowbird 1 l-12 (3) 10 6 es. Nestlings were cross-fostered when a nest was Red-winged found containing young nestlings and appropriate host Blackbird 10-12 (3) 10-11 6 nests were available, or wher, the age of a previously Bank Swallow 12-16 (4) 19 10 candled clutch was misjudged and the eggs hatched Eastern Phoebe 14-16 (4) 15-17 9 before the cross was made. Gray Catbird 12-13 (3) 10 6 Each experimental parasite and conspecific control a From Harrison (1978). egg was matched to a host nest in such a way that the h Parentheses contain the time in day? considered one-quarter the incubation period. egg was expected to hatch before the host clutch. In r Minimum age that had to be attaIned by nettling in order to qualify for “nestling survival” category. only 16 of 222 crosses were the host young older than the bird placed in the nest. Once an egg was matched to a host, the cross was not carried out until late in the experimental parasite 1974 and 1977. Eggs and nestlings were cross-fostered or conspecific control eggs’ incubation period. Of the in June and July, and nests were checked and fledg- eggs crossed, 4 were switched in the first quarter of lings observed until mid-August. their incubation period, 14 in the second quarter, 28 in the third quarter, and 139 in the last quarter. We SUBJECTS tried to switch eggs or nestlings that were not too far We selected host species according to several consid- out of synchrony with the stage of incubation of the erations: they had to he locally abundant, not subject host clutch, so that the cross-fostered young did not to heavy predation, and they had to accept foreign eggs hatch extremely early in the hosts’ incubation period. placed in their nest. Although we also wanted to use Of 208 egg and nestling crosses for which information a species commonly parasitized by the cowbird, the on both the host and parasite hatch dates were known, above criteria were met by only two species, both of 16 host clutches were one to four days older than the which are not normally victimized by the cowhird- cross-fostered egg or nestling, 29 parasites hatched the the Barn Swallow (Hirundo rustica) and the House same day as the host clutch, 76 parasites were one to Sparrow (Passer domesticus). four days older than the host, 53 parasites were five to Species used as experimental parasites were the eight days older, and 33 were nine to 13 days older Red-winged Blackbird (Agelaius phoeniceus) and the than the host young. cowbird (placed in both host species ’ nests), the Barn Eggs and nestlings were taken from their own nest Swallow (into the House Sparrow nests), and the Gray and transported to their host nest in small vials filled Catbird (Dumetella carolinensis), Eastern Phoebe with cotton. No eggs or nestlings were kept out of a (Snyornis phoebe), Bank Swallow (Riparia riparia), nest for more than two hours, and most crosses were and the House Sparrow (into the Barn Swallow nests). completed in less than one-half hour. Each host clutch The term “experimental parasite” refers to any egg or was parasitized with only one egg, although we tested nestling placed in the nest of another species. The subsequent clutches at the same nest. species in whose nest they were placed, either a Barn After completing the cross, we visited each nest at Swallow or a House Sparrow nest, is referred to as the least once, and none more than three times. At each “host.” In addition to these experiments, we switched nest check, we recorded the status of the fostered egg Barn Swallow eggs into other Barn Swallow nests, and or nestling as either present (with estimated age), or House Sparrow eggs into other House Sparrow nests absent. We also noted the state of the host young at the in an effort to determine the effects of the cross-foster- time of the last check for 100 of the nests. ing technique on hatching success and nestling surviv- Nests were checked until 1) the switched egg was al. judged to have failed to hatch, 2) the fostered egg or All of the eggs and nestlings were collected in the nestling was gone, 3) some catastrophe had befallen field, except for the cowbird eggs, which were taken the nest, such as predation, desertion, or nest destruc- from a group of captive cowbirds. The cowbird eggs tion, or 4) the parasite nestling had survived to its cri- were incubated by Canaries (Serinus canuria) for 9 to terion age for successful passage through the nestling 10 days and hatched one to two days after being placed stage. in the host nest. The disappearance of the contents of the nest was attributed to predation. Desertion was indicated by a PROCEDURE cold clutch and the absence of an incubating female at An egg was removed from each nest found and candled the nest. The criterion for nestling survival was half to determine the stage of incubation. In no case was the nestling period, the age in days depending upon our estimate of the incubation stage off by more than the nestling period of