Egg-laying behaviour by shiny cowbirds parasitizing brown-and-yellow marshbirds.

MERMOZ, M. E.; REBOREDA,JC

ANIMAL BEHAVIOUR

Animal Behavior Society

Año: 1999 vol. 58 p. 873 - 882

0003-3472

We studied the laying behaviour of shiny cowbirds, Molothrus bonariensis, parasitizing brown-and-yellowmarshbirds, Pseudoleistes virescens. Shiny cowbirds lay two egg morphs, spotted and white immaculate.Brown-and-yellow marshbirds eject the white egg morph but accept the spotted morph. The incidence ofparasitism in this host was 66.5%, and half of the parasitized nests had more than one shiny cowbird egg.There was a positive relationship between the number of parasitic events and the availability of nestsin the laying stage, but parasitic events reached a plateau when the availability of nests was high. Thedistribution of parasitic eggs per nest was more clumped than expected by chance. Shiny cowbird femalessynchronized parasitism with host laying in 80% of the cases. They seldom parasitized nests before thehost started laying or after the nest had been deserted or predated. The majority of multiply parasitizednests were parasitized by more than one female. Females that lay white eggs did not avoid parasitizingbrown-and-yellow marshbird nests. Egg pecking by cowbird females resulted, on average, in one egg lostper parasitic event and the probability of being broken was greater for host eggs when host and parasiticeggs where both in the nest.