Zebra finch nestlings, rather than parents, suffer from raising broods under low nutritional conditions
In sexually reproducing species, parents and offspring have different optima in terms of the amount of parental investment. For offspring, higher investment than the parental optimum generally increases their fitness. However, such higher investment will, in theory, result in net parental fitness loss because increased benefits from current offspring will be more than offset by a potential decrease in parental survival or future reproduction. Whether and how parents respond to a shortfall in resources might therefore have important fitness consequences. Here, we manipulate the nutritional condition of captive zebra finch families to investigate how variation in environmental conditions influences parental resource allocation between themselves and their offspring. By allowing the same zebra finch pairs to raise one brood under high and another brood under low nutritional conditions, we found that parents lost more body mass and their offspring grew at slower rates under low nutritional conditions. Therefore, both parents and their offspring were affected by the nutritional treatments. Offspring incurred greater costs, because slower growth rates under low nutritional conditions resulted in an overall lower body mass of nestlings after reaching independence. Nutritional treatments had sex-specific effects on parental body mass, because, in contrast to fathers, only mothers recouped their losses under high nutritional conditions. Furthermore, both parents spent less time brooding their nestlings under low nutritional conditions. Parental strategies hence vary with the nutritional quality of their food, even in captivity, supporting the idea that parental resource allocation may be driven by the expected costs and benefits from current offspring.