Breeding Ecology of Noethern Pintails in Prairie Landscapes: Tests of Habitat Selection and Reproductive Trade-Off Models

Ecologists and conservation biologists are interested in explaining why animal abundance and reproductive success vary among habitats. Initial motivation for this research arose from concerns for Northern Pintail (Anas acuta) populations in North America. Unlike many prairie-nesting dabbling duck populations, pintails failed to increase during periods of excellent wetland conditions, and remained below conservation goals. Low pintail populations have been linked to degraded landscape conditions on the Canadian prairies. Current habitat management for pintails aims to protect and create larger areas of perennial cover either by encouraging better management of grazing lands, by converting cropland to grassland, or by promoting adoption of fall-seeded crops like winter wheat. The central premise is that larger areas of natural grassland cover will attract breeding pintails to nest earlier in the season in low-predation-risk habitat. I studied pintail nesting ecology near the Milk River Ridge, Alberta, 2004-2006, in terms of a life-cycle perspective, from spring arrival and settling on breeding areas, to assess age and quality of nesting females, to determine timing and investment in reproduction, and finally to measure nesting success. A gradient in presumed high (grassland) to low (agriculture) habitat quality provided a landscape template for testing habitat selection models. Pintail breeding pair densities were 1.5-3 times higher in grassland than agricultural landscapes in all three years, regardless of regional population size, with pairs occupying grassland landscapes at higher densities immediately upon arrival in early spring. Northern Shoveler (A. clypeata), gadwall (A. strepera) and blue-wing teal (A. discors) had similar settlement patterns as pintails, but mallard (A. platyrhynchos) pair density was higher in agricultural areas. Relatively more, older female pintails were captured at nests in grassland landscapes whereas yearling females were encountered more often in agricultural areas, a pattern that was not detected in female shovelers. This response suggests that older female pintails may be better able to recognize and settle in higher quality grassland habitats. Body mass of pintail females did not vary among years, decreased seasonally, and was positively related to body size index and incubation stage. Furthermore, pintail body mass did not differ between grassland (650 ± 24 g), ecotone (678 ± 27 g) and agriculture (672 ± 33 g). In female shovelers, body mass varied among years (555 ± 29 g in 2004, 481 ± 18 g in 2005, 508 ± 21 g in 2006), and increased with nesting date. Shoveler body mass did not differ between grassland (519 ± 32 g), ecotone (519 ± 44 g), or agriculture (507 ± 35 g). Nest initiation dates did not vary by landscape for pintail, shoveler or mallard, but all species nested earlier in 2006 versus 2004. In pintail, shoveler and mallard, clutch size was negatively related to nest initiation date. Pintail and shoveler clutch sizes were generally larger in a wet year with abundant wetlands (2006) when compared with a dry year (2004), but no landscape differences were detected. Mallard clutch size did not vary by year or landscape. Female reproductive timing and investment (in terms of clutch size) were unrelated to upland habitat characteristics, counter to a hypothesis that predicts larger pintail clutch sizes in agricultural landscapes. However, pintail and shoveler invested in larger clutches in 2006, a wet year with abundant wetlands, possibly due to greater abundance of aquatic foods. Finally, nest survival rates of duck species, except mallard, tended to be higher in grassland landscapes and lower in agricultural landscapes. Pintail nest survival was consistently higher in grassland than in agricultural landscapes and was highest in 2006 when wetland conditions were excellent. Shoveler and blue-winged teal nest survival rates did not vary strongly with landscape, but were also higher in 2006, whereas mallard and gadwall nest survival estimates did not vary with landscape or year. Overall, pintails settled at higher densities in grassland landscapes where breeding success was higher (indexed by nesting success). This suggests that pintails respond appropriately to cues that enable them to recognize suitable habitat, at least in regions where large contiguous areas of grassland habitat remain. Furthermore, assuming that findings for pintails reflect those of other grassland bird species, large remnant areas of intact natural grassland seem particularly in need of protection or restoration, and management regimes that maintain their habitat integrity. By integrating applied and theoretical aspects of pintail reproductive ecology, I attempted to provide deeper insights into the processes that could shape behavioral decisions by breeding pintails and other duck species. Older pintails may occupy wetlands in higher quality grassland habitat early in spring, forcing subordinate or later-arriving individuals into poorer quality habitat (i.e., where nesting success is lower); however, mechanisms involved in this putative process are unknown. Overall, results suggest that grassland restoration or enhancement (e.g., managing grazing intensity) could improve reproductive success of pintails and possibly other grassland bird species