Helped ground tit parents in poor foraging environments reduce provisioning effort despite nestling starvation

Abstract

Life history theory views parental effort as a dynamic response to current productivity benefits and personal survival costs. Classical provisioning models specific to avian cooperative breeding systems predict that nestling starvation caused by local food limitation will induce helped parents to maintain their effort to ensure fledging success. Yet, food shortages may impose such a high provisioning cost that selection favours parents that lighten their workload at the expense of offspring productivity. We tested this alternative prediction with the ground tit, Parus humilis, which is an insectivorous, facultative cooperative breeder; helpers are mostly philopatric male offspring. Our study was based on data from three breeding seasons over which rainfall, and hence food resources, varied greatly. Total amount of food delivered to the young was highest for group-fed broods in food-rich environments, although helped male and female parents invested less than their counterparts with no helpers, and provisioning rate of the latter in both types of environmental conditions was similar to that of group-fed broods in food-poor environments. The variation in care level was mirrored by the observed pattern of partial brood loss, the single largest cause limiting productivity. The lowered effort in association with increased brood reduction under poor foraging conditions suggests ground tit parents with helpers trade future fitness against current reproduction to cope with harsh, unpredictable environments. This finding broadens our understanding of interspecific variability of parental response to the presence of helpers across ecological gradients.