Heterothermy in Caprimulgid Birds: A Review of Inter- and Intraspecific Variation in Free-Ranging Populations

Caprimulgid birds represent one of the best studied endotherm taxa in terms of the occurrence of heterothermy in free-ranging populations. In this chapter, we review currently available data on heterothermy in this group, and examine potential ecological correlates of these responses. To date, heterothermic responses have been examined in at least one population of each of six species, ranging in body mass (Mb) from 40 to 450 g and occurring in habitats from deserts to mesic woodlands. Patterns of heterothermy vary from infrequent, shallow bouts to periods of uninterrupted torpor lasting several days, during which body temperature may be reduced below 5C. Overall levels of heterothermy, as quantified using a recently proposed metric, do not show statistically significant relationships with Mb nor with ecological variables such as minimum air temperature or habitat aridity. Nevertheless, it is striking that the two most heterothermic species recorded to date, the Common Poorwill and the Freckled Nightjar, both inhabit arid habitats. Moreover, the former species remains the only bird known to hibernate. Within species, patterns of heterothermy may vary considerably among populations, with lunar cycles, temporal fluctuations in insect abundance and roost site characteristics being among the ecological determinants of heterothermy.