Global drivers of biodiversity change can operate at different spatial scales, and these drivers may not be operating independently. Cross-scale interaction (CSI) effects on a response variable involve multiplicative influences of variables at different spatial scales. We investigated CSI effects of sub-regional climate (breeding season temperature and precipitation, spring precipitation, and winter temperature) and landscape-scale deciduous forest patch size on eight forest bird species. Climate can influence food availability and avian breeding success, and forest patch size affects the amount of habitat available for forest-interior and forest-edge birds. Yet, ecologists do not know whether CSIs between these variables influence forest bird abundance. Identification of CSIs is the first step in understanding CSI mechanisms and developing appropriate conservation strategies. We used 5-year (2009-2013) counts of total abundance for birds along North American Breeding Bird Survey (BBS) routes within six EPA Level II Ecoregions in the eastern United States. Species’ maximum and median natal dispersal distances were used, respectively, to create sub-regional- and landscape-scale buffers around the BBS routes, within which environmental variables were measured. For each species, the fits of twenty-one a priori negative-binomial regression models were compared using AICc statistics.
Results/Conclusions
Five species (American Redstart [Setophaga ruticilla], Kentucky Warbler [Geothlypis formosa], Scarlet Tanager [Piranga olivacea], Eastern Kingbird [Tyrannus tyrannus], and Song Sparrow [Melospiza melodia]) were influenced by CSIs, but three species (Black-and-white Warbler [Mniotilta varia], Field Sparrow [Spizella pusilla], and Indigo Bunting [Passerina cyanea]) were not. CSIs were less important than were many of the climate and land-cover variables in the models. As one example of a CSI, Song Sparrow abundance increased with increasing precipitation in landscapes with smaller patch sizes but decreased with increasing precipitation in landscapes with larger patch sizes. Patch size was negatively correlated with edge density. In landscapes with smaller patches, the Song Sparrow (forest-edge species) may benefit from more habitat, and rainfall-induced increases in available food may outweigh the negative effect of higher rainfall on nest success, whereas in high-rainfall landscapes with less habitat (larger patch size), reduced nest success from increased rainfall may outweigh its positive effect on food. Our results imply that CSIs involving climate and forest patch size may be influencing the abundance of various forest birds. Identifying and understanding these CSIs may improve the effectiveness of management efforts for declining bird species.