Migration tactics and connectivity of a Nearctic-Neotropical migratory shorebird

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.