An Investigation of the Host Specificity of the Brown-headed Cowbird

We attempted to determine the factors associated with the lack of parasitism of the Common Grackle (Quiscalus quiscula) by the Brown-headed Cowbird (Molothrus ater). We investigated the breeding phenology of the two species, the responses of colonial- and noncolonial-nesting grackles to female cowbird models, the frequency of artificial egg rejection by grackles, incubation success of cowbird eggs transferred into grackle nests, and the survival rates of cowbirds cross-fostered into grackle nests. By the time cowbirds began egg-laying at our study sites, 88.5 % of all grackle nests were beyond the point of successful parasitism. Grackles responded much more aggressively toward female cowbird models than to Fox Sparrow (Passerella iliaca) models. Grackles rejected artificial cowbirds eggs more frequently during the prelay stage of the nesting cycle (13 rejections at 32 nests) compared to later stages. However, the rejection frequency during the later stages of nesting (lay and incubation) was virtually the same as in Rothstein\u27s original study (1975) (12.4 % vs. 11.3 %). A total of 14 cowbird eggs and nestlings were cross-fostered into grackle nests. Data were collected on six cowbird nestlings, none of which survived to fledging. Five of the nestlings died after two days, and the sixth nestling survived five days despite having two grackle nestmates that were each a day older. Grackle nestlings weighed more than twice as much as cowbird nestlings at hatching (5.4 ± 0.91 g vs. 2.5 ± 0.72 g), and had significantly greater gape widths and culmen lengths for the first two days after hatching. The lack of survival of cowbird nestlings in grackle nests may be partially due to this size asymmetry. However, one cowbird nestling died after two days despite having no grackle nestmates to compete with, thus suggesting the possibility of some behavioral incompatibility. This was unexpected as it is generally believed that nestling passerines have similar dietary requirements, with the exception of those species that feed their young primarily seeds or fruit. Of the eight eggs that did not hatch, four appeared to be the result of ineffective incubation. These clutches contained between four and six eggs total, whereas the clutches in which cowbird eggs hatched contained a total of three eggs or fewer. These data support the host incubation hypothesis for egg removal by female cowbirds. If Brown-headed Cowbirds preferred larger hosts in the past (as indicated by the fact that all but one of the species that regularly eject cowbird eggs are larger than the cowbird), then it may have been advantageous for a female cowbird to remove at least one host egg to ensure more effective incubation of her own smaller egg. Mourning Doves (Zenaida macroura) like Common Grackles exhibit a high rate of parasitic egg rejection behavior (31.2 %) for an accepter species. Despite Rothstein’s (1975a) conclusion that no geographic variation in egg rejection behavior exists, we found Mourning Doves in central Illinois rejected artific ial cowbird eggs at nearly twice the rate (58.6 %, x2 = 3.7, df = 1, p \u3c 0.06) of those in Rothstein’s trials. The reason for the lack of geographic variation in Rothstein\u27s trials may be the result of small sample sizes

