Abstract The flexible-investment hypothesis posits that parents should allocate their resources according to offspring needs. The available evidence suggests that this is valid in most avian biparental care systems, yet experiments designed to quantify the degree to which caregivers respond to the demands of offspring are rare in cooperative breeding systems, where offspring are reared by individuals additional to the breeding pair (i.e. helpers). By cross-fostering Iberian magpie (Cyanopica cooki) chicks of different ages, we tested whether breeders and helpers adjust both the duration and intensity of care in response to brood developmental stage. Both breeders and helpers (all males) prolonged their nestling care period when raising chicks younger than their own and shortened it when raising chicks older than their own, with no detectable effect on reproductive success. Overall, provisioning increased when the chick-rearing period was experimentally shortened and decreased when it was prolonged. Yet the magnitude of these responses differed among carer statuses. In the shortening experiment, female breeders showed the strongest increase in provisioning, whereas helpers showed little or no change. In contrast, during brood prolongation, provisioning declined in all carer types but was most pronounced in breeders. Overall, these results support the flexible-investment hypothesis while indicating that helpers exhibit more limited flexibility in their feeding responses than breeders. Combined with previous observational work, our results are consistent with the idea that helping behaviour may function in part as a signal to gain direct benefits through reciprocity.
Espinosa-Colín et al. (2026) studied this question.