Floral resource availability and hummingbird territorial behaviour on a Neotropical savanna shrub

The influence of floral resource availability on behaviour and dominance hierarchy of hummingbirds has seldom been investigated in open tropical environments. We evaluated the behavioural changes of hummingbirds associated with Palicourea rigida, a shrub common in the Cerrado, the open savanna areas in Central Brazil. We observed the foraging behaviour as a function of resource availability and experimentally manipulated floral display and nectar in order to define resource thresholds associated with territorial behaviour and territory intruding behaviour. Territorial behaviour was observed only in patches with high floral abundance and was restricted to some hummingbird species. The large Eupetomena macroura seems to be the dominant species in the community, excluding other hummingbirds and holding the richest patches. The experimental reduction in the floral abundance and nectar led to the abandonment of territory by these hummingbirds, which reinforces the idea that a certain threshold is necessary to maintain territorial defence. Higher floral display, on the other hand, seems to lead to higher frequency of intrusions and higher behavioural complexity of territory defenders, possibly meaning greater energy expenditure. The flower number and nectar manipulation affected visitation rates of territorial hummingbirds, but the frequency of intruders was affected only by the former manipulation. This indicates that territorial intruder use visual cues (floral abundance) as indicators of resource availability in order to estimate cost-benefit of territory invasion. Although territorial behaviour could limit pollen flow in patches with larger floral displays, this may be compensated by the territory invaders, which bring pollen from other patches.