Combined effects of heat waves and droughts on avian communities across the conterminous United States 1,5, 1 1 2 THOMAS P. A LBRIGHT, ANNA M. PIDGEON, CHADWICK D. RITTENHOUSE, MURRAY K. CLAYTON, BRIAN D. 3 4 1 1 WARDLOW, CURTIS H. FLATHER, PATRICK D. CULBERT, AND VOLKER C. RADELOFF 1Department of Forest and Wildlife Ecology, University of Wisconsin, Madison, Wisconsin 53706 USA 2Department of Statistics, University of Wisconsin, Madison, Wisconsin 53706 USA 3National Drought Mitigation Center, University of Nebraska, Lincoln, Nebraska 68583 USA 4Rocky Mountain Research Station, United States Department of Agriculture, Forest Service, Fort Collins, Colorado 80526 USA Abstract. Increasing surface temperatures and climatic variability associated with global climate change are expected to produce more frequent and intense heat waves and droughts in many parts of the world. Our goal was to elucidate the fundamental, but poorly understood, effects of these extreme weather events on avian communities across the conterminous United States. Specifically, we explored: (1) the effects of timing and duration of heat and drought events, (2) the effects of jointly occurring drought and heat waves relative to these events occurring in isolation, and (3) how effects vary among functional groups related to nest location and migratory habit, and among ecoregions with differing precipitation and temperature regimes. Using data from remote sensing, meteorological stations, and the North American Breeding Bird Survey, we used mixed effects models to quantify responses of overall and functional group abundance to heat waves and droughts (occurring alone or in concert) at two key periods in the annual cycle of birds: breeding and post-fledging. We also compared responses among species with different migratory and nesting characteristics, and among 17 ecoregions of the conterminous United States. We found large changes in avian abundances related to 100-year extreme weather events occurring in both breeding and post-fledging periods, but little support for an interaction among time periods. We also found that jointly-, rather than individually-occurring heat waves and droughts were both more common and more predictive of abundance changes. Declining abundance was the only significant response to post-fledging events, while responses to breeding period events were larger but could be positive or negative. Negative responses were especially frequent in the western U.S., and among ground-nesting birds and Neotropical migrants, with the largest single-season declines (36%) occurring among ground-nesting birds in the desert Southwest. These results indicate the importance of functional traits, timing, and geography in determining avian responses to weather extremes. Because dispersal to other regions appears to be an important avian response, it may be essential to maintain habitat refugia in a more climatically variable future. Key words: birds (Aves); climate change; drought; extreme weather; heat wave; land surface temperature; mixed effects models; MODIS; North American Breeding Bird Survey; standardized precipitation index; United States. Received 26 August 2010; revised 15 October 2010; accepted 26 October 2010; published 23 November 2010. Corresponding Editor: B. Wolf. Citation: Albright, T. P., A. M. Pidgeon, C. D. Rittenhouse, M. K. Clayton, B. D. Wardlow, C. H. Flather, P. D. Culbert, and V. C. Radeloff. 2010. Combined effects of heat waves and droughts on avian communities across the conterminous United States. Ecosphere 1(5):artX. doi:10.1890/ES10-00057.1 Copyright: Ó 2010 Albright et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits restricted use, distribution, and reproduction in any medium, provided the original author and sources are credited. 5 Present address: Laboratory for Conservation Biogeography, Department of Geography, University of Nevada-Reno, 104A Mackay Science Hall MS0154, Reno, Nevada 89557-0154 USA. E-mail: [email protected] v www.esajournals.org 1 November 2010 v Volume 1(5) v Article 12 ALBRIGHT ET AL. INTRODUCTION depend on whether drought occurs before, during, or after the main period of vegetation Episodes of extreme weather can alter biotic growth (Heitschmidt et al. 1999). The timing of communities by affecting survival, reproduction, events may also be important to birds, because habitat selection, and resources on which organ- they have different requirements at different isms depend. Under a changing climate, extreme times in their annual cycles. Caloric needs may weather events such as heat waves and droughts be higher while caring for nestlings (Williams are likely to become more frequent and intense in 1988) and prior to and during migration (Jenni- many