Male house sparrows deliver more food to experimentally subsidized offspring

We explored behavioural adjustments made by parent house sparrows, Passer domesticus, when the nutritional condition of their dependent nestlings was improved experimentally. Male parents responded to artificially supplemented broods by increasing food deliveries, whereas female parents continued matching the already high rate of control females. Thus, parental care was not truncated in the face of fortified offspring, but actually escalated (ca. 17% more adult food deliveries overall). Supplemented nestlings showed a nonsignificant tendency to recruit into the adult breeding population more than controls. We propose that one important reason why male parents responded more strongly than their female partners centres on the lower marginal costs for additional male posthatching investment, specifically by demonstrating that increasing paternal investment is likely to confer higher fitness than alternative male activities. (c) 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.