Influence of introduced peregrine falcons on the distribution of red knots within a spring staging site

Trace element contamination in three shorebird species migrating through Delaware Bay, New Jersey: arsenic, mercury and selenium are increasing

Many shorebird populations are declining, and contaminants may be partly contributing to the decline by interfering with feeding, migration, and breeding success. The goal of our study was to determine whether there was a temporal change in concentrations of trace elements of red knot (Calidris canutus rufa), sanderling (Calidris alba), and ruddy turnstone (Arenaria interpres) during spring migration in Delaware Bay, New Jersey, USA. We sampled blood to 1) determine levels of trace elements in 2019, 2) compare 2019 trace element levels with those from shorebirds in 2011-2012, and 3) examine variability in blood levels of trace elements among species. In 2019: 1) trace element levels were significantly different among species (except cadmium[Cd]), 2) Cd was lowest in all species, and arsenic (As) and selenium (Se) were highest, and 3) sanderlings had the highest levels of As and Se, and knots had the highest levels of chromium (Cr) and lead (Pb). Se was higher in these shorebirds than reported for other shorebirds from elsewhere. As, mercury (Hg), and Se increased significantly between 2011-2012 and 2019 in all three species. There were no significant temporal changes in Cd. Chromium (Cr) decreased in knots and sanderling. The temporal increases in As, Se, and Hg bear watching as they are toxic in vertebrates, and each can decrease the toxicity of the others. The data indicate that shorebirds can be bioindicators of changing trace element levels in estuaries, potentially providing early warning of increasing levels of As, Hg, and Se in the environment.

Avian wings can lengthen rather than shorten in response to increased migratory predation danger

Increasing predation danger can select for safety-enhancing modifications to prey morphology. Here, we document the multi-decade wing lengthening of a Pacific flyway migrant, the western sandpiper (Calidris mauri), and contrast this with contemporaneous wing shortening of the closely related semipalmated sandpiper (C. pusilla) on the Atlantic flyway. We measured >12,000 southbound western sandpipers captured from 1978 to 2020 at a major stopover site in British Columbia. Wing length increased at 0.074 mm year-1 (SE = 0.017; p < .0003) for adults, and 0.087 mm year-1 (SE = 0.029; p < .007) for juveniles. These rates are of similarly large magnitude (4%-5% overall), but opposite in direction, to the rate we previously reported for semipalmated sandpiper adults (-0.103 mm year-1). In both species, the change is specific to wings rather than being part of a general body size change. We interpret both trends as responses to the ongoing strong increase of peregrine falcon (Falco peregrinus) populations since the mid-1970s, an important predator encountered by these species in contrasting ways during migration. Western sandpipers and peregrine migrations have temporal and spatial overlap. Longer wings enhance migratory speed and efficiency, enabling western sandpipers to decrease overlap by advancing to safer zones ahead of falcon passage. In contrast, semipalmated sandpipers primarily encounter peregrines as residents at migratory staging sites. Shorter wings improve acceleration and agility, helping migrants to escape attacks. Juvenile western sandpiper wing length also shows a component additive to the lengthening trend, shifting between years at 0.055 mm day-1 with the highly variable snowmelt date, with wings shorter following early springs. On the Pacific flyway, the timing of peregrine southward passage advances with snowmelt, increasing the relative exposure of juveniles to post-migratory resident peregrines. We interpret this annual wing length adjustment as an induced defense, made possible because snowmelt timing is a reliable cue to danger in the upcoming migration.

Timing and duration of stopovers affects propensity to breed, incubation periods, and nest success of different wintering cohorts of red knots in the Canadian Arctic during the Years 2009 to 2016

Recent interest in migratory connectivity of shorebirds leads to examining the role stopovers and connectivity play in reproductive success. Since many shorebird species are declining, there is a need to determine factors affecting reproductive success. We used light records from 104 geolocators recovered from red knots (Calidris canutus rufa) to examine-incubation success as a function of temporal patterns at stopovers, year (2009-2016), and wintering cohort. Geolocators were attached on leg flags, mainly in Delaware Bay (New Jersey) and Texas. Successful incubation in different years ranged from 21 to 76%; from 8 to 43% of knots did not attempt incubation on breeding grounds. This is the first estimate of the number of shorebirds going to the Arctic that did not breed, since all other estimates of success were derived from field studies based on birds that initiated nests. High breeding success (76%) occurred in only one year (2011). Nearly all of knots stopped in Hudson Bay region. Arrival date and time in the Pre-Arctic, arrival on breeding grounds, year, and wintering cohort were associated with successful incubation. The percentage attempting incubation, and the percentage that were successful, varied by wintering cohort. When only the east coast rufa knots are considered, time spent on Delaware Bay showed a significant duration difference. Knots wintering in Texas had the highest propensity to initiate incubation (96%), and the highest incubation success (67%). Of rufa knots using the Atlantic flyway, those wintering in southern South America had the lowest incubation success rate (25%). We discuss the importance of quality of flyways, opportunities for refuelling during migration, and importance of time at stopovers. These data can be used to understand migratory connectivity, distinguish factors affecting reproductive success of long-distance migrants, determine which parts of the annual cycle require protection, and aid in recovery plans for long-distance migrants.

Predation landscapes influence migratory prey ecology and evolution

Migratory prey experience spatially variable predation across their life cycle. They face unique challenges in navigating this predation landscape, which affects their perception of risk, antipredator responses, and resulting mortality. Variable and unfamiliar predator cues during migration can limit accurate perception of risk and migrants often rely on social information and learning to compensate. The energetic demands of migration constrain antipredator responses, often through context-dependent patterns. While migration can increase mortality, migrants employ diverse strategies to balance risks and rewards, including life history and antipredator responses. Humans interact frequently with migratory prey across space and alter both mortality risk and antipredator responses, which can scale up to affect migratory populations and should be considered in conservation and management.