Brood parasitism causes female‐biased host nestling mortality regardless of parasite species

Abstract

Inequality in male and female numbers may affect population dynamics and extinction probabilities and so has significant conservation implications. We previously demonstrated that Brown‐headed Cowbird Molothrus ater brood parasitism of Song Sparrows Melospiza melodia results in a 50% reduction in the proportion of female host offspring by day 6 post‐hatch and at fledging, which modelling demonstrated is as significant as nest predation in affecting demography. Many avian brood parasites possess special adaptations to parasitize specific hosts so this sex‐ratio effect could be specific to the interaction between these two species. Alternatively, perturbations associated with brood parasitism per se (e.g. the addition of an extra, larger, unrelated nestling), rather than a Cowbird nestling specifically, may be responsible. We experimentally eliminated the effects of Cowbird‐specific traits by parasitizing nests with a conspecific nestling rather than a Cowbird, while otherwise emulating the circumstances of Cowbird parasitism by adding an extra, larger (2‐day‐older), unrelated Song Sparrow nestling to Song Sparrow nests. Our parasitism treatment led to few host offspring deaths and no evidence of male‐biased sex ratios by day 6 post‐hatch. However, after day 6, female nestling mortality rates increased significantly in experimentally parasitized nests, resulting in a 60% reduction in the proportion of females fledging. Cowbird‐specific traits are thus not necessary to cause female‐biased host nestling mortality and far more general features associated with Cowbird parasitism instead appear responsible. Our results suggest, however, that Cowbird‐specific traits may help accelerate the pace of female host deaths. The conservation implications of our results could be wide reaching. Cowbirds are unrelated to all their hosts, are larger than the great majority, and a Cowbird nestling's presence can mean there is an extra mouth to feed. Thus, sex‐biased mortality in parasitized nests could be occurring across a range of host species.

Fear affects parental care, which predicts juvenile survival and exacerbates the total cost of fear on demography

“…, DeCaire et al. ), and the total number of times the offspring was fed during each 1‐h treatment period. How concealed the offspring kept itself between parental visits was quantified by recording if it was visible or not, every 10 min, at which time the height above ground of the perch it was on was also estimated, to the nearest 1 m. The distance the offspring moved was quantified by two means: (1) measuring the straight‐line distance (m) between its position at the beginning and end of each 1‐h playback treatment and (2) estimating the distance it moved within each 2‐min interval during each 1‐h treatment and calculating the median “step” distance.…”