Finer-scale habitat predicts nest survival in grassland birds more than management and landscape: a multi-scale perspective.
Abstract
Birds may respond to habitat at multiple scales, ranging from microhabitat structure to landscape composition. North American grassland bird distributions predominantly reside on private lands, and populations have been consistently declining. Many of these lands are enrolled in U.S. federal conservation programmes, and properly guided management policies could alleviate declines. However, more evaluative research is needed on the effects of management policies juxtaposed with other multi-scale habitat features. Furthermore, research focused on nest survival is arguably more valuable because habitat associations with avian densities can sometimes be deceptive. We investigated nest survival of a grassland facultative (red-winged blackbird Agelaius phoeniceus) and obligate species (dickcissel Spiza americana), and two nesting communities (ground and above-ground nesters) relative to management and multi-scale habitat (nest-site characteristics, in-field microhabitat, patch metrics, and landscape context). Our study was conducted on private lands in Illinois (2011-2014) and directly linked to policy-based management (discing, herbicidal spraying, spray/interseeding) and landowner decisions. Multi-scale models explained more variation in nest survival compared to single scales or management in three of four analyses (blackbirds, dickcissels, and above-ground nesters). Finer-scale habitat variables, such as nest-site characteristics, were more often in top and among the competitive models relative to landscape factors. Compared with other management types, discing (i.e., tractor-pulled disc harrows removed approximately 50% of vegetation) displayed distinct effects and positively influenced nest survival in above-ground nesters. Also, greater proportions of a field managed cumulatively and yearly, regardless of type, generally improved nest survival for dickcissels and above-ground nesters. All groups except above-ground nesters had generally higher nest survival in native-grass dominated fields. Synthesis and applications. Habitat practitioners can improve nest survival for certain grassland birds by directly affecting in-field microhabitat vegetation and structure. However, characteristics associated with specific nest locations often drive nest survival. We suggest habitat managers and agency staff promote native grass practices and management, such as discing, to enhance nest survival of grassland bird populations. Management will likely be most effective in favourable unfragmented grassland landscapes with less surrounding forested areas, which also promote other important responses (e.g., colonization and persistence) of target species.