Ethan D. Clotfelter

Impact of brood parasitism by Brown-headed Cowbirds on Red-winged Blackbird reproductive success

We examined brood parasitism by Brown-headed Cowbirds (Molothrus ater) on a prairie-nesting population of Red-winged Blackbirds (Agelaius phoeniceus) from 1984 to 1997 and the extent to which parasitism affected host reproductive success. During this 14-year period, 2-32% of redwing nests were parasitized per year. Two measures of cowbird parasitism, the proportion of nests parasitized and the number of cowbird eggs laid per year. significantly increased over time. Increases in these estimates of the intensity of parasitism were independent of annual changes in nesting density of Red-winged Blackbirds. Clutch size was reduced in parasitized nests, most likely as a result of host egg removal by cowbirds, but hatching success was similar between parasitized and unparasitized nests. Red-winged Blackbirds abandoned more than one-third of all parasitized nests. We estimated the cost of abandonment in terms of increased failure rate and decreased fledgling production of re-nesting attempts. When we excluded abandoned nests from the analysis, there was no difference in the probability of success between parasitized and unparasitized nests. Nestlings from parasitized nests were similar in mass and tarsus length to nestlings from unparasitized nests, but parasitized nests produced fewer redwing fledglings per successful nest. The success of unparasitized nests was negatively related to the intensity of cowbird parasitism, suggesting that cowbirds are predators as well as brood parasites of redwing clutches.

MECHANISMS OF FACULTATIVE SEX-RATIO VARIATION IN ZEBRA FINCHES (TAENIOPYGIA GUTTATA )

The offspring sex ratio at independence in a colony of Zebra Finches (Tae- niopygia guttata) was significantly male biased. Three possible proximate causes of this bias were examined. (1) The first was that there is a sex-biased laying sequence followed by brood reduction. Males were significantly more likely to hatch from earlier-laid eggs than females, but there was no evidence of increased mortality among late-laid nestlings. (2) The second was that there is sex-biased mortality due to sex-specific growth rates. Male and female offspring had similar rates of mass increase, tarsus growth, and wing-chord growth. (3) The third was that there is sex-biased provisioning by one or both parents, increasing survival probabilities of preferred sex. There was evidence that males fed male-biased broods more frequently than female-biased broods. Fledging age, fledging mass, and the number of days to independence were similar for male and female offspring, suggesting that sex-biased provisioning had no significant biological effect. Although none of the hypotheses adequately explained the observed sex-ratio bias, results from this and other studies suggest that sex- biased laying sequences followed by brood reduction are most likely to cause skewed offspring sex ratios in nature. The implications of a sex-biased laying sequence and of sex-biased provisioning by the male are discussed in reference to the potential evolutionary conflict between males and females over control of offspring sex ratios. Received 5 October 1994, accepted 21 June 1995.

The effect of aggregated nesting on red-winged blackbird nest success and brood parasitism by brown-headed cowbirds

We examined temporal and spatial nesting aggregations in a prairie-nesting population of Red-winged Blackbirds (Agelaius phoeniceus). In particular, we were interested in the effects of aggregated nesting on blackbird nest success and brood parasitism by Brown-headed Cowbirds (Molothrus ater). Most cowbird parasitism occurred early in the breeding season during the peak in Red-winged Blackbird nesting. As a result, more parasitized nests were active simultaneously with other nests than were unparasitized nests, even when we controlled statistically for the effects of nest initiation date. The probability of parasitism across the entire study site did not decrease on days when many nests were initiated, suggesting no swamping effect of aggregated nesting on cowbirds. However, individual parasitized nests were less synchronous with their nearest neighbors and farther from the nearest simultaneously active nest than were unparasitized nests. These data are consistent with the hypothesis that brood parasitism by Brown-headed Cowbirds may select for aggregated nesting in Red-winged Blackbirds as a result of greater parasitism pressure on isolated nests. Nests that succeeded in producing at least one fledgling were initiated significantly earlier in the breeding season than were nests that failed. Nest aggregation had no effect on nest success; successful nests were no different from failed nests in the number simultaneously active nests, their distance to nearest simultaneously active nests, the laying interval with their nearest neighbors, and the distance to their nearest neighbors. These data suggest that nest predation (the primary cause of nest failure), in contrast to nest parasitism, does not necessarily select for aggregated nesting, or that predation and nest aggregation are related only at high nest-densities.