each species (Table 1). one quarter of the incubation period. The‘ absolute val- We watched three individual young immediately af- ue in days of one-quarter of the incubation period dif- ter they left their Barn Swallow nest. The birds were fered among the species (Table 1). The location, con- a Red-winged Blackbird, a catbird, and a cowbird, all COWBIRD NESTLINGS 419 of which were placed in the host nest as eggs. A blind TABLE 2. Hatching success of the experimental par- was set up at least one day before the expected depar- asite and control cross species for each host species. ture of the parasite young so that the swallow foster parents could become accustomed to it. We were care- Total number Number ful not to influence the departure of the experimental of eggs of Hatch- fledglings from the nest. The young were watched un- CTOSS- Parasite species foqtered ha?%d suc”c”,ss’ til nightfall and, in the cases of the cowbird and the catbird, for several hours the next morning. Their beg- Host: Barn Swallow ging behavior and vocalizations were recorded in con- Brown-headed Cowbird 24 20 83% junction with the presence and behavior of their foster Red-winged Blackbird 22 20 91% parents. House Sparrow 19 17 89% Gray Catbird In order to determine if the cowbird did significantly 11 10 91% Eastern Phoebe 14 13 93% better in hatching success or nestling survival than any Bank Swallow 1 1 100% other experimental parasite species placed in a given Barn Swallow control 28 24 86% host nest, a Fischer Exact Probability Test was per- formed on each experimental parasite/cowbird pair. Barn Swallow host subtotal 119 105 88% Host: House Sparrow Brown-headed Cowbird RESULTS 7 6 86% Red-winged Blackbird 8 6 75% EGG STAGE Barn Swallow 7 7 100% House Sparrow control 18 15 83% Eggs not included in hatching analysis. We were unable to learn if 26 of the switched House Sparrow host subtotal 40 34 85% eggs hatched. Five nests were robbed be- Total 159 139 87% fore the first visit and nine were deserted or destroyed. We used eggs from seven nests that were cold when they were found; the eggs probably were dead before they were nests were robbed and one nest fell after cross-fostered. In five cases, the fate of the the eggs had hatched. switched egg was unknown, as these nests Of the 37 nestlings cross-fostered after were not visited until the expected fledge hatching, seven yielded no data. Five were date; the absence of the young bird could taken by predators, one nest was destroyed, not be explained with certainty. Thus, 159 and one nest was deserted. cross-fostered eggs provided data for the Combining of nestling data. Before pool- calculation of hatching success. ing these sources of data, we first had to de- Hatching success. The overall hatching termine that the two manipulations did not rate for the experiment was 87% (Table 2), differentially affect nestling survival. Of 129 individual species ranging from 75% to informative nestlings that hatched in host 100% in House Sparrow nests, and from nests, 63% survived to the criterion age of 83% to 100% in the Barn Swallow nests. one-half their nestling period. Of the 30 in- The conspecific control crosses had hatch- formative nestlings that hatched before ing rates of 86% for the Barn Swallow and being placed in host nests, 40% survived to 83% for the House Sparrow. No significant criterion age. Although this suggests that differences were found (P < .05) in the the latter were less successful than the for- hatching success of the cowbird relative to mer, an examination of the survival of the that of the other experimental species in cross-fostered nestlings by species reveals either host nest. that this was not the case (Table 3). Ten of the cross-fostered nestling failures were NESTLING STAGE four Red-winged Blackbird and six Barn Nestlings not included in survival analysis. Swallow nestlings placed in the House The two sources of information about nest- Sparrow nests, which corresponds to the ling survival were cross-fostered eggs that failure of all 11 nestlings of these two hatched in the host nests, and young placed species that hatched in the House Sparrow in the host nests as nestlings. Of the 139 nests. Conversely, 88% of the House Spar- nestlings that hatched in the host nests, 10 row nestlings placed in the Barn Swallow provided no data on nestling survival. The nest survived, as compared to a survival rate fate of seven nestlings was ambiguous, as at of 76% of nestlings which hatched in the the first check the nestling was present but host Barn Swallow nest. In addition, the one had not reached criterion age, and the next phoebe (in a Barn Swallow nest) and two visit was on the date of expected departure, House Sparrow (in House Sparrow nests) so the nestlings’ absence could have been nestlings placed in the host nest all sur- due to either death or fledging. Two other vived. Also, the failure of seven out of eight 420 D. EASTZER, P. R. CHU AND A. P. KING

TABLE 3. Survival of the experimental and control nestlings for each host species.

Nestlings hatched Nestlings crossed in host nest into host nest

Total number Number Total number Number of nestlings of nestlin s of nestlings of nestlings Nestlin Pararite species hatched survive h! cross-fostered survived surviva P

Host: Barn Swallow Brown-headed Cowbird 18 12 - 67% Red-winged Blackbird 17 10 - 59% House Sparrow 17 13 9 8 81% Gray Catbird 10 6 - - 60% Eastern Phoebe 12 10 1 1 85% Bank Swallow 1 1 8 1 22% Barn Swallow control 22 20 - - 91%

Barn Swallow host subtotal 97 72 18 10 72%

Host: House Sparrow Brown-headed Cowbird 6 0 - - 0% Red-winged Blackbird 4 0 4 0 0% Barn Swallow 7 0 6 0 0% House Sparrow control 15 9 2 2 65%

House Sparrow host subtotal 32 9 12 2 25%

Total 129 81 30 12 58%

one- to two-day old Bank Swallows cross- and weighed them daily. One died by day fostered into Barn Swallow nests (for which 7, and the other two lost weight until they we have no comparable data for nestlings died on day 9. Necropsies disclosed atro- crossed as eggs) was apparently not due to phied musculature although the stomach the fact that they were crossed as nestlings; and gizzard were full of insect and seed five of the seven nestlings that failed to matter, indicating that the nestlings had reach criterion age were alive at seven days been fed by their foster parents. Unlike the of age (five days after the switch), and died experimental parasites, the House Sparrow subsequently. The failure was thus due to conspecific control crosses had a relatively events in the host nest, rather than the high survival rate of 65%. switching of the birds as nestlings. Because Only the Bank Swallow had significantly of this lack of systematic differences be- lower nestling survival (P < .05) than the tween the two sources of nestling survival cowbird in the Barn Swallow nest. There information, the data were pooled for anal- were no significant differences (at the P < ysis. .05 level) in survival between the cowbird Nestling survival. The nestling survival and the two experimental parasites in the rates in the Barn Swallow nests for five of House Sparrow nest, as all three species the experimental species ranged from 59% died. for the Red-winged Blackbird to 85% for the Asynchrony of hatching between parasite phoebe. The sixth experimental parasite, and host species. We examined the relative the Bank Swallow, had a nestling survival ages of the parasite nestling and the host rate of only 22%. Sixty-seven percent of the brood in order to determine if the discrep- cowbirds survived to criterion age, and the ancy affected the survival of the parasite. Barn Swallow had the expected highest rate We analyzed 150 cases, measuring the age of success at 91% (Table 3). of the host brood from the hatching date of In the House Sparrow nests, by contrast, the oldest young. In I4 cases, the cross was none of the nestlings of any of the three ex- made such that the host clutch was one to perimental species lived to criterion age. four days more advanced than the individ- Nineteen of the 27 experimental parasite ual placed in the nest, owing to either a nestlings died within six days after hatch- miscalculation of the age of the candled egg ing; the rest died subsequently. All ap- or the unavailability of a host clutch at the peared to be losing weight until they dis- appropriate stage. Four of the parasite nest- appeared from the nest or were found dead. lings survived, an observed survival rate of In order to determine if older nestlings 29%. The expected nestling survival rate would do better, we put three four-day old (see legend, Table 4, for formula) for this blackbird nestlings in House Sparrow nests sample was 71%. Thus, the observed sur- COWBIRD NESTLINGS 421

TABLE 4. Parasite nestling survival in relation to the degree of hatching asynchrony of the parasite and the host clutch.

Observed Expected Total number NeStbIlgS nestlin nestlin of crosses survived surviva P surviva F

Parasite 14 days younger than host brood 14 4 29% 71% Parasite same age as host brood 18 9 50% 49% Parasite 14 days older than host brood 49 31 63% 55% Parasite 5-8 days older than host brood 43 27 63% 59% Parasite 9-13 days older than host brood 26 17 65% 65%

a SR, = ZnrlN where SR, is the expected survival rate for each sample, n is the number of crosses from each parasite-host condition in the sample, r is the overall nestling survival rate for that parasite host condition, and N is the total number of crosses in the sample.

viva1 rate was 42% lower than that expected four days of age were not included in the in a random sample of that species compo- calculations). Of the 16 nests in which the sition. The same procedure was carried out cross-fostered egg did not hatch, all host for the samples of nestlings that were the clutches were successful. In addition, in the same age as the host young, one to four days 40 nests in which the cross-fostered individ- older than the host young, five to eight days ual died, 39 of the host clutches were suc- older, and nine to 13 days older than the cessful. However, of the 44 nests in which host young. The survival rates for these four the cross-fostered nestling survived to cri- samples did not differ greatly from their ex- terion age, only 31 host clutches were suc- pected values, the largest discrepancy cessful (Table 5). being 8% (Table 4). Host nestling survival. Data on the effect FLEDGLING STAGE of the cross-fostered bird on the status of the Fledglings’ behavior. One blackbird and host young at the time of the last nest check one catbird were each observed for seven were available for 100 nests. A host clutch hours immediately after leaving the host was rated a success if half or more of the Barn Swallow nest, and the cowbird was fol- number of host eggs originally in the nest lowed for a total of 9.5 h after fledging. Each were alive and at least four days old at the fledgling spent most of its time vocalizing. time of the last check. If no host young, or As measured by the proportion of one-min- less than half the original number of eggs ute intervals during which the fledgling was were alive at the last check, the nest was calling, the blackbird called during two- considered a failure (nests in which the host thirds of the observation period, and the cat- young had not yet hatched or were less than bird and cowbird each vocalized for more

TABLE 5. Success of host clutcha in relation to the fate of the cross-fostered bird

Fate of the parasite

Did not hatch Died Survived

Parasite snecies Successi ‘ Failure S”ClXSS” Failure SWXXSS” Failure

Host: Barn Swallow Brown-headed Cowbird 2 0 2 0 2 1 Red-winged Blackbird 1 0 5 1 2 2 House Sparrow 2 0 3 0 6 5 Gray Catbird 1 0 4 0 3 0 Eastern Phoebe 0 0 1 0 9 1 Bank Swallow 0 0 6 0 1 0 Barn Swallow control 4 0 0 0 7 2

Barn Swallow host subtotal 10 0 21 1 30 11

Host: House Sparrow Brown-headed Cowbird 1 0 0 0 0 0 Red-winged Blackbird 2 0 6 0 0 0 Barn Swallow 0 0 8 0 0 0 House Sparrow control 3 0 4 0 1 2