locations (IPCC 2007). In much of temper- Eiermann and Jenni 1996), which may affect the ate North America, changes in the interannual sensitivity of birds to extreme weather events. variability in temperature and precipitation are The periods of the year during which bird species predicted to be the most drastic aspects of are most sensitive to heat waves may also climate change in the 21st century (Diffenbaugh depend on the functional characteristics of the et al. 2008). Extreme events, which exceed species (Albright et al., in press). ecological or physiological tolerances of some Heat waves often, but not always, accompany species, may have greater influence on popula- droughts (de Boeck et al. 2010). In most non- tion persistence than changes in mean conditions polar terrestrial regions, summer temperature (Jentsch et al. 2007). As societies and ecosystems anomalies are negatively correlated with precip- are confronted with a changing climate, it is itation (Trenberth and Shea 2005). A key contrib- critical to understand how events such as heat utor to this relationship is that low soil moisture waves and drought affect biodiversity (Archaux associated with drought results in an enhanced and Wolters 2006). However, such an under- ratio of sensible-to-latent heat (the Bowen ratio) standing is often limited to individual species, leading to greater surface and air temperatures. sites, and disturbance events, while a broader In Europe, for instance, the extreme heat wave of perspective that considers communities, diverse 2003 was associated with both sustained elevated regions, and interactions among events is lack- temperatures and below-normal precipitation ing. (Fischer et al. 2007). However, heat waves may Drought has been associated with lower also be accompanied by normal or even above- habitat quality (Mueller et al. 2005), higher normal precipitation (Gershunov et al. 2009). mortality (Mooij et al. 2002), and reduced Similarly, summer droughts may occur during reproductive effort (Christman 2002), and can normal or abnormally cool periods (Trenberth decrease abundance and species richness of avian and Shea 2005). For most organisms, the co- communities (Albright et al. 2010). Heat waves occurrence of drought and heat waves may be can also stress avian communities by increasing especially challenging as water requirements are water requirements (Guthery et al. 2005), elicit- greatly increased when temperatures are elevat- ing altered behavior on birds (Guthery et al. ed. Thus, knowledge of the effects of temperature 2001), and reducing reproduction and survival and precipitation extremes, occurring both sep- (Becker et al. 1997, Christman 2002), resulting in arately and in concert, is important for under- altered community structure and lower species standing biotic responses to contemporary and richness (Albright et al., in press). There is some future environmental variability. evidence that the effects of both heat waves and Here, our goal was to elucidate the fundamen- drought vary among birds according to their tal, but poorly understood, effects of heat waves migratory strategy (Albright et al. 2010), body and droughts on avian communities across the size (McKechnie and Wolf 2010) and other conterminous United States at key time periods functional traits (Jiguet et al. 2006), and among of the annual cycle of birds (bioperiods): early regions with differing climate regimes (Albright breeding and post fledging. First, we explored et al., in press). the temporal dimensions of these extreme weath- The timing of disturbances influences their er events by asking whether avian assemblages effects on vegetation and ecological communities are more responsive during a particular biope- (Pickett and White 1985). For example, the riod and whether the effects of extreme events impacts of drought on primary productivity can occurring in consecutive bioperiods are greater v www.esajournals.org 2 November 2010 v Volume 1(5) v Article 12 ALBRIGHT ET AL. than the sum of their effects individually. We peak breeding season (most often during June) in hypothesized that especially deleterious effects which all birds seen or heard within 400 m are would follow a sustained period of high temper- recorded. We removed route-years collected by atures and moisture deficits. While drought and first-year observers and those having inclement heat waves are often coincident, we also asked weather at the time of the survey (Link and Sauer whether the effects on avian assemblages vary 1997, Sauer et al. 2004). For each suitable route- according to whether these extreme weather year, we summed counts of individual birds for events occur in concert or alone (e.g., a drought (1) North American landbirds