Breeding territory quality and agonistic behavior: effects of energy availability and intruder pressure in hummingbirds

Summary

The relative importance of food a vailability and intruder pressure on breeding territorial behavior was studied in two experiments with male calliope hummingbirds. In the first experiment, extra food was provided in inconspicuous feeders. Territory owners who fed from the feeders increased their display rates (power-diving and hovering) when on their territories, and spent less time out of sight (mostly off their territories). Food addition by this method had no significant effect on intruder pressure, as measured by number and duration of chases. In the second experiment, intruder pressure was increased by attracting feedertrained males to feeders in territories of individuals who did not feed from feeders. In this experiment, display activity of residents did not change but chasing activity increased. These results suggest that display rates by male calliope hummingbirds depend at least in part on the amount of food available, but are not strongly affected by the number of feeding attempts by male intruders. No attempt was made to determine whether displays can deter competitors, attract mates, or do both. However, since these energetically expensive displays depend at least in part on the energy available to owners, they provide readily available indices of male and/or territory quality which could potentially be used for different purposes by different individuals.