When do hummingbirds use torpor in nature?

Abstract

The physiology of torpor in hummingbirds is well known from laboratory studies, but we still do not know when or how often this means of energy conservation is used in nature, whether regularly (the "routine" hypothesis) or only in response to inadequate food intake (the "emergency-only" hypothesis). I used body masses of broad-tailed hummingbirds (Selasphorus platycercus) to evaluate these hypotheses. Wild birds were weighed on electronic balances (1) on capture in mist-nets or traps and (2) when perched at feeders containing sucrose solutions. Masses followed predictable patterns within a season (breeding or wintering). Estimated energy equivalents of daily cycles of storage and depletion did not support the routine hypothesis. Converted to energy equivalent after adjusting for water content of dusk feeding, the estimated energy equivalent of overnight mass decrease (adjusted for water content of dusk feeding) averaged 15 times the demand for torpor during the breeding season, normally equalling or exceeding the demand for high nocturnal body temperature. In the annual cycle of the broadtail, the times when torpor might become necessary are (1) on arrival at breeding or wintering sites before flowers are abundant and (2) in midseasons (when nectar production fails), when daytime feeding is interrupted by storms, or when birds are otherwise prevented from storing adequate reserves by dusk.

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