Migratory shorebird adheres to Bergmann’s Rule by responding to environmental conditions through the annual lifecycle

The inverse relationship between body size and environmental temperature is a widespread ecogeographic pattern. However, the underlying forces that produce this pattern are unclear in many taxa. Expectations are particularly unclear for migratory species, as individuals may escape environmental extremes and reorient themselves along the environmental gradient. In addition, some aspects of body size are largely fixed while others are environmentally flexible and may vary seasonally. Here, we used a long-term dataset that tracked multiple populations of the migratory piping plover Charadrius melodus across their breeding and non-breeding ranges to investigate ecogeographic patterns of phenotypically flexible (body mass) and fixed (wing length) size traits in relation to latitude (Bergmann’s Rule), environmental temperature (heat conservation hypothesis), and migratory distance. We found that body mass was correlated with both latitude and temperature across the breeding and non-breeding ranges, which is consistent with predictions of Bergmann’s Rule and heat conservation. However, wing length was correlated with latitude and temperature only on the breeding range. This discrepancy resulted from low migratory connectivity across seasons and the tendency for individuals with longer wings to migrate farther than those with shorter wings. Ultimately, these results suggest that wing length may be driven more by conditions experienced during the breeding season or tradeoffs related to migration, whereas body mass is modified by environmental conditions experienced throughout the annual lifecycle

Migratory shorebird adheres to Bergmann’s Rule by responding to environmental conditions through the annual lifecycle

The inverse relationship between body size and environmental temperature is a widespread ecogeographic pattern. However, the underlying forces that produce this pattern are unclear in many taxa. Expectations are particularly unclear for migratory species, as individuals may escape environmental extremes and reorient themselves along the environmental gradient. In addition, some aspects of body size are largely fixed while others are environmentally flexible and may vary seasonally. Here, we used a long-term dataset that tracked multiple populations of the migratory piping plover Charadrius melodus across their breeding and non-breeding ranges to investigate ecogeographic patterns of phenotypically flexible (body mass) and fixed (wing length) size traits in relation to latitude (Bergmann’s Rule), environmental temperature (heat conservation hypothesis), and migratory distance. We found that body mass was correlated with both latitude and temperature across the breeding and non-breeding ranges, which is consistent with predictions of Bergmann’s Rule and heat conservation. However, wing length was correlated with latitude and temperature only on the breeding range. This discrepancy resulted from low migratory connectivity across seasons and the tendency for individuals with longer wings to migrate farther than those with shorter wings. Ultimately, these results suggest that wing length may be driven more by conditions experienced during the breeding season or tradeoffs related to migration, whereas body mass is modified by environmental conditions experienced throughout the annual lifecycle

Radio Transmitters did not Affect Apparent Survival Rates of Adult Piping Plovers (Charadrius melodus)

Assessments of possible adverse effects of transmitters on marked individuals is an important component of individual-based tracking studies, particularly for species that are listed under the U.S. Endangered Species Act. The breeding and post-breeding movements of adult Piping Plovers (Charadrius melodus) from the federally-threatened Atlantic Coast Population were studied by gluing miniature, 1.0-g, digital VHF radio-transmitters on their interscapular region. Mark-resighting data from 2015-2018 was used to estimate apparent survival rates for 289 adult Piping Plovers in Massachusetts, Rhode Island, and New Jersey in order to compare survival estimates between individuals with a transmitter attached and control individuals without a transmitter. Cormack-Jolly-Seber models were used for live-encounter data in a Bayesian framework to estimate apparent survival rates based on resightings of uniquely marked individuals. There was no evidence that mean apparent survival rates differed between adults with transmitters (0.756; 95% CI = 0.611 - 0.877) and without transmitters (0.673; 95% CI = 0.607 - 0.740). In addition, there was no evidence of differences in apparent survival rates between breeding location (state) or years. This study provides further evidence that radio transmitters glued temporarily to the inter-scapular region can be an effective tool to monitor local and regional movements of sensitive shorebirds, such as Piping Plovers

The effect of top predator removal on the distribution of a mesocarnivore and nest survival of an endangered shorebird

For trophic systems regulated by top-down processes, top carnivores may determine species composition of lower trophic levels. Removal of top predators could therefore cause a shift in community composition. If predators play a role in limiting the population of endangered prey animals, removing carnivores may have unintended consequences for conservation. Lethal predator removal to benefit prey species is a widely used management strategy. Red foxes (Vulpes vulpes) are a common nest predator of threatened piping plovers (Charadrius melodus) and are often the primary target of predator removal programs, yet predation remains the number one cause of piping plover nest loss. Predator exclusion cages (hereafter, exclosures) around nests are often used to keep foxes from eating eggs, as an alternative to predator removal. Fox removals may increase the presence or activity of smaller predators, including American mink (Neovison vison), which can enter exclosures. We predicted that when foxes were removed from plover breeding sites, mesopredator release of mink would occur and lead to increased predation levels of plover nests. Average probability of mink occupancy was higher when foxes were absent (0.26 [95% BCI = 0.16, 0.38]) than when they were present (0.04 [0.01, 0.09]). For nests not protected by exclosures, nest predation was similar at sites with (0.42 [0.12, 0.68]) and without foxes (0.38 [0.10, 0.64]). At sites where foxes were absent, however, predation rates of exclosed nests were 3-fold higher (foxes absent [0.25 {0.06, 0.52}]) compared to sites where foxes remained [0.06 {0.01, 0.18}]). Our results suggest that removal of foxes from plover breeding sites allowed American mink to expand their local range. In turn, removal of foxes led to a decrease in nest survival of an endangered ground-nesting bird. Conservation within complex trophic systems may fail if interactions among species are not well understood when implementing lethal predator removal

Radio transmitters did not affect daily nest and chick survival of Piping Plovers (Charadrius melodus)

Biologists interested in using radio telemetry to track the movements of birds should concurrently conduct studies to assess potential impacts on study organisms, particularly when monitoring threatened or endangered species. We investigated the effects of traditional and digital very high frequency (VHF) radio transmitters on daily nest survival and chick survival rates of Piping Plovers (Charadrius melodus) along the Atlantic Coast in 2012, 2013, and 2015. We attached 1.0-1.2 g transmitters to 110 plovers and monitored their 160 nest attempts. We also monitored 221 nest attempts by 161 control pairs with no transmitters. There was no evidence that nest or chick survival differed between tagged and control pairs. Transmitters did not seem to adversely impact Piping Plover daily nest and chick survival and are a valuable tool to monitor movements of this threatened species

Migratory shorebird adheres to Bergmann’s Rule by responding to environmental conditions through the annual lifecycle

The inverse relationship between body size and environmental temperature is a widespread ecogeographic pattern. However, the underlying forces that produce this pattern are unclear in many taxa. Expectations are particularly unclear for migratory species, as individuals may escape environmental extremes and reorient themselves along the environmental gradient. In addition, some aspects of body size are largely fixed while others are environmentally flexible and may vary seasonally. Here, we used a long-term dataset that tracked multiple populations of the migratory piping plover Charadrius melodus across their breeding and non-breeding ranges to investigate ecogeographic patterns of phenotypically flexible (body mass) and fixed (wing length) size traits in relation to latitude (Bergmann’s Rule), environmental temperature (heat conservation hypothesis), and migratory distance. We found that body mass was correlated with both latitude and temperature across the breeding and non-breeding ranges, which is consistent with predictions of Bergmann’s Rule and heat conservation. However, wing length was correlated with latitude and temperature only on the breeding range. This discrepancy resulted from low migratory connectivity across seasons and the tendency for individuals with longer wings to migrate farther than those with shorter wings. Ultimately, these results suggest that wing length may be driven more by conditions experienced during the breeding season or tradeoffs related to migration, whereas body mass is modified by environmental conditions experienced throughout the annual lifecycle

plover_bergmann

R data file containing the data required to determine geographic patterns in piping plover body size as a function of latitude or seasonal temperature