Atmospheric pressure predicts probability of departure for migratory songbirds

BACKGROUND

Weather can have both delayed and immediate impacts on animal populations, and species have evolved behavioral adaptions to respond to weather conditions. Weather has long been hypothesized to affect the timing and intensity of avian migration, and radar studies have demonstrated strong correlations between weather and broad-scale migration patterns. How weather affects individual decisions about the initiation of migratory flights, particularly at the beginning of migration, remains uncertain.

METHODS

Here, we combine automated radio telemetry data from four species of songbirds collected at five breeding and wintering sites in North America with hourly weather data from a global weather model. We use these data to determine how wind profit, atmospheric pressure, precipitation, and cloud cover affect probability of departure from breeding and wintering sites.

RESULTS

We found that the probability of departure was related to changes in atmospheric pressure, almost completely regardless of species, season, or location. Individuals were more likely to depart on nights when atmospheric pressure had been rising over the past 24 h, which is predictive of fair weather over the next several days. By contrast, wind profit, precipitation, and cloud cover were each only informative predictors of departure probability in a single species.

CONCLUSIONS

Our results suggest that individual birds actively use weather information to inform decision-making regarding the initiation of departure from the breeding and wintering grounds. We propose that birds likely choose which date to depart on migration in a hierarchical fashion with weather not influencing decision-making until after the departure window has already been narrowed down by other ultimate and proximate factors.

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

The Bay of Fundy, Canada is a critical staging area for Semipalmated Sandpipers (Calidris pusilla) during post-breeding migration. Recent range-wide population declines and changes in diet and migratory timing in the Bay of Fundy prompted a re-examination of staging ecology, including length of stay (last estimated in 1981), which is used in calculating migratory population estimates. We used radio-telemetry and the Motus Wildlife Tracking System to estimate individual length of stay and departure conditions for 159 Semipalmated Sandpipers in 2013 and 2014. Using tracking data we compared two estimation methods, minimum length of stay and mark-recapture modelling. Using minimum length of stay, the mean length of stay was approximately 21 days, an increase from the previous estimate of 15 days. Mark-recapture models suggested a much longer staging period that is inconsistent with other data. Sandpipers captured early in the staging period stayed longer on average than those captured later. Departures from the staging area were correlated with north-westerly winds, moderate to high wind speeds and low but rising atmospheric pressures. We suggest that Semipalmated Sandpipers in the Bay of Fundy are not operating on a time-selected migration schedule and instead wait for favourable weather conditions to depart, which occur more often later in the migratory period. Population trends in the Bay of Fundy should be re-evaluated in light of the increased length of stay.

A place to land: spatiotemporal drivers of stopover habitat use by migrating birds

Migrating birds require en route habitats to rest and refuel. Yet, habitat use has never been integrated with passage to understand the factors that determine where and when birds stopover during spring and autumn migration. Here, we introduce the stopover-to-passage ratio (SPR), the percentage of passage migrants that stop in an area, and use 8 years of data from 12 weather surveillance radars to estimate over 50% SPR during spring and autumn through the Gulf of Mexico and Atlantic coasts of the south-eastern US, the most prominent corridor for North America's migratory birds. During stopovers, birds concentrated close to the coast during spring and inland in forested landscapes during autumn, suggesting seasonal differences in habitat function and highlighting the vital role of stopover habitats in sustaining migratory communities. Beyond advancing understanding of migration ecology, SPR will facilitate conservation through identification of sites that are disproportionally selected for stopover by migrating birds.

Both short and long distance migrants use energy-minimizing migration strategies in North American herring gulls

BACKGROUND

Recent studies have proposed that birds migrating short distances migrate at an overall slower pace, minimizing energy expenditure, while birds migrating long distances minimize time spent on migration to cope with seasonal changes in environmental conditions.

METHODS

We evaluated variability in the migration strategies of Herring Gulls (Larus argentatus), a generalist species with flexible foraging and flight behaviour. We tracked one population of long distance migrants and three populations of short distance migrants, and compared the directness of their migration routes, their overall migration speed, their travel speed, and their use of stopovers.

RESULTS

Our research revealed that Herring Gulls breeding in the eastern Arctic migrate long distances to spend the winter in the Gulf of Mexico, traveling more than four times farther than gulls from Atlantic Canada during autumn migration. While all populations used indirect routes, the long distance migrants were the least direct. We found that regardless of the distance the population traveled, Herring Gulls migrated at a slower overall migration speed than predicted by Optimal Migration Theory, but the long distance migrants had higher speeds on travel days. While long distance migrants used more stopover days overall, relative to the distance travelled all four populations used a similar number of stopover days.

CONCLUSIONS

When taken in context with other studies, we expect that the migration strategies of flexible generalist species like Herring Gulls may be more influenced by habitat and food resources than migration distance.

Compensation for wind drift in the nocturnally migrating Song Thrushes in relation to altitude and wind

Compensation for wind drift in relation to the side-wind velocity and altitude was investigated in Song Thrushes during autumn migration. The birds were recorded at night flying above the prominent leading line of a marine spit which coincided with the general direction of their migration. Among the large size passerine species, Song Thrushes were identified by a combination of five flight characteristics typical only of this species during particular periods of autumn. The thrushes showed different reactions to the crosswinds: complete and partial compensation for the displacement and drift. Under normal visibility, the completeness of compensation depended both on the velocity of the side-wind and altitude. The degree of compensation achieved was reduced with an increase of altitude, regardless of the wind. Under the same wind conditions, the angle of drift (the angle between the track direction and the leading line) increased with altitude, but the number of birds that compensated for drift decreased. On average, at heights below 300 m agl, the thrushes were capable of compensating completely for moderate winds; between 300 and 600 m agl compensation was partial; but above 600 m the birds drifted completely. Birds of the same species flying above the same terrain may demonstrate different reactions to the same crosswind depending on altitude. Meanwhile, flight tracks gradually deviated from the leading line with an increase in altitude, the headings of the birds got closer to the general migratory direction. It is more likely that the birds control displacement using the visual flow regulation principle by the angular velocity of the landmarks below them running aside in relation to their flight direction, which is inversely proportional to the altitude. Low flying thrushes promptly reacted to the shifting of the leading line of the spit with an average angular velocity of more than 0.8°/s perpendicular to the direction of flight and compensated completely for drift. Shifting of the leading line with an angular velocity of less than 0.4°/s, the high flying birds did not seem to notice or did not try to compensate for displacement deliberately.

The impacts of climate change on the biomechanics of animals: Themed Issue Article: Biomechanics and Climate Change

Anthropogenic climate change induces unprecedented variability in a broad range of environmental parameters. These changes will impact material properties and animal biomechanics, thereby affecting animal performance and persistence of populations. Climate change implies warming at the global level, and it may be accompanied by altered wind speeds, wave action, ocean circulation, acidification as well as increased frequency of hypoxic events. Together, these environmental drivers affect muscle function and neural control and thereby movement of animals such as bird migration and schooling behaviour of fish. