Food availability, intrusion pressure and territory size: an experimental study of Anna's hummingbirds (Calypte anna)

Abstract We studied breeding male Willow Flycatchers (Empidonax traillii) to test a model of optimal territorial behavior for feeding-time minimizers proposed by Hixon (1980). Specific predictions were that feeding time, defense time, and territory size should decrease with increasing food availability and increase with increased pressure from both conspecific and heterospecific competitors. The constant reproductive output of 4 and 3 eggs for first and second clutches, respectively, and the large component of uncommitted (sitting) time (62.6 ± 3.2%) over the breeding season confirmed that the Willow Flycatcher conformed to a time-minimization strategy. Data analyzed over three stages of the breeding cycle, and over all stages, however, showed that only 5 of the 36 possible relationships were significant as predicted by the model. In general, food availability and competitor pressure were not important influences on the territorial behavior of these birds. Variations in territory size could not be attributed to constraints on feeding time, but correlated closely with the energetic requirements of all birds occupying the territory. We believe that breeding insectivorous passerines, including Willow Flycatchers, maintain a large component of uncommitted time, as well as a larger than necessary territory, to minimize the impact of short-term variations in competitor pressure and food supply. Such birds need not conform to the predictions of models that optimize foraging time.

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Seasonal changes in territory use by red squirrels, Tamiasciurus hudsonicus, and responses to food augmentation

Canadian Journal of Zoology ◽

10.1139/z02-185 ◽

2002 ◽

Vol 80 (11) ◽

pp. 1957-1965 ◽

Cited By ~ 6

Author(s):

Kara L Vlasman ◽

John M Fryxell

Keyword(s):

Habitat Use ◽

Food Availability ◽

Territory Size ◽

Tamiasciurus Hudsonicus ◽

Red Squirrel ◽

Red Squirrels ◽

Treatment Groups ◽

Conspecific Density ◽

Effect Of Food ◽

Seasonal Influences

Traditionally, the importance of food availability and intruder pressure on habitat use have been investigated through their influences on territory size. Food-augmentation studies are more prevalent; however, they are potentially confounded by the indirect effect of conspecific attraction to local food additions. Frequent ambiguous results may be attributed to the scale of investigation. Furthermore, such investigations have not considered potential seasonal influences. In this study, controlled experimental manipulations of food availability and conspecific density were performed within red squirrel (Tamiasciurus hudsonicus) territories in Algonquin Park, Ontario, to tease apart the relative importance of direct and indirect effects of food augmentation. Effects were investigated at two levels of scale, using territory size and giving-up densities (GUDs) within activity locations as response variables. Augmenting food, with or without simultaneously removing conspecifics, did not influence territory size. However, territory size decreased in control and treatment groups following manipulations, which is suggestive of seasonal influences associated with caching behaviour and midden defence. Despite overwhelming seasonal influences, GUDs were more sensitive to treatments, revealing that competitors had a greater influence on habitat use than the direct effect of food addition. Ambiguous results of previous studies may due to the use of territory-size estimators that are insensitive to responses occurring at smaller scales.

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