Reduced body mass gain in small passerines during migratory stopover under simulated heat wave conditions Ulf Bauchinger a,b, ⁎, Scott R. McWilliams b , Berry Pinshow a a Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, 84990 Midreshet Ben-Gurion, Israel b Dept. Natural Resources Science, 105 Coastal Institute in Kingston, University of Rhode Island, Kingston, RI 02881, USA abstract article info Article history: Received 1 October 2010 Received in revised form 10 November 2010 Accepted 10 November 2010 Available online 21 December 2010 Keywords: Ambient temperature Body temperature Climate Fuel deposition Global change Heterothermy Mass gain Stopover performance For birds that migrate long distances, maximizing the rate of refueling at stopovers is advantageous, but ambient conditions may adversely influence this vital process. We simulated a 3-day migratory stopover for garden warblers (Sylvia borin) and compared body temperatures (T b ) and rates of refueling under conditions of a heat wave (T a = 40 °C by day, and 15 °C at night) with those under more moderate conditions (T a = 27 °C by day, and 15 °C at night). We measured T b with implanted thermo-sensitive radio transmitters. Birds had significantly lower rates of body mass gain on the first day of stopover (repeated measures mixed model ANOVA, p = 0.002) affecting body mass during the entire stopover (p = 0.034) and higher maximum T b during the day when exposed to high T a than when exposed to moderate T a (p = 0.002). In addition, the birds exposed to high T a by day had significantly lower minimum T b at night than those exposed to moderate daytime T a (p = 0.048), even though T a at night was the same for both groups. We interpret this lower nighttime T b to be a means of saving energy to compensate for elevated daytime thermoregulatory requirements, while higher T b by day may reduce protein turnover. All effects on T b were significantly more pronounced during the first day of stopover than on days two and three, which may be linked to the rate of renewal of digestive function during stopover. Our results suggest that environmental factors, such as high T a , constrain migratory body mass gain. Extreme high T a and heat waves are predicted to increase due to global climate change, and thus are likely to pose increasing constraints on regaining body mass during stopover and therefore migratory performance in migratory birds. © 2010 Elsevier Inc. All rights reserved. 1. Introduction In order to exploit favorable habitats during different seasons of the year, hundreds of bird species migrate thousands of kilometers, often flying over broad ecological barriers. Depending on the distance, birds are on the move from weeks to months, typically alternating between flight and sojourn at stopover sites where they spend time refueling. Indeed, migrants spend about 85% of their migration time at stopovers compared to 15% in active flight, and twice as much energy at stopovers than in flight (Hedenstöm and Alerstam, 1997, Wikelski et al., 2003). During spring migration, when early arrival at the breeding area is beneficial in terms of annual reproductive success (Newton, 2006; Newton, 2008), efficient rebuilding of body tissues is likely to be under selective pressure (Alerstam and Lindström, 1990; Lindström and Alerstam, 1992; Hedenström and Alerstam, 1997). Therefore maximizing the rate of refueling at a stopover is adaptive because it facilitates early resumption of migration. Adaptive behavioral and physiological responses such as hyper- phagia, diet selection (Bairlein 2002; McWilliams et al., 2004) and/or heterothermy (increased body temperature (T b ) at high ambient temperature (T a )(Tieleman and Williams, 1999) and decreased T b at low T a (Prinzinger et al., 1991; McKechnie and Lovegrove, 2002)) make it possible for birds to maximize their rate of refueling. Whereas heterothermic responses depend on the environment, food selection and rate of intake may depend on the state of the digestive tract (Karasov and Pinshow, 2000; McWilliams et al., 2004; Karasov and McWilliams 2005; McWilliams and Karasov, 2005). Passerine birds within the Afro-Palearctic migration system face the challenge of flying across a major ecological barrier, the Sahara desert. This effort has been reported to result in the catabolism of half of the digestive tract and liver mass (Hume and Biebach, 1996; Biebach, 1998; Karasov Comparative Biochemistry and Physiology, Part A 158 (2011) 374–381 Abbreviations: m b , body mass; m b change , body mass change over 24 h; T a , ambient temperature; T b , body temperature; T b day , daytime body temperature; T b night , nighttime body temperature; T b range day/night , range of body temperature between daytime and nighttime; T b min , ten minute mean around the T b minima of the night; T b max , ten minute mean around the T b maxima of the day; T b range peaks , range of T b between T b min and T b max ; T a 40/15 °C , ambient temperature regime with peak value of 40 °C during day and constant value of 15 °C during night; T a 27/15 °C , ambient temperature regime with peak value of 27 °C during day and constant value of 15 °C during night. ⁎ Corresponding author. Dept. Natural Resources Science, 105 Coastal Institute in Kingston, University of Rhode Island, Kingston, RI 02881, USA. Tel.: + 1 4018747531; fax: + 1 4018744561. E-mail address: ulf@etal.uri.edu (U. Bauchinger). 1095-6433/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2010.11.030 Contents lists available at ScienceDirect Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa