Building the Full Annual Cycle Picture for Long-Billed Curlews: Correlates of Nest Success in the Breeding Grounds and Spatial Distribution and Site Fidelity in the Wintering Grounds

Migratory birds face threats throughout the annual cycle, and cumulative effects from linkages between the breeding and non-breeding grounds may impact species at the population level. Long-billed Curlews (Numenius americanus) are a migratory shorebird of conservation concern associated with grasslands that show breeding population declines at some regional and local scales. Curlews exhibit high site fidelity to breeding territories, but also spend approximately 75% of the year on the wintering grounds. Therefore, localized population declines could indicate localized threats, in the breeding or wintering grounds. However, little information is available regarding the spatial distribution of curlews on the wintering grounds, especially for Mexico. Furthermore, breeding ground studies which examine habitat selection and nest success in the context of predator and anthropogenic pressures are lacking. We add critical information that could help pinpoint conservation issues, including understanding limitations to nesting success and mapping spatial distribution and habitat use patterns during the non-breeding season. On the breeding grounds, we used a conditional logistic regression model to compare used nest-sites to available random sites and examine habitat selection within territories. We also studied correlates of nesting success with a generalized linear model for 128 curlew nests at five sites in the Intermountain West. During the non-breeding season, we attached satellite transmitters to track 21 curlews that bred in the Intermountain West and wintered in California and Mexico and quantified 95% home range and 50% core use size via utilization distributions created with dynamic Brownian Bridge Movement Models. For 14 individuals, we tracked multiple winter seasons and compared inter-annual site fidelity among winter areas, sexes, and habitat type with a Utilization Distribution Overlap Index. We documented four main wintering areas: (1) Central Valley of California, (2) the adjoining Imperial and Mexicali Valleys of California and Mexico, (3) the Chihuahuan Desert of inland Mexico, and (4) coastal areas of western Mexico and the Baja Peninsula. Curlews wintering in coastal areas had significantly smaller home ranges and fewer core use areas than inland-wintering curlews. Home ranges in the Central Valley were larger than other inland areas, and Central Valley females had larger home ranges than Central Valley males. Inter-annual site fidelity for wintering curlews was high, regardless of habitat type or sex. On the breeding grounds, curlews selected habitats for nest-sites with lower vegetation height and lower percent cover of grasses, bare ground, and shrubs than available sites. Nest-sites were six times more likely to have a cowpie within 50 cm than random sites. Higher probability of nest success was associated with higher curlew density in the nesting area, increasing percent cover of conspicuous objects such as cowpies within approximately two meters of the nest, and – surprisingly – higher densities of American Crows and Black-billed Magpies in the breeding area. In a separate analysis with a subset of nests (n = 100), we found nests had higher probability of success when they were farther from roads and perches. Given the central role of working lands to breeding curlews in much of the Intermountain West, an understanding of limitations to nesting success in these diverse landscapes is necessary to guide adaptive management strategies in increasingly human-modified habitats. Similarly, foundational understanding of winter spatial ecology is essential for understanding population declines which may be related to linkages between breeding and non-breeding seasons. Overall, these findings provide valuable information for full annual cycle conservation and will be particularly constructive for conservation planning once range-wide migratory connectivity is mapped