House Sparrow host subtotal 6 0 18 0 1 2

Total 16 0 39 1 31 13

a Host clutch success measured as survival (to at least 4 days of ape) of half or more the number of host eggs originally in the nest. 422 D. EASTZER, P. R. CHU AND A. P. KING than 90% of the observation period. The ap- DISCUSSION proach of the foster parents often elicited EGG STAGE calling from a silent fledgling, or increased the rate of calling in a bird already vocal- The hatching success of all the species izing. cross-fostered into both the Barn Swallow The calls and the rate of calling differed and the House Sparrow host nests was quite among the three fledglings. The blackbird high. The cowbird eggs, however, did not vocalized least, usually at a rate of 15 to 25 do significantly better than those of the oth- calls per minute, with a maximum rate of 35 er foreign species placed in either host calls per minute. The catbird called 25 to 50 species ’ nest. Thus, although the pattern times per minute (maximum, 60/min) and and conditions of incubation differed some- the cowbird usually called between 40 and what among the species studied, as among 55 times per minute (maximum, 75/min). passerines in general, these differences did When one of the foster parents came near, not significantly affect hatching. the fledgling sometimes assumed a begging When given the advantage of hatching posture. The Red-winged Blackbird often before the host young, nonparasitic young gaped towards the adult silently, whereas appeared to fare as well as the cowbird the catbird and the cowbird frequently ac- through the egg stage. However, the crosses companied their begging with vocaliza- were made late in the incubation period, tions. and avian embryos may be more susceptible Foster parents ’ behavior. The three pairs to variations in the pattern of incubation of Barn Swallow parents appeared to be during early development. Although our very nervous and excitable during the ob- data do not suggest that the cowbird has any servation period. They frequently flew into special adaptations allowing it to be suc- and out of the area where the fledgling was cessfully incubated by so many species, fur- calling, and occasionally panted (a sign of ther work on this subject would be useful. stress). The parents spent much time ori- The hatching success of the conspecific ented towards, perched near, flying by and control crosses corresponds well with sometimes circling the fledgling. hatching rates reported for normal clutches Barn Swallows characteristically feed of these species (See1 1968, Snapp 1976). their young while hovering slightly above Thus, the cross-fostering technique ap- and facing them. The Barn Swallow foster peared to cause little or no decrease in parents of the catbird hovered near it 18 hatching success. times, and the blackbirds’ foster parents hovered 28 times, but on no occasion did NESTLING STAGE these parents attempt to feed the fledgling. The nestling survival rates for the conspe- In fact, neither of these fledglings was fed cific control crosses correspond well to by their foster parents during the observa- those reported for normal broods (See1 tion period. The foster parents of the cow- 1970, Snapp 1976), indicating that the via- bird, in contrast, hovered without attempt- bility of the nestlings was not affected by ing to feed it only twice, whereas they the manipulation. hovered while attempting to exchange food It was practical to record nestling mortal- 54 times during the observation period. Al- ity for only the first half of the nestling stage though many of the attempted exchanges (criterion age). Studies of normal broods were unsuccessful (e.g., the cowbird suggest that the major cause of death in the dropped the insect), the fledgling swal- second half of the nestling period is star- lowed after other exchanges and clearly was vation, reflecting the adequacy of the food fed. The cowbird was first fed 4.5 h after supply available to the female (Robertson leaving the nest; hovering near the other 1973, Caccamise 1976). As the earlier-hatch- fledglings first occurred one hour after the ing experimental parasite could easily out- blackbird fledged and three hours after the compete the host young and was often the catbird fledged. The hovering behavior ap- only nestling left in the nest, the Barn Swal- peared identical whether or not an ex- low hosts probably had no difficulty in pro- change of food with the fledgling was at- viding the parasite with sufficient food. Un- tempted. less there were incompatibilities between In summary, all three fledglings were per- the Barn Swallow and the experimental par- sistent yet different in their begging. Only asite that did not appear until the second the cowbird was able to elicit feeding from half of the nestling stage, which seems un- its Barn Swallow foster parents. likely, survival to criterion age provides a COWBIRD NESTLINGS 423

