Brown-Headed Cowbird Parasitism on Endangered Species: Relationships with P = ) but was not correlated with abundance of other cowbird hosts. Larger than the Brown-headed Cowbird and mostly restricted to the Southwest, this species is another brood parasite. It may be more specialized in its choice of . Cuckoos are not the only birds that fool other species into feeding their chicks: . hypothesis in Brown-headed Cowbird hosts, they did not find any support. relationship between body size and egg size in birds in general and cuckoos in.
Typically double-brooded, prothonotary warblers lay 4—5 eggs during each nesting attempt Petit As a host, the prothonotary warbler appears to lack adaptive responses to brood parasitism Hoover band although able to raise a few warbler offspring with a cowbird nestling, parasitism events typically result in a reduction of host fecundity Hoover c. Common throughout North America, brown-headed cowbirds are obligate brood parasites that have been successfully raised by approximately host species Lowther Telemetry Dufty ; Raim and genetic studies Hahn et al.
In prothonotary warbler nests, cowbird nestlings increase the rate of food provisioning by adult warblers Hoover and Reetz and, weigh on average 2—3 times more than warbler nestlings Hoover c.
Each year, approximately nest-boxes 1.
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Nest-boxes were placed 1. To limit nest predation, a majority of the nest-boxes were attached to greased conduit poles. We monitored nest-boxes every 3—6 days throughout the breeding season and we recorded the number of eggs and nestlings of warblers and cowbirds present at each visit. Nest initiation dates i. We assumed nestlings fledged if they reached 10—11 days of age and the nest was empty and intact.
Additional evidence of fledging included the presence of trampled droppings in the nest, alarm calls from adults, and observations of appropriately aged fledglings in the territory. Adult warblers were captured and fitted with a unique color-band combination and a numbered aluminum band.
The ages for female warblers were identified by wing and tail feather criteria Pyle ; aged as either second-year i. The identities of ca. Experimental parasitism Inwe devised an experiment where warbler nests were randomly chosen to be parasitized or not on 6 study sites to eliminate the potential confound that female cowbirds selectively parasitize female warblers that have a greater likelihood of double-brooding. During May, we experimentally added 2 nonincubated cowbird eggs to approximately half of the active nest-boxes while female warblers were still laying.
We added 2 eggs to account for cowbird hatching failure and to ensure experimentally parasitized nests fledged at least 1 cowbird. Opening sizes of nest-boxes were made to be small enough 38mm to exclude most female cowbirds, but still large enough to encourage use by the warblers.
If female cowbirds, in general, selectively parasitize female warblers that are more likely to double-brood, then our random assignment of parasitism status should have eliminated this confound and resulted in 2 treatment groups that were not different in the probability of double-brooding.
Radio-telemetry To determine whether cowbird postfledging survival influences the double-brooding decisions of warbler hosts, we attached radio-transmitters 0. Evidence indicating juvenile cowbird mortality included plucked feathers and bite marks on recovered transmitters.
Postfledging mortality generally occurs within the first 3 weeks after fledging, typically during the first few days Cox et al. To identify the factors correlated with double-brooding within the nonexperimental dataset, we used generalized linear mixed models GLMM with a binomial response, logit link function, and Laplace likelihood approximation Bolker et al.
For these analyses, we limited the potential effects of making multiple nesting attempts on females and their ability to double-brood by only including successful nests initiated from the beginning of the breeding season to June 1.
There could be some females in this sample that are renesting after early nest failure i. The potential effect of nonindependence for the same female nesting across several years was controlled for by including female warbler identity as a random variable. Furthermore, GLMMs investigating the factors correlated with double-brooding probability included the effects of year categoricalnest initiation date ordinaland female warbler age continuous; Bulluck et al. Fecundity reduction and double-brooding To determine if the likelihood of double-brooding for female warblers is correlated with fecundity reduction, we used a model including only nonparasitized nests and examined the relationship between residual brood size number of fledglings — number of warbler eggs and the probability of initiating a second clutch.
Brood parasitism and double-brooding Because warblers fail to recognize foreign eggs Hoover b and provide care for cowbird offspring throughout the nestling and postfledging stages, even at the expense of adult and juvenile warbler survival Hoover and Reetz ; McKim-Louder et al.
Therefore, we estimated the effect of parasitism on perceived fecundity with a GLMM that included the total number of fledglings both cowbird and warbler as the dependent variable normal distribution and the parasitism status as an independent variable, whereas including female warbler identity as a random effect and controlling for the fixed effects of year categoricaldate, and female age.
We then used a GLMM to compare the probability of double-brooding among successful parasitized nests that fledged at least 1 cowbird offspring and nonparasitized nests that fledged at least 1 warbler offspring, whereas controlling for the potential effects of female identity random effect and including date, year, and warbler age as covariates.
For the experimental data collected inwe used a Pearson chi-square test to compare the frequency of double-brooding among experimentally parasitized broods that fledged at least 1 cowbird with nest-boxes, where no cowbird eggs were added i. To identify if double-brooding females that raised a cowbird in their first brood are more likely to be parasitized again, thus enhancing parasite transmission, we compared the parasitism rates of the second breeding attempt between double-brooded females that were either parasitized or nonparasitized during their first brood.
We examined the parasitism status during the second breeding attempt binomial response variable using a GLMM with female identity included as a random effect, while including year and initiation date of the second brood as covariates.
Because cowbirds are native to the U. However, unpermitted control of cowbirds is occasionally permissible under special circumstances outlined in the Migratory Bird Treaty Act. Some species, such as the Yellow Warbler, can recognize cowbird eggs and will reject them or build a new nest on top of them.
Those species which accept cowbird eggs either do not notice the new eggs, or as new evidence suggests, accept them as a defense against total nest destruction.
Use feeders that are made for smaller birds, such as tube feeders that have short perches, smaller ports, and no catch basin on the bottom. Avoid platform trays, and do not spread food on the ground.
Cowbirds prefer sunflower seeds, cracked corn, and millet; offer nyjer seeds, suet, nectar, whole peanuts, or safflower seeds instead. Clean up seed spills on the ground below feeders.
First, look for any eggs that appear different or out of place. Cowbird eggs are sometimes, but not always, larger than those of the host bird. This is especially true of warblers and small birds, but cowbird eggs are the same size as Northern Cardinal eggs. Cowbird eggs are white to grayish-white with brown or gray spots or streaks. Look for intact eggs on the ground under active nests.
Female cowbirds often evict one or more of the host eggs before they lay their own.