Patterns of brood division and an absence of behavioral plasticity in a neotropical passerine

Studies of parental behavior in various habitats provide an opportunity to gain insight into how different environments may mold strategies of parental care. Brood division by parents has been hypothesized to occur facultatively within and among species. Brood division occurs when each parent cares for specific offspring within a brood. We studied brood division in a neotropical passerine, the western slaty antshrike (Thamnophilus atrinucha). Our results present a unique picture of a highly specialized example of avian brood division. Division was a fixed behavioral pattern in the population studied: all broods divided by fledging and remained divided during the entire post-fledging period. Brood division before fledging, a previously unreported phenomenon, occurred in 40% of nests observed. Parents that preferentially fed a certain offspring (defined as their focal offspring) in the nest fed the same individual after fledging. Each parent fed only its focal offspring in broods of one and two. The male parent cared for the heavier offspring and the first offspring to leave the nest. Siblings were segregated spatially during the time of highest predation risk. These observations suggest that a consistently high risk of predation on offspring has favored initial spatial segregation and inflexibility of brood division behavior in this species. Factors other than predation risk alone may explain the observed patterns of long-term, perfect brood division. Because high predation is common and relatively predictable in the tropics, selection for fixed brood division may be stronger in tropical birds than in the temperate zone.