reasonable measure of the cowbirds’ nest- the Purple Finch (Carpodacus purpureus) ling survival rate relative to the other and the American Goldfinch (Carduelis species studied. tristis) are host species that feed their The cowbird nestling was successfully young regurgitated plant seeds. The Cedar reared by the Barn Swallow, as were four of Waxwing (Bombycilla cedrorum) feeds its the other five foreign species placed in the young a partly frugivorous diet; although it Barn Swallow nest. On the other hand, nei- usually ejects cowbird eggs, over 50 cases ther the cowbird, the Barn Swallow, nor the of parasitism of this species are known Red-winged Blackbird survived in the (Friedmann et al. 1977). Field studies of House Sparrow nest. This suggests that cowbird parasitism of the American Gold- cowbirds are no better equipped to cope finch (Middleton 1977) and Cedar Waxwing with parent-young incompatibilities than (Rothstein 1976) support the contention that are nonparasitic species. the cowbird nestling cannot survive on a It is interesting to note that the Bank diet of seeds or fruit. Swallow was the least successful of the six The low survival rate of parasite nestlings species cross-fostered into the Barn Swal- that were one to four days younger than the low nest although it was the closest relative. host brood demonstrates the role of com- One might expect that these species would petition in nestling survival. In many pas- have had the most similar developmental serines, hatching is asynchronous and late- requirements. Differences in their ecology hatching young suffer higher mortality and the microenvironment of their nests, (Ricklefs 1965, Willson 1966, See1 1970, however, may have led to incompatibilities Hussell 1972, Dyrcz 1974, Howe 1976, in the parent-young interaction. The Bank Strehl 1978). The female cowbirds’ tenden- Sw*allow nests in holes whereas the Barn cy to lay in nests that have one or two host Swallow does not. The behavior and com- eggs (Ficken 1961, Hann 1941, King 1979) munication between parents and nestlings can be seen as a behavior insuring that her are probably quite different in these two sit- egg is not laid in a nest in an advanced stage uations. of incubation. In addition, her tendency to The failure of the three fostered species use nests containing eggs smaller than her to be reared by the House Sparrow presents own (King 1979) also provides an advan- a more dramatic demonstration of the in- tage in competition with the host young. ability of some avian young to be reared by When the nonparasitic species ’ nestlings in a different species. Although Friedmann this study were provided with a similar ad- (1963) reported two observations of House vantage in another species ’ nest, they were Sparrow adults feeding cowbird fledglings, as successful as the cowbird nestling. the survival rate from the nest of this rare The reduction of host brood success when host species appears to be low. We specu- the parasite nestling survived to criterion late that the nutritional requirements of the age, as opposed to when it did not hatch or fostered young were not met. The loss of died, also demonstrates the impact of com- weight and muscular atrophy of the two petition on nestling survival. The nests blackbird nestlings transferred at four days were checked only until the cross-fostered of age, despite their full stomachs, argue nestling reached criterion age, and in some that the diet was insufficient. However, 19 cases the host broods included in our cal- of the 27 nestlings died within six days post- culation were between four days old and hatching. Data from four studies on House criterion age (eight days for the House Spar- Sparrows, compiled by Summers-Smith row, ten days for the Barn Swallow). Only (1963), show that 70-100% of the food those broods in which the parasite survived brought to the nestlings during the first six remained in competition with it. Hence, days is animal matter, compared to only 33- these host broods probably would have 50% for the remainder of the nestling peri- been even less successful if they had been od. Thus, the high rate of mortality during followed to their criterion age. The reduc- the early part of the nestling period suggests tion of host brood success of naturally par- that there may be other incompatibilities asitized nests is well documented, and this besides nutrition between the House Spar- study shows that the earlier hatching of non- row parents and the cross-fostered young. parasitic species will also reduce the suc- If the House Sparrow diet is insufficient cess of the host brood. to rear cowbirds, this implies that some It is important that the cowbird did not species that are commonly parasitized can- fare better than other, nonparasitic species not raise nestling cowbirds. For example, placed in a given host nest. Wallace 424 D. EASTZER. P. R. CHU AND A. P. KING

