A generalist brood parasite modifies use of a host in response to reproductive success

Abstract

Avian obligate brood parasites, which rely solely on hosts to raise their young, should choose the highest quality hosts to maximize reproductive output. Brown-headed cowbirds (Molothrus ater) are extreme host generalists, yet female cowbirds could use information based on past reproductive outcomes to make egg-laying decisions thus minimizing fitness costs associated with parasitizing low-quality hosts. We use a long-term (21 years) nest-box study of a single host, the prothonotary warbler (Protonotaria citrea), to show that local cowbird reproductive success, but not host reproductive success, was positively correlated with the probability of parasitism the following year. Experimental manipulations of cowbird success corroborated that female cowbirds make future decisions about which hosts to use based on information pertaining to past cowbird success, both within and between years. The within-year pattern, in particular, points to local cowbird females selecting hosts based on past reproductive outcomes. This, coupled with high site fidelity of female cowbirds between years, points to information use, rather than cowbird natal returns alone, increasing parasitism rates on highly productive sites between years.

Keywords: brood parasitism; cognition and reproduction; cowbird; host selection; reproductive performance information; statistical decision theory.

Figure 1.

The relationship between the site-specific cowbird reproductive success in year t and the probability of parasitism for nests within that given study site the following year (t + 1). Results of a GLMM (n = 1458 nests) and the mean predicted probability of parasitism (±s.e.) are presented while holding additional explanatory variables at mean observed values; data includes nests from non-manipulated study sites (i.e. without egg removal) and both study site and female warbler identity were included as random effects.

Figure 2.

The comparison between the site-specific cowbird reproductive success in year t and the probability of parasitism the following year (t + 1) for nest-boxes used by warblers in consecutive years; nest-boxes parasitized (grey dotted line) in year t and non-parasitized (black line) in year t. Results of a GLMM (n = 245 nests) and the mean predicted probability of parasitism (±s.e.) are presented while holding additional explanatory variables at mean observed values; data include nests from non-manipulated study sites (i.e. without egg removal) and both study site and female warbler identity were included as random effects.

Figure 3.

The comparison between parasitized nests where all cowbird eggs were removed versus nests where ≥1 cowbird offspring fledged in year t and the probability of parasitism the following year (t + 1) for nest-boxes used by warblers in consecutive years. Results of a GLMM (n = 280 nests) and the mean predicted probability of parasitism (±s.e.) are presented while holding additional explanatory variables at mean observed values; study site included as a random effect.

Figure 4.

The relationship between parasitized first broods where all cowbird eggs were removed versus nests where ≥1 cowbird offspring fledged and the probability of parasitism for the second brood (i.e. double-brooding). Results of a GLMM (n = 312 nests) and the mean predicted probability of parasitism (±s.e.) are presented while holding additional explanatory variables at mean observed values; study site included as a random effect.