UV reflectance of eggs of brown-headed cowbirds (Molothrus ater) and accepter and rejecter hosts |
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Authors: | Todd J Underwood and Spencer G Sealy |
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Institution: | (1) Department of Biological Sciences, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada;(2) Department of Biology, Kutztown University, Kutztown, PA 19530, USA |
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Abstract: | 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. |
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Keywords: | Brood parasitism UV reflectance Egg discrimination Brown-headed cowbird Molothrus ater |
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