(1979:124) claimed that “the host may at- coax its Barn Swallow foster parents to feed tend to the young (cowbird) parasite in pref- it tempts us to speculate that it does possess erence to its own young” and that the cow- adaptations that enable it to deal with a bird is a “supernormal releaser” for parental foster parents’ reluctance to feed a foreign care. This implies that there is some set of fledgling. P. Woodward (pers. comm.) sug- characteristics that are integral to parent-off- gested that a cowbirds’ loud and persistent spring interactions among all passerines, calling may serve this function. Neither the and that the young cowbird has improved catbird nor the blackbird, however, was these features. Our data, however, suggest fed despite their loud and almost continu- that in fact there may not be such a set of ous vocalizing. This suggests that if, indeed, universal features. Rather, the young cow- a fledgling cowbird does something special, bird may be an “average” nestling in the it is more complex and subtle than simply sense that it is moderately successful in its begging more often than the host young. interactions with its most frequent host par- Whether the cowbird fledgling is able to in- ents, while it may be a failure with some of duce feeding from a broader range of its rarer hosts. species than the young of nonparasitic pas- Rothstein (1976) has shown that cowbirds serines needs further study. do not employ the best reproductive behavior since they sometimes parasitize species ACKNOWLEDGMENTS that eject cowbird eggs. We suggest further We thank R. E. Johnston for helpful suggestions during that cowbirds sometimes parasitize species this project. We are grateful to M. J. West, whose in- that accept the egg but cannot rear the nest- sights helped improve the paper. M. Ficken, R. S. Mil- ling to independence. Gochfeld (1979) ler and R. Robertson provided critical comments and suggestions on the manuscript. W. Kuenzel kindly found that the Shiny Cowbird (Molothrus demonstrated the technique for necropsy of the nest- honariensis), which also parasitizes many lings. We are indebted to all the property owners who species, commonly lays in the nest of the allowed us access to the nests, especially H. Levin, Long-tailed Meadowlark (Sturnella Zoyca), Mrs. B. Saunders, A. Reed, G. Campbell and the care- takers of the Ithaca Game Farm. Our work was sup- although the cowbird apparently cannot be ported by a grant from the Department of Psychology, reared to fledging by this host. Thus, occa- Cornell University. sionally wasting eggs by parasitizing un- suitable hosts appears to be one of the costs LITERATURE CITED of a generalist manner of brood parasitism. CACCAMISE, D. F. 1976. Nesting mortality in the Red- winged Blackbird. Auk 93:517-534. FLEDGLING STAGE DYRCZ, A. 1974. Factors affecting the growth rate of Our most salient finding was that the cow- nestling Great Reed WarbIers and Reed Warblers at Milicz, Poland. Ibis 116:330-339. bird fledgling was fed by the Barn Swal- FICKEN. M. S. 1961. Redstarts and cowbirds. Kingbird., lows, while the Red-winged Blackbird and 11:83-85. Gray Catbird fledglings were not. This sug- FRIEDMANN, H. 1929. The cowbirds: A study in social gests that a fledgling that has been raised oarasitism. Charles C. Thomas. Sorinefield. IL. FRIEDMANN, H. 1963. Host relations ofyhe parasitic by another species may be unable to learn cowbirds. U.S. Natl. Mus. Bull. 233. the parent-offspring communication system FRIEDMANN, H., L. F. KIFF, AND S. I. ROTHSTEIN. of the foster species. The habit of not feed- 1977. A further contribution to knowledge of the ing fledglings of a different species may host relations of the parasitic cowbirds. Smithson. have evolved in response to selection pres- Contrib. Zool. 235: l-75. GOCHFELD, M. 1979. Begging by nestling Shiny Cow- sures other than cowbird parasitism (as the birds: Adaptive or maladaptive. Living Bird 17:41- Barn Swallow is not a usual cowbird host). 48. Thus, species that have not evolved specific HANN, H. W. 1941. The cowbird at the nest. Wilson defenses against cowbird parasitism early in Bull. 53:209-221. HARRISON, C. 1978. A field guide to the nests, eggs, the nesting cycle (Rothstein 1975; e.g., and nestlings of North American birds. Collins, aggression against adult cowbirds to pre- Cleveland. vent egg laying [Robertson and Norman HOWE, H. 1976. Egg size, hatching asynchrony, sex 19751 or egg ejection [Rothstein 1970]), may and brood reduction in the Common Grackle. still present a potential problem to the Ecology 57: 1195-1207. HUSSELL, D. J. T. 1972. Factors affecting clutch size young cowbirds’ survival at the fledgling in arctic passerines. Ecol. Monogr. 42:317-364. stage. The extent of this reluctance to feed KING, A. P. 1979. Variables affecting parasitism in the nonconspecific fledglings among species North American cowbird (Molothrus ater). Ph.D. that the cowbird normally parasitizes will diss., Cornell University, Ithaca, NY. MIDDLETON, A. L. A. 1977. Effect of cowbird parasit- determine the severity of this problem for ism on American Goldfinch nesting. Auk 94:304- the cowbird as a species. 307. The ability of the fledgling cowbird to MULLER, R. E., AND D. G. SMITH. 1978. Parent-off- COWBIRD NESTLINGS 425

