Abstract
A new technique for identifying eggs of brood parasites to individual females is presented. Twenty-six undeveloped brown-headed cowbird eggs were removed from nests on a small study area in eastern Kansas. Proteins from their yolks were subjected to starch gel electrophoresis. Electromorphs are completely of maternal origin, and results from a separate sample from California showed that electromorphs do not differ among eggs from an individual cowbird. Combinations of electromorphs at four polymorphic “loci” identified eggs to seven hypothetical females. The binomial probability that two or more of eight females (fieldwork suggested no more than 7–8 females laying on the site) share a particular combination was calculated from an off-site, independent sample of eggs. Six of seven of these probabilities were below 5%. Eggs collected directly from two females matched two of the electromorph combinations. Discriminant function analyses on 11 egg variables correctly classified 95.7% of the eggs to the group in which they were placed by electromorph combination. Patterns of dispersion of the eggs of individuals over host species, time and space were assessed. Despite small sample sizes, it is clear that individuals are not host-specific and overlap considerably in their laying areas.
Published Version
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