An Investigation of the Host Specificity of the Brown-headed Cowbird

We attempted to determine the factors associated with the lack of parasitism of the Common Grackle (Quiscalus quiscula) by the Brown-headed Cowbird (Molothrus ater). We investigated the breeding phenology of the two species, the responses of colonial- and noncolonial-nesting grackles to female cowbird models, the frequency of artificial egg rejection by grackles, incubation success of cowbird eggs transferred into grackle nests, and the survival rates of cowbirds cross-fostered into grackle nests. By the time cowbirds began egg-laying at our study sites, 88.5 % of all grackle nests were beyond the point of successful parasitism. Grackles responded much more aggressively toward female cowbird models than to Fox Sparrow (Passerella iliaca) models. Grackles rejected artificial cowbirds eggs more frequently during the prelay stage of the nesting cycle (13 rejections at 32 nests) compared to later stages. However, the rejection frequency during the later stages of nesting (lay and incubation) was virtually the same as in Rothstein\u27s original study (1975) (12.4 % vs. 11.3 %). A total of 14 cowbird eggs and nestlings were cross-fostered into grackle nests. Data were collected on six cowbird nestlings, none of which survived to fledging. Five of the nestlings died after two days, and the sixth nestling survived five days despite having two grackle nestmates that were each a day older. Grackle nestlings weighed more than twice as much as cowbird nestlings at hatching (5.4 ± 0.91 g vs. 2.5 ± 0.72 g), and had significantly greater gape widths and culmen lengths for the first two days after hatching. The lack of survival of cowbird nestlings in grackle nests may be partially due to this size asymmetry. However, one cowbird nestling died after two days despite having no grackle nestmates to compete with, thus suggesting the possibility of some behavioral incompatibility. This was unexpected as it is generally believed that nestling passerines have similar dietary requirements, with the exception of those species that feed their young primarily seeds or fruit. Of the eight eggs that did not hatch, four appeared to be the result of ineffective incubation. These clutches contained between four and six eggs total, whereas the clutches in which cowbird eggs hatched contained a total of three eggs or fewer. These data support the host incubation hypothesis for egg removal by female cowbirds. If Brown-headed Cowbirds preferred larger hosts in the past (as indicated by the fact that all but one of the species that regularly eject cowbird eggs are larger than the cowbird), then it may have been advantageous for a female cowbird to remove at least one host egg to ensure more effective incubation of her own smaller egg. Mourning Doves (Zenaida macroura) like Common Grackles exhibit a high rate of parasitic egg rejection behavior (31.2 %) for an accepter species. Despite Rothstein’s (1975a) conclusion that no geographic variation in egg rejection behavior exists, we found Mourning Doves in central Illinois rejected artific ial cowbird eggs at nearly twice the rate (58.6 %, x2 = 3.7, df = 1, p \u3c 0.06) of those in Rothstein’s trials. The reason for the lack of geographic variation in Rothstein\u27s trials may be the result of small sample sizes

Rejection of Cowbird Eggs by Mourning Doves: A Manifestation of Nest Usurpation? Brian D. Peer and Eric K. Bollinger

This article was printed in The Auk, Volume 115, No.4 (1998)

EXPLANATIONS FOR THE INFREQUENT COWBIRD PARASITISM ON COMMON GRACKLES

We determined the factors responsible for the lack of parasitism on Common Grackles (Quiscalus quiscula) by Brown-headed Cowbirds (Molothrus ater). We found no evidence of parasitism on the 401 grackle nests we monitored in east-central Illinois. By the time cowbirds began laying eggs, 88.5% of all grackle nests were beyond the point of successful parasitism. Grackles rejected cowbird eggs more frequently during the prelaying stage of the nesting cycle (38.2%) compared to later stages (12.3%). Thirty-three cowbird eggs and nestlings were cross-fostered into grackle nests. Data were collected on 15 cowbird nestlings, of which three fledged. The cross-fostered cowbird eggs that did not hatch appeared to have been incubated ineffectively. These eggs were in clutches that contained four to six eggs, whereas the cowbird eggs that hatched were in clutches of three eggs or less. Grackles also responded more aggressively toward female cowbird models than to Fox Sparrow (Passerella iliaca) models in five of seven response categories. Cowbirds avoid parasitizing grackles even though the opportunity existed. Nest abandonment and infestation of grackle nests with ectoparasitic mites may decrease the suitability of grackles as hosts; however the primary reason for the infrequent cowbird parasitism on grackles appears to be past rejection behavior. Grackles likely rejected cowbird eggs at a higher frequency in the past and as a consequence grackles were avoided by cowbirds. Rejection apparently decreased in the absence of parasitism due to the high degree of intraclutch egg variability exhibited by grackles whiwhich would increase the chances of them mistakenly rejecting their own discordant eggs

