Peripheral Foraging by Territorial Rufous Hummingbirds: Defense by Exploitation

Rufous Hummingbirds in the Sierra Nevada Mountains were observed to forage in the edge areas of their territories during the first few hours of the day, evenly over their territories during midday, and in the territory cores in late afternoon. This distribution of foraging effort appeared to be inconsistent with maximizing their rates of net energy gain during foraging. If Rufous Hummingbirds were to maximize their net energy gain during individual foraging bouts, foraging effort would have to be distributed evenly over the territory at all times of the day. We present a model which suggests that the observed feeding behavior of territorial Rufous Hummingbirds leads to higher daily net energy gains and lower food losses than if the birds maximized only net energy gain during each foraging bout during the day. Intrusion pressure accounts for the early—morning bias of foraging of edge areas. Intruders attempted to rob nectar from flowers in the edge areas of territories much more frequently than in the cores, and a higher proportion of these attempts were successful. Intruders frequently returned within 5—10 min to the sites that they had just robbed, and returned most frequently to those territories where they were able to rob for the longest time (on average) before being detected and chased. Nectar levels on territories were highest at dawn, and since edges were more prone to food losses than were cores, the territory owner minimized losses by feeding in the edge areas early in the morning. After chasing intruders from their territories, owners often returned and fed in the site just robbed by the intruder, which also reduced food losses by depleting nectar levels in areas prone to intrusion. According to the model, reduction of reintrusion rates by lowering the yield per intrusion and thereby discouraging robbers contributes more to the owner's benefit than does simple pre—emption of the resource by the owner. Thus, the peripheral foraging behavior of hummingbirds and their feeding response following intrusions are components of territory defense. Such d efense by exploitation may be common when food resources are being defended, and should be considered along with aggressive exclusion of intruders ( d efense by interference ) in the overall strategies of territorial animals. We close with a speculation that the late—afternoon foraging bias to the core may raise foraging efficiency during the time of day when the birds are heaviest and therefore have the highest foraging cost.