tive abilities to locate resources (e.g., the "shepherd's
hypothesis" [Rohwer and Ewald 1981], or "produc-
ers versus scroungers" [Barnard and Sibly 1981]).
My experiments failed to reveal any potential
costs of having a dominant foraging mode, consis-
tent with other experimental work that has unsuc-
cessfully attempted to find the "downside" of ter-
ritoriality in hummingbirds (Tiebout 1991a, 1992,
1993). These experiments leave open the question of
how hummingbirds can benefit from a subordinate
rank during interspecific competitive encounters.
More experiments are needed to evaluate food locat-
ing and acquiring abilities, especially under com-
petitive conditions with cryptic or low-density food
sources. We must also discover whether experiments
with free-living birds produce results similar to ex-
periments on captive animals. Such studies should
help determine whether there are any advantages of
low dominance rank and will contribute to our un-
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218 Birds