Offspring number and composition influence parental care strategy and offspring survival in the azure-winged magpie

Abstract

Examining parental optimization of their caregiving strategies is crucial for understanding the evolution of parental care because it benefits offspring while incurring costs to parents. This study hypothesized that besides offspring number, the composition of offspring in a nest, including the numbers of core (first-day hatching) and marginal offspring (subsequent-day hatching), can affect parental care. We tested this hypothesis using the azure-winged magpie, Cyanopica cyanus, breeding on the Tibetan Plateau. Initially, we categorized azure-winged magpie nests into different structures based on their offspring composition. Subsequently, we compared the fledging rates of core or marginal offspring among different-structured nests. Our findings showed that the fledging rate of core offspring was strongly affected by offspring numbers rather than their composition; it increased with an increase in marginal offspring numbers. Marginal offspring showed lower fledging rates than their core brood-mates and hatching sequences determined survival rates. Younger marginal offspring had consistently lower brood fledging rates. Our investigation of parental provisioning behaviours found that nest visitors provided more food to broods with more marginal offspring but allocated more food to core offspring, depriving marginal offspring of their fair share. Overall, our findings suggest that azure-winged magpies can optimize their caregiving strategy based on both the number and composition of offspring. This ultimately ensures greater survival for core offspring at a cost primarily borne by younger marginal offspring.