Migration tactics and connectivity of a Nearctic–Neotropical migratory shorebird

Abstract

During long‐distance spring migrations, birds may rest and refuel at numerous stopover sites while minimizing the time to reach the breeding grounds. If habitat is limited along the migration route, pre‐breeding birds optimize flight range by having longer stopovers at higher quality sites compared to poorer quality sites. Stopover duration also depends on distance remaining to breeding grounds, ecological barriers and individual characteristics. We assessed spring migration tactics and connectivity of a Nearctic–Neotropical migratory shorebird, the semipalmated sandpiper Calidris pusilla, at two sites with known relative habitat quality on the northern Gulf of Mexico (NGOM) coast, the first land encountered after crossing the Gulf of Mexico (GOM). We used automated radio telemetry (Motus) to estimate stopover duration and probability of departure. Migration speed was estimated for individuals detected at subsequent receivers on the Motus network. To measure migratory connectivity, we used morphometrics and the Motus network to assign general breeding regions. Additionally, feather stable isotope ratios of C and N provided coarse information about overwintering regions. Stopover duration declined with higher fuel loads at capture as expected under a time‐minimizing strategy. After accounting for fuel load, stopover duration was approximately 40% longer at the higher quality site. We found no detectable effect of age, sex or breeding location on stopover behaviour. The probability of departure was strongly affected by humidity and also by tailwind and weather conditions. Birds stopping at the higher quality site had earlier apparent arrival to the breeding grounds. The Louisiana coast is an apparent stopover hub for this species, since the individuals were departing to range‐wide breeding regions and isotope values suggested birds were also using widespread wintering regions. Our study shows how high‐quality, coastal wetlands along the NGOM coast serve a critical role in the annual cycle of a migratory shorebird. Stopover behaviour indicated that high‐quality habitat may be limited for this species during spring migration. As threats to the GOM coast increase, protection of these already limited wetlands is vitally important.

Length of stay and departure strategies of Semipalmated Sandpipers (Calidris pusilla) during post-breeding migration in the upper Bay of Fundy, Canada

“…We found strong evidence that departure timing is correlated with suitable weather conditions, consistent with results from studies of this species in different parts of their range (Anderson et al, 2019;Herbert et al, 2022). Given the high cost of a non-stop flight of up to 60 h, favourable weather conditions may be crucial for successful migration (Lank, 1983;Stoddard et al, 1983;Butler et al, 1997).…”

“…Prey availability and predation pressure may also explain differences in length of stay between early and late migrating 10.3389/fevo.2022.897197 sandpipers. Semipalmated Sandpipers have been found to stay longer at high quality sites (Herbert et al, 2022). Density of C. volutator, a historically important prey item for sandpipers in the Bay of Fundy, declines later in the season due to natural mortality and predation by shorebirds (Schneider and Harrington, 1981;Hamilton et al, 2006;Barbeau et al, 2009), although this seasonal pattern seems less strong in other prey items (Gerwing et al, 2015).…”

“…Linscott and Senner (2021) discussed non-fuelling behaviours, such as sleeping, recovery and social interactions, regularly observed at staging sites which could decouple length of stay from rate of weight gain. Further, the relationship between fuel load on capture and length of stay can vary with time in the season (Anderson et al, 2019) and staging site quality (Herbert et al, 2022). Finally, in addition to extensive within-season variation, length of stay may also vary among years or increase or decrease over a period of many years as a result of changes in habitat quality, prey availability, predator pressure or climate conditions (e.g., Ydenberg et al, 2004;Gordo, 2007).…”

“…There are different methods for estimating length of stay. Minimum length of stay is commonly used in tracking studies (Kennedy et al, 2017;Anderson et al, 2019;Herbert et al, 2022), though alternative methods like mark-recapture modeling, which can account for time at a site prior to capture, could provide more accurate estimates (Pradel et al, 2005;Pledger et al, 2009). To date there has been limited work directly comparing these methods in shorebirds, though what has been done suggests substantial discrepancies (Howell et al, 2019).…”

Movement and habitat use of non‐breeding Semipalmated Sandpiper (Calidris pusilla) at the Banco dos Cajuais in Northeast Brazil

“…Although there is some understanding of migration patterns for the different breeding populations within North America (Gratto‐Trevor et al, 2012; Herbert et al, 2022), little is known about stopover locations in South America, with only a few complete pathways recorded (Brown et al, 2017). Semipalmated sandpipers overwintering at the Banco dos Cajuais are likely fueling for migration at other important locations in South America.…”