Parental care in relation to brood size in the House Finch

Studies that quantify parental care in birds are often faced with the confounding effects of variation in brood size. That is, nestlings from broods of varying sizes may receive different quantities of food for reasons not entirely related to the parental quality of the adults. To control for variation in brood size, researchers often divide feeding visitation rates by brood size to yield a per-nestling feeding rate. This presents problems, however, if adults adjust food load size in response to variation in blood size. We examined the relationship between brood size and parental care in the herbivorous House Finch (Carpodacus mexicanus) by considering not only visitation rates but also food load sites. As brood size increased, the overall visitation rate increased but both per-nestling visitation rate and per-nestling food load rate decreased. The relationship between brood size and per-nestling care was similar regardless of whether we considered visitation rate or actual food loads, suggesting that in the House Finch per-nestling visitation rate serves as a reasonable index of total mass of food received by the nestlings. However, we urge caution in assuming that per-nestling visitation rate is an adequate measure of parental care in other species.