GIANT COWBIRD EGGS IN THE NESTS OF 2 ICTERID HOSTS - THE USE OF MORPHOLOGY AND ELECTROPHORETIC VARIANTS TO IDENTIFY INDIVIDUALS AND SPECIES

The Giant Cowbird (Scaphidura oryzivora) is a brood parasite of several icterid hosts in Central and South America. Cowbird eggs can visually resemble those of their hosts. Evidence of egg mimicry requires quantification of the degree of similarity of host and parasite eggs. It is also unclear if nests found with more than one cowbird egg were parasitized by more than one female cowbird. We used electrophoresis of egg proteins and egg morphometric analyses to discriminate among the eggs of two host species (Cacicus cela and Zarhynchus wagleri) and those of the parasite, and to document the number of females that laid in a single host nest. Electromorphs of transferrin differed between the hosts ("a" alleles only) and the parasite ("b" and "c" alleles only), and thus serve as a species-specific marker. Multivariate assessment of egg measurements and markings indicate significant and non-overlapping differences in morphology between eggs of the three species as well. Electromorph and color morph differences showed that two or more female cowbirds definitely laid in six of 10 nests containing two or more cowbird eggs. In the other four nests, paired cowbird eggs could not be differentiated by color or electromorph and may have been laid by the same female.