A future cost of misdirected parental care for brood parasitic young?

Parental care is advantageous because it typically increases the survival of genetically related young. In contrast, parental care given to unrelated young incurs no benefit. A further cost of parental investment is that it reduces the future reproductive potential of the caregiver. I examined whether eastern phoebes' Sayornis phoebe future reproductive effort was related to interspecific brood parasitism by brown-headed cowbirds Molothrus ater in prior broods. In 2000 absolute and relative measures of change in clutch sizes from first to second breeding attempts were similar in parasitized and non-parasitized broods, while the latency to renest was several days shorter for parasitized broods. In addition, the relative change in clutch size was more negative for phoebe nests with more cowbird chicks per brood. In 2001 these statistical relationships between absolute and relative measures of residual effort and prior parasite load were also confirmed in control but not in experimentally manipulated clutches. The experimental data support previous findings that parasitism per se does not seem to influence residual reproductive output of adult phoebe hosts. These data also emphasize that intragenerational residual costs of parental care should be measured by the use of a relative measure of reproductive effort or other statistical methods that take into account the biological and statistical non-independence of clutch sizes from subsequent breeding attempts.