Provisioning adjustments by male and female fairy martins to short-term manipulations of brood size

In biparental birds, the relative contribution of the sexes to feeding their brood (provisioning share) is sometimes reported to vary with brood size. However, the explanation for changes in provisioning share are often ambiguous (particularly in correlational studies), while the variation in findings between studies remains poorly understood. In this study we examined how short-term, within-pair manipulations of brood size (reduced, original and enlarged) affected provisioning rate in the fairy martin, Hirundo ariel. Following each manipulation, provisioning rates were monitored continuously for a two day period. Total provisioning rate increased when broods were enlarged and decreased when broods were reduced, though increases were inadequate to meet demand in enlarged broods because per chick feeding and growth rates declined. Both sexes responded similarly to brood size change over the course of the two days following the manipulation and there was no overall difference in provisioning share between treatments. Provisioning rate was correlated with wind strength (negatively) and ambient temperature (positively) but the response of the sexes to these variables was also similar. The absence of any change in provisioning share with brood demand suggests that alternative activities that could be traded-off against provisioning (e.g., self-maintenance) were similar for both sexes. Nevertheless, provisioning share may vary with the interval (i.e., hours, days, weeks) between the manipulation and measurement of provisioning, and these dynamics may contribute to explaining the inconsistent findings reported in the many previous studies examining the relationship between brood size and provisioning share.