Ecological factors affect the level and scaling of avian BMR Brian Keith McNab ⁎ Department of Zoology, University of Florida, Gainesville, FL 32611, USA abstract article info Article history: Received 26 May 2008 Received in revised form 14 August 2008 Accepted 20 August 2008 Available online 27 August 2008 Keywords: Avian flight Ecological factors Energetics Geography Mammalian flight Migration Non-passerines Passerines Phylogeny The basal rate of metabolism (BMR) in 533 species of birds, when examined with ANCOVA, principally correlates with body mass, most of the residual variation correlating with food habits, climate, habitat, a volant or flightless condition, use or not of torpor, and a highland or lowland distribution. Avian BMR also correlates with migratory habits, if climate and a montane distribution is excluded from the analysis, and with an occurrence on small islands if a flightless condition and migration are excluded. Residual variation correlates with membership in avian orders and families principally because these groups are behaviorally and ecologically distinctive. However, the distinction between passerines and other birds remains a significant correlate of avian BMR, even after six ecological factors are included, with other birds having BMRs that averaged 74% of the passerine mean. This combination of factors accounts for 97.7% of the variation in avian BMR. Yet, migratory species that belong to Anseriformes, Charadriiformes, Pelecaniformes, and Procellariiformes and breed in temperate or polar environments have mass-independent basal rates equal to those found in passerines. In contrast, penguins belong to an order of polar, aquatic birds that have basal rates lower than passerines because their flightless condition depresses basal rate. Passerines dominate temperate, terrestrial environments and the four orders of aquatic birds dominate temperate and polar aquatic environments because their high BMRs facilitate reproduction and migration. The low BMRs of tropical passerines may reflect a sedentary lifestyle as much as a life in a tropical climate. Birds have BMRs that are 30–40% greater than mammals because of the commitment of birds to an expensive and expansive form of flight. © 2008 Elsevier Inc. All rights reserved. 1. Introduction A variety of approaches have been used to analyze avian basal rates of metabolism (BMR), which are the minimal rates in the zone of thermoneutrality when birds do not increase heat production for temperature regulation. The argument to be made here is that avian BMR varies with a variety of factors, especially body mass, but also with avian behavior and conditions in the environment, a pattern that was seen in a recent analysis of BMR in mammals (McNab, 2008). This pattern has ecological consequences for birds. Analyses of avian BMR began with Brody and Procter (1932), who described the combined BMR of birds and mammals as a power function of body mass, an approach partially followed by Benedict (1938), even though he argued that the use of logarithms diminished species differences. King and Farner (1961) subsequently demon- strated that birds have higher basal rates than mammals of the same mass, especially at masses less than 100 g. Lasiewski and Dawson (1967) found that passerines had basal rates, corrected for body mass, that were 65% greater than those of other birds, which in turn had basal rates that were approximately 11% greater than mammals. Then, Aschoff and Pohl (1970a,b) noted that thermoneutral rates of metabolism in birds and mammals were minimal during the period of inactivity, a condition that since has been incorporated into the definition of basal rate of metabolism. They also showed that passerines during the rest period have basal rates that averaged 65% greater than other birds. The Aschoff–Pohl relationship has been widely used as the standard scaling relationship for birds, but it was derived from only 17 species, 3 of which were domesticated and many repeatedly measured, for a total of 31 measurements. Kendeigh et al. (1977), using data from 172 species, demonstrated that basal rate in birds is 15–25% greater in winter than summer and that passerines have basal rates that are 57–70% greater than non-passerines. Thus, the consensus until the mid-1990s was that passerines have appreciably higher basal rates than other birds of the same mass and that birds collectively have higher basal rates than mammals. The conclusion that birds have higher basal rates than mammals was challenged by Gillooly et al. (2001) and White and Seymour (2004), who argued that birds and mammals have the same basal rates after their rates were ‘corrected’ through the use of a Q 10 function for their difference in body temperature (T b ), birds generally having higher body temperatures than mammals. Even if this argument were correct, it does not account for the higher body Comparative Biochemistry and Physiology, Part A 152 (2009) 22–45 ⁎ Tel.: +1352 392 1178. E-mail address: bkm@zoo.ufl.edu. 1095-6433/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2008.08.021 Contents lists available at ScienceDirect Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa