| Summary: | Shifts in the duck community composition in the Canadian Prairie Pothole Region (PPR) have been evident over the past 25 years for reasons that are not entirely clear. Several duck species have increased in abundance whereas others have remained stable, or have declined, despite shifts in agricultural land use from annually tilled cropland to more pasture and haylands, and increases in spring pond counts since 2008. I considered three main hypotheses that could account for species-specific changes in duck abundances within the Canadian PPR; (i) redistribution; (ii) climate and (iii) land use. First, I examined whether shifts in populations of eleven duck species (seven dabbling ducks, four diving ducks) could be due to redistribution from other regions either to or from the Canadian PPR. I found no support for this hypothesis for any species. Trends and timing of changes in duck abundances were generally similar and coincident in US prairie, Canadian PPR and southern boreal forest biomes in all species. To test for effects of variations in climate, I used >20,000 nesting records for eight upland nesting duck species and related the timing of nesting and subsequent nest success to annual variations in spring temperature and moisture conditions. Furthermore, I tested for the relative importance of antecedent winter climate on nesting activities using winter El Niño Southern Oscillation indices. In general, nesting occurred earlier in warmer springs and following warmer, wetter winters. However, the magnitude of responses varied, indicating species-specific responses to variations in climate. Early nesting was the most influential factor for increasing nest survival and because timing advanced following wetter winters, this may provide a cross-seasonal mechanism for how winter climate can subsequently impact offspring recruitment in species like mallard (Anas platyrhynchos). To evaluate the land use change hypothesis, I related changes in duck abundances to changes in ponds and upland habitat composition using unique monitoring data collected in the PPR in 1985 and 2011. I used data from 42 transects distributed across the PPR to first characterize changes in upland habitat and then relate these changes to abundances of four common dabbling duck species. Pond abundances increased between ~1985 and ~2011, and cropland was converted to tame grass (i.e., pastureland). Changes in abundances of mallard and northern shoveler (A. clypeata) were primarily driven by changes in pond abundances. In blue-winged teal (A. discors), abundances of breeding birds increased more rapidly than other species and there was some evidence that conversion of cropland to tame grass resulted in greater teal abundances. Change in abundance of northern pintail (A. acuta) was not related to changes in ponds or upland habitat conditions. Overall, I found no evidence for the redistribution hypothesis; however, I did find evidence for species-specific responses to both variations in climate and changes in land use. This implies that species respond differently to various drivers and may be a partial explanation for observed differences in population trajectories. Although further research should seek to explain increases in certain species over others, my work provides insights into potential species-specific population drivers. This information is critical in informing managers about possible areas where conservation actions could be implemented to sustain waterfowl populations.
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