Dogs increase recovery of passerine carcasses in dense vegetation

Wildlife managers use carcass searches to assess mortality resulting from biological (e.g., diseases) and physical (e.g., structure collisions) sources. Carcass searches may occur over large areas and need to be completed rapidly because of scavenging and decomposition. However, small carcasses are often missed when dense vegetation is searched. We placed carcasses of house sparrows (Passer domesticus) in dense cover of residual and newly grown vegetation and compared searching efficiency of humans and canines. Dogs received no special training in searching for passerine carcasses. In 36 trials conducted in 5 x 40-m plots, human searchers found 45% (SD=19) of the carcasses compared to 92% (SD=13) for dogs (P=0.005). The ratio of recovered to missed carcasses was approximately 12:l for dogs and 1 :I for humans. The improvement in searching efficiency using dogs was similar (P=0.58) between residual cover (searched in April) and new growth cover (searched in August). A greater rate of searching efficiency is obtained per unit of time by using dogs. Greater efficiency improves quantitative and qualitative assessments of avian mortality in the field

A method to measure lactate recycling in cultured cells by edited 1H nuclear magnetic resonance spectroscopy

http://www.sciencedirect.com/science/article/B6W9V-4PF6B5M-1/1/385b6c0836057ee00a92ea234317f1e

Management of the Brown-Headed Cowbird: Implications for Endangered Species and Agricultural Damage Mitigation

The brown-headed cowbird (Molothrus ater; cowbird) is unique among North American blackbirds (Icteridae) because it is managed to mitigate the negative effects on endangered songbirds and economic losses in agricultural crops. Cowbird brood parasitism can further affect species that are considered threatened or endangered due to anthropogenic land uses. Historically, cowbirds have often been culled without addressing ultimate causes of songbird population declines. Similar to other North American blackbirds, cowbirds depredate agricultural crops, albeit at a lower rate reported for other blackbird species. Conflicting information exists on the extent of agricultural damage caused by cowbirds and the effectiveness of mitigation measures for application to management. In this paper, we reviewed the progress that has been made in cowbird management from approximately 2005 to 2020 in relation to endangered species. We also reviewed losses to the rice (Oryza sativa) crop attributed to cowbirds and the programs designed to reduce depredation. Of the 4 songbird species in which cowbirds have been managed, both the Kirtland’s warbler (Dendroica kirtlandii) and black-capped vireo (Vireo atricapilla) have been removed from the endangered species list following population increases in response to habitat expansion. Cowbird trapping has ceased for Kirtland’s warbler but continues for the vireo. In contrast, least Bell’s vireo (V. bellii pusillus) and southwestern willow flycatcher (Empidonax traillii extimus) still require cowbird control after modest increases in suitable habitat. Our review of rice depredation by cowbirds revealed models that have been created to determine the number of cowbirds that can be taken to decrease rice loss have been useful but require refinement with new data that incorporate cowbird population changes in the rice growing region, dietary preference studies, and current information on population sex ratios and female cowbird egg laying. Once this information has been gathered, bioenergetic and economic models would increase our understanding of the damage caused by cowbirds

HAS AN INTEGRATED PEST MANAGEMENT APPROACH REDUCED BLACKBIRD DAMAGE TO SUNFLOWER?

Since the mid- 1970s many new and modified damage abatement methods have been used to reduce blackbird damage to ripening sunflower in the northern Great Plains. To assess the overall impact of these techniques, we analyzed the dynamic relationship between breeding blackbird densities and sunflower damage. Breeding density estimates were made at both the regional and county levels, whereas, sunflower damage estimates were made at the county level only. Periodic regional estimates of breeding densities between 1967 and 1998 for red-winged blackbirds (Agelaius phoeniceus), common grackles (Quiscalus quiscula), and yellow-headed blackbirds (Xanthocephalus xanthocephalus) showed no differences among years. To increase our ability to detect changes in breeding density, we started intensive county-level surveys in 1996. These surveys, in four counties in North Dakota and South Dakota, showed that blackbird densities were greater in 1998 and 1999 than during the previous two years. We surveyed sunflower damage in two of these counties from 1994 to 1998 and found no difference in damage (F= 1.8%) among years. In 1997 and 1998, with the two other counties added to the survey, we found that damage was similar between years, averaging 2.2%. Dollar loss per hectare was trending lower in three of the study counties that had a historical database for comparison. This supports the idea that \u27local\u27 breeding densities are not correlated with damage levels. We will continue to use annual estimates of breeding densities and sunflower damage to assess the effects of an evolving Integrated Pest Management program