Brood division in a marsh-dwelling bird and its relation with the increase in offspring survival, acceleration of renest, and reduced competition for food resources

Parental care involves strategies that increase the offsprings survival. In some birds, each adult only feeds one young (brood division). Six hypotheses explain the functions of brood division: 1) reduced predation; 2) selection of a young by the parents; 3) selection of an adult by the young; 4) reduction of offspring competition; 5) efficiency improvement in parent-offspring interactions; and 6) social specialization. We studied the reproductive behavior and nestling morphology of the species to evaluate the possible functions associated with the brood division in Formicivora acutirostris, a bird of tidal marshes of southern Brazil. Females lay two eggs, but nests with one nestling may occur. We monitored 899 nests, nestlings, and parental care of young until they reached independence. Brood division occurred when the fledglings left the nest and continued until their independence. We did not observe a difference in the frequency with which sex feed single fledglings clutches. The fledglings remain in separated areas (pro hypothesis #1). There is a sex bias in the division (pro hypothesis #2). Adults fed the siblings equally (contra hypothesis #3). There is no aggression between siblings (contra hypothesis #4). Each fledgling remained close to their adult and responds only to it (pro hypotheses #5 and #6). We propose that brood division in the species may 7) reduce drowning of fledglings, 8) spatially divide food consumption, and 9) allow concomitant reproduction. We emphasize flooding as a structuring force for F. acutirostris, which raises concerns about the effects of climate change on this species.