Reproductive Ecology of Long-billed Curlews Breeding in Grazed Landscapes of Western South Dakota

Long-billed curlews (Numenius americanus) are currently undergoing significant population declines mainly attributed to the destruction of important breeding habitat for agriculture and development. Uncultivated native rangelands and pastures support a majority of long-billed curlew breeding populations and reproductive success in these areas plays a major role in population dynamics. The western portion of South Dakota provides important mixed-grass prairie breeding habitats for long-billed curlews and most of these habitats are used as pastureland for grazing domestic livestock. The main objectives of this study were to estimate nesting success and brood survival and to characterize nesting and brood-rearing habitats of a population of long-billed curlews breeding in western South Dakota. This study was conducted during the spring and summer of 2005 and 2006 on the Triple U Buffalo Ranch in Stanley County, South Dakota. Nest sites were located by dragging a rope through pastures to flush incubating adults. Broods were monitored by tracking radio-marked adults that successfully hatched young. Habitat preferences were characterized by comparing habitat at nest sites and brood location points to habitat at points randomly distributed throughout the study site. A total of 48 nests were located and 43 adult curlews (21 males, 22 females) were radio marked over the 2 years of this study. Habitat measurements were taken at 48 nest sites, 80 brood location points, and 154 random points. In 2005, curlews used nest sites similar to random points with an average of 55% grass cover, 47% forb cover and an average visual obstruction reading (VOR) of 27 cm. In the fall of 2005, a natural range fire burned a large area of the study site. The following year, forb cover and VOR’s at nest sites, brood points, and random points decreased significantly (p\u3c0.05). This reduction in vegetative cover also corresponded to a severe drought and a non-peak year for the biennial forb yellow sweetclover (Melilotus officinalis). Nest success decreased from 0.39 in 2005 to 0.15 in 2006 due to a large increase in the rate of nest predation which accounted for 64% of nest failure in 2006. Daily nest survival rates for 2005 and 2006 combined were positively related to average VOR’s taken at nest sites (β=6.45) and negatively related to the density of bison grazed in pastures containing nests (β=-1.29). Despite the negative impact of reduced vegetative cover on reproductive success, curlews selected nest sites in shorter vegetation with lower VOR’s than random points in 2006. Curlews selected nest sites composed of a greater proportion of junegrass (Koeleria macrantha) and buffalograss (Buchloe dactyloides) than random points in 2005 and 2006 and daily survival rate at nest sites dominated by these species was 100% during both years. In 2006, nest sites also had a greater amount of forb cover than random points and daily survival rate for 2005 and 2006 combined was higher at nest sites dominated by forb cover (98%) than at nest sites dominated by grass cover (91%). Of the 15 broods produced by radio-marked curlews in 2005 and 2006, only 33% were known to produce fledglings. Possible causes of chick mortality included avian predation and heat prostration. Broods used habitats composed of a greater proportion of sixweeks fescue (Vulpia octoflora), indianwheat (Plantago patagonica), junegrass, and American vetch (Vicia americana) than random points in 2005 and a greater proportion of creeping spikerush (Eleocharis palustris) and water than random points in 2006. In 2006, broods also used areas with less grass cover and more bareground than random points. Nest success and brood survival was low for the population of long-billed curlews breeding on the Triple U Buffalo Ranch during both years of this study. Uncontrollable climatic factors that affect the vegetative structure of habitat appear to have a large impact on the reproductive success of long-billed curlews breeding in South Dakota. Controllable factors such as grazing pressure should be manipulated to help maximize nesting success and brood survival. This may require reducing livestock densities in pastures used by breeding curlews during the nesting period in order to reduce the risk of nest trampling