The function of early onset of nocturnal incubation in red-winged blackbirds

We examined variation in the onset of nocturnal incubation by female Red-winged Blackbirds (Agelaius phoeniceus). Many females began nocturnal incubation after laying the prepenultimate egg, which resulted in asynchronous hatching. This pattern of incubation is consistent with a model that predicts hatching asynchrony when the ratio of nest predation in the egg stage to predation in the nestling stage is near unity. Using 13 years of nest data, however, we found no significant relationship between the natural degree of hatching asynchrony and fledging success, and a significant negative relationship between hatching asynchrony and nestling quality. Therefore, we considered three hypotheses to explain the early onset of nocturnal incubation in this species: (1) increased egg viability, (2) protection of clutches from predators, and (3) protection of clutches from brood parasitism by Brown-headed Cowbirds (Molothrus ater). We found no evidence that early onset of nocturnal incubation increased egg viability. Incubation was not related to ambient temperature or precipitation, and early onset of incubation was not associated with increased hatching success. Nocturnal incubation was similar at depredated and nondepredated nests, which does not support the hypothesis that nest predation selects for early onset of incubation. Onset of nocturnal incubation occurred significantly later at nests parasitized by cowbirds than at unparasitized nests, however, in one of two years. These data suggest that protection against cowbird parasitism favors early onset of nocturnal incubation in Red-winged Blackbirds.

Courtship displaying and intrasexual competition in the bronzed cowbird

This note reports observations of courtship behavior in Bronzed Cowbirds (Molothrus aeneus) and uses these observations to test predictions regarding mate choice and intrasexual competition. Little is known of the social behavior of this obligate brood parasite, but they are thought to be promiscuous (Carter 1984). Female home ranges greatly overlap one another, and males defend localized areas of dominance analogous to exploded leks (Bradbury 1981, Carter 1984). Recent observations (Clotfelter, unpubl. data), including some of those reported here, support Carters (1984) conclusions.

Red-winged blackbird parental investment following brood parasitism by brown-headed cowbirds : is parentage important ?

Abstract Parental investment by red-winged blackbirds (Agelaius phoeniceus) in response to natural and experi‐mental parasitism by brown-headed cowbirds (Molothrus ater), and in response to freeze-dried, female cowbird mounts presented near redwing nests during the egg-laying period was measured. Two measures of redwing parental investment were used: nest defense effort toward a model predator, and rate of feeding nestlings. There were no significant differences in levels of parental investment among unparasitized nests, naturally parasitized nests, or experimentally parasitized nests. Similarly, parental investment did not differ between redwings that were exposed to the cowbird mount and those that were not exposed to the mount, or among redwings exposed to the cowbird mount at different distances from the nest. This suggests that red-winged blackbirds do not recognize when they have been parasitized, and hence do not associate parasitism with a decrease in their parentage, or that parentage is not an important predictor of parental investment in this species.

UNUSUAL PARASITISM BY THE BRONZED COWBIRD

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TEMPORAL AND GEOGRAPHIC PATTERNS OF AGE-SPECIFIC GULL PLUMAGE IN RELATION TO POTENTIAL EXPOSURE TO POLYCHLORINATED BIPHENYLS

Abstract Study skins of Herring Gulls (Larus argentatus) and Great Black-backed Gulls (L. marinus) collected over a period of ∼150 years in the northeastern United States were used to test the hypothesis that potential exposure to polychlorinated biphenyls (PCBs) has an effect on age-related plumage coloration. We found no changes in the average age class (as determined by plumage) of gulls collected before and after 1930 (date PCBs began to be used) and no differences in average age class in birds collected near PCB-contaminated sites versus those collected far from such sites. We found significant skews in the age distributions of birds in our sample because of an under-representation of birds of intermediate age classes, but those skews were similar in pre- and post-1930 data sets and thus likely represent sampling errors and not the effects of PCBs. There was no difference in the age distribution of gulls collected near or far from contaminated sites. Our study, albeit indirectly, shows no evidence that PCB exposure affects plumage maturation rate in piscivorous gulls. Patrons Géographiques et Temporels du Plumage chez les Laridés en Fonction de l’bcpAecpge et de l’Exposition Potentielle aux Polychlorobiphényles