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Brown-headed cowbirds (Molothrus ater) are obligate brood parasites. Only females search for host nests and they find host nests one or more days before placing eggs in them. Past work has shown that females have a larger hippocampus than males, but sex differences in spatial cognition have not been extensively investigated. We tested cowbirds for sex and seasonal differences in spatial memory on a foraging task with an ecologically relevant retention interval. Birds were trained to find one rewarded location among 25 after 24 h. Females made significantly fewer errors than males and took more direct paths to the rewarded location than males. Females and males showed similar search times, indicating there was no sex difference in motivation. This sex difference in spatial cognition is the reverse of that observed in some polygynous mammals and is consistent with the hypothesis that spatial cognition is adaptively specialized in this brood-parasitic species.  相似文献   

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Many hosts of obligate brood parasites accept parasitic eggs despite the high costs of parasitism. Acceptance is particularly perplexing in brown-headed cowbird (Molothrus ater) (hereafter “cowbird”) hosts because the eggs of cowbirds and most hosts do not appear to match closely in visual characteristics detectable by humans. However, recent evidence suggests that parasite and host eggs may match in their ultraviolet (UV) reflectance, undetectable by humans, and that birds may use UV signals for egg discrimination. We determined whether egg colour matching in UV reflectance separates accepters and rejecters of cowbird parasitism by comparing the total UV (300–400 nm) reflectance of the eggs of 11 host species to cowbird eggs. Eggs of three of five accepter species and five of five rejecter species differed significantly from cowbird eggs in UV reflectance. We found no significant difference in the UV reflectance of the eggs of three closely related pairs of accepter and rejecter species. There also was no significant difference in the UV reflectance of cowbird eggs laid in nests of five host species, and the UV reflectance of cowbird eggs was not significantly correlated with that of host clutches. Thus, we found no support for the UV-matching hypothesis in brown-headed cowbirds and UV reflectance does not appear to separate accepters and rejecters of parasitism. Differences in UV reflectance between cowbird and host eggs, however, provide potential cues for use in egg discrimination. Experimental testing is needed to determine the relative importance of UV reflectance compared to other visual cues.  相似文献   

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Garamszegi LZ  Avilés JM 《Oecologia》2005,143(1):167-177
Interspecific brood parasites may use the secondary sexual characters of the hosts to decide which species to parasitize. Hence, species with conspicuous and well-recognisable traits may have higher chances of becoming parasitised. Using North American birds and their frequent brood parasite, the brown-headed cowbird Molothrus ater, we tested the relationship between features of song and plumage coloration of hosts and the frequency of brood parasitism while controlling for several potentially confounding factors. Relying on two sets of analysis, we focused separately on the evolutionary view of the parasite and the host. From the cowbirds perspective, we found that males of heavily parasitized species posit songs with low syllable repertoire size, shorter inter-song interval and have brighter plumage. From the hosts perspective, a phylogenetic analysis revealed similar associations for features of song, but not for plumage characteristics that were unrelated to brood parasitism. These comparative findings may imply that brood parasites choose novel hosts based on heterospecific signals; and/or host species working against sexual selection escape from brood parasitism by evolving inconspicuous sexual signals. Although our data do not allow us to distinguish between these two evolutionary scenarios, our results suggest that selection factors mediating cowbird parasitism via host recognition by heterospecific signals may have an important role in the evolutionary relationship between brood parasites and their hosts.Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

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