spring interactions in Zebra Finches. Auk 95:485- SEEL, D. C. 1968. Clutch size, incubation and hatch- 495. ing success in the House Sparrow and Tree Spar- NICE, M. M. 1937. Studies in the life history of the row (Passer spp.) at Oxford. Ibis 110:270-282. Song Sparrow, I. Trans. Linn. Sot. N. Y. 4:1-247. SEEL, D. C. 1970. Nestling survival and nestling NICE, M. M. 1939. Observations on the behavior of a weights in the Honse Sparrow and Tree Sparrow young cowbird. Wilson Bull. 51:233-239. Passer spp. at Oxford. Ibis 112:1-14. NICOLAI, J. 1974. Mimicry in parasitic birds. Sci. Am. SKUTCH, A. F. 1976. Parent birds and their young. 231:92-98. University of Texas Press, Austin. NORRIS, R. T. 1947. The cowbirds of Preston Frith. SNAPP, B. D. 1976. Colonial breeding in the Barn Wilson Bull. 59:83-103. Swallow (Hirundo rzlstica) and its adaptive sig- RICKLEFS, R. E. 1965. Brood reduction in the Curve- nificance. Condor 78:471-480. billed Thrasher. Condor 67:505-510. STREHL, C. 1978. Asynchrony of hatching in Red- RICKLEFS, R. E. 1968. Patterns of growth in birds. Ibis winged Blackbirds and survival of late and early 110:419-451. hatching birds. Wilson Bull. 90:653-655. ROBERTSON, R. J. 1973. Optimal niche space of the SUMMERS-SMITH, D. 1963. The House Snarrow. Col- Red-winged Blackbird. 3. Growth rate and food of lins, London. nestlings in marsh and upland habitat. Wilson WALLACE, R. A. 1979. Animal behavior. Its develop- Bull. 85:209-222. ment, ecology, and evolution. Goodyear Publ., ROBERTSON, R. J., AND R. F. NORMAN. 1976. Behav- Santa Monica, CA. ioral defenses to brood parasitism by potential WILLSON, M. F. 1966. Breeding ecology of the Yel- hosts of the Brown-headed Cowbird. Condor low-headed Blackbird. Ecol. Monogr. 36:51-77. 78: 166-173. ROTHSTEIN, S. I. 1970. An experimental investigation Department of Psychology, University of North Car- of the defenses of the hosts of the parasitic Brown- olina, Chapel Hill, North Carolina 27514. Address of headed Cowbird. Ph.D. diss., Yale University, second author: Ecology Program, Division of Enuiron- New Haven, CT. mental Biology, National Science Foundation, Wash- ROTHSTEIN, S. I. 1975. Evolutionary rates and host ington, D.C. 20550. Address of third author: Depart- defenses against avian brood parasitism. Am. Nat. ment of Psychology, Duke University, Durham, North 109: 161-176. Carolina 27705. Accepted for publication 23 Novem- ROTHSTEIN, S. I. 1976. Cowbird parasitism of the Ce- ber 1979. dar Waxwing and its evolutionary implications. Auk 93:498-509.