The Effect of Food Availability on Time and Energy Expenditures of Territorial and Non-Territorial Hummingbirds

We studied the time and energy allocations related to territorial behavior in male Blue-throated Hummingbirds (Lampornis clemenciae; about 8.3 g) under conditions of unlimited and restricted food availability. When food was unlimited. territorial males avoided inter-specific aggression, chasing only 11% of the inter-specific’intruders (Blackchinned Hummingbirds, Archilochus alexandri; about 3.5 g). Thus, when food was unlimited, inter-specific intruders were able to forage efficiently, meeting their estimated daily energy requirement with ease (27 Id/day). Conversely, 8 1% of intraspecific intruders were chased, and intra-specific intruders were able to feed at territorial feeders only when the territorial male was away. Chases of intra-specific intruders were longer and appeared to be more intense than chases of inter-specific intruders. When food was restricted territorial activity, including the total number of chases engaged in by the territory owner, was significantly reduced, although the basic characteristics of territorial behavior (e.g., chases and displays) did not change. Territory owners chased a higher proportion of inter-specific intruders when food was restricted (48%), suggesting an increase in inter-specific competition. A high proportion of intra-specific competitors continued to be chased (80%), although the total number of intra-specific intruders was lower. We believe that variations in the cost of territoriality are dependent primarily on the level of intra-specific competition. This is supported by the fact that when food was restricted to an amount that could support a maximum of 1.4 L. clemenciae(based on doubly labeled water measurements offield metabolic rate in a previous study), energy intake by the territory owner decreased from 114 kJ/day to 64 Id/day, with the primary difference being number of intra-specific chases. These data also suggest hat the exclusion of other hummingbird species might be based strictly on the amount of available food (energy). When food is restricted, inter-specific competition is more costly to the territory owner causing the exclusion of a higher proportion of inter-specific intruders. The high proportion of intra-specific intruders that are chased in either experimental condition suggests that territorial behavior in L. clemenciae might have functions other than resource protection per se, such as social functions related to their mating system.