GLOBAL CHANGE AND CONSERVATION ECOLOGY Vanessa M. Torti Æ Peter O. Dunn Variable effects of climate change on six species of North American birds Received: 19 March 2004 / Accepted: 26 May 2005 / Published online: 12 August 2005 Ó Springer-Verlag 2005 Abstract Many recent studies have shown that birds are advancing their laying date in response to long-term increases in spring temperatures. These studies have been conducted primarily in Europe and at local scales. If climate change is a large-scale phenomenon, then we should see responses at larger scales and in other re- gions. We examined the effects of long-term temperature change on the laying dates and clutch sizes of six eco- logically diverse species of North American birds using 50 years of nest record data. As predicted, laying dates for most (four of six) species were earlier when spring temperatures were warmer. Over the long-term, laying dates advanced over time for two species (red-winged blackbirds, Agelaius phoeniceus and eastern bluebirds, Sialia sialis). Laying date of song sparrows (Melospiza melodia) also advanced with increasing temperature when the analysis was restricted to eastern populations. Neither laying date nor clutch sizes changed significantly over time in the remaining species (American coot, Fu- lica americana, killdeer, Charadrius vociferous, and American robin, Turdus migratorius), an unsurprising result given the lack of increase in temperatures over time at nest locations of these species. This study indi- cates that the relationship between climate change and breeding in birds is variable within and among species. In large-scale analyses of North American birds, four of seven species have shown advances in laying dates with increasing temperature (including song sparrows in the east). These variable responses within and among species highlight the need for more detailed studies across large spatial scales. Keywords Climate change Æ Clutch size Æ Laying date Æ Temperature Æ Timing of breeding Introduction Air temperatures at ground surface have increased over the past 100 years by a global average of 0.6°C and are expected to continue to increase at a faster rate over the next 100 years (IPCC 2001). Many species of plants and animals have responded to the increase in air tempera- tures by advancing the time of spring events, such as flowering and egg-laying (Walther et al . 2002; Parmesan and Yohe 2003; Root et al. 2003). The evidence for these phenological responses comes mainly from studies that have been conducted at small spatial scales, where temperature changes may actually be much less (or greater) than the global average. Such regional hetero- geneity in temperature change may explain some of the variation among studies. Indeed, a review of 109 species found that phenological shifts were stronger north of 50°, where temperatures have warmed more than at lower latitudes over the past 50 years (Root et al. 2003). Furthermore, most evidence of phenological shifts in animals has come from local studies in western Europe, where there have been some of the largest increases in temperature (Walther et al. 2002; Parmesan and Yohe 2003). If climate change is a widespread phenomenon, then we should expect to see its influence on phenology at larger scales and in other regions with warming temperatures. To date, however, there have been rela- tively few large-scale studies of the effects of climate change on breeding phenology (e.g., Dunn and Winkler 1999; Schwartz and Reiter 2000; Visser et al. 2003). Birds provide an excellent opportunity to study large- scale effects of climate change on breeding phenology because volunteers have contributed thousands of nest- ing records to collections in Europe and North America. In an analysis of over 92,000 nesting records from 36 species in the UK, Crick and Sparks (1999) found that Communicated by Scott Robinson V. M. Torti Æ P. O. Dunn (&) Department of Biological Sciences, University of Wisconsin—Milwaukee, P.O. Box 413, Milwaukee, WI, 53201 USA Oecologia (2005) 145: 486–495 DOI 10.1007/s00442-005-0175-4