Competition for resources modulates cell-mediated immunity and stress hormone level in nestling collared doves (Streptopelia decaocto).

Competitive stress imposed by hatching asynchrony may affect developmental trajectories of offsprings by regulating resource allocation between growth and other fitness-related traits. For instance, the down-regulation of immunity is a commonly observed phenomenon under stressful conditions. However, physiological mechanisms that regulate resources allocation to growth and immune functions in response to competition for resources, as well as inter-sexual differences in physiological strategies, are still poorly investigated. To partially fill this gap, we first conducted a descriptive study on chicks of the collared dove (Streptopelia decaocto), a species producing two chicks per brood. Our results show that first hatchlings (seniors) were bigger, showed lower baseline corticosterone levels (CORT) and showed a higher cell-mediated immunoresponsiveness (CMI) than late hatchlings (juniors). However, when controlling for body size, only CMI remained weaker in junior chicks suggesting differences in strategies of resource allocation between siblings. Interestingly, CORT in juniors increased with increasing within-brood hierarchy. But, while within-brood variation in CMI followed the opposite pattern of variation in CORT, we found no evidence that inter-individual variation in CMI was directly related to CORT. In addition hatching-rank related differences in body size, CMI and CORT was similar between sexes. To ensure that the lower phenotypic value expressed by juniors was not fully related to a lower quality of the late-laid egg, we experimentally suppressed the competitive stress experienced by juniors by removing the first-laid egg (i.e. the egg normally producing a senior chick). In the absence of their brood mates, juniors were bigger, had lower levels of CORT and showed a higher CMI than juniors raised in two-brood chicks, suggesting that body size, CMI and CORT in juniors were modulated by the competitive stress. Overall, this study suggests that juniors respond to within-brood competition by elevating CORT and down-regulating CMI. In this context, the role of CORT, as a mechanism regulating physiological strategies related to growth and immunocompetence is discussed.