Is broodmate aggression really associated with direct feeding?

The Feeding Method hypothesis (FMH) proposes that the way parents transfer food to chicks influences whether broodmates compete for it aggressively or non-aggressively. The FMH assumes that aggression is more efficient for securing a large share of food when prey items pass from bill to bill (direct feeds) than when prey is deposited on the nest floor (indirect feeds). In species with a developmental transition from indirect to direct feeding, the hypoth- esis predicts more aggression during direct than indirect feeds and an increase in rates of aggression as feeding becomes increasingly direct. We quantified development of aggression and feeding in two-chick cattle egret (Bubulcus ibis) broods in order to test the FMH’s as- sumption and its two developmental predictions. We also examined whether changes in rates of aggression early in the nestling period are better predicted by the Feeding Method, Food Amount or Early Dominance Establishment hypotheses. Neither the assumption nor either of the predictions of the FMH was supported and, if anything, senior broodmates were more aggressive early in the nestling period when feeding was indirect. These observations cast doubt on the ultimate influence of feeding method on use of aggression and, especially, on the role of direct feeding as a proximate trigger for aggression. Rates of aggression better fitted the temporal patterns predicted by Early Dominance Establishment and Food Amount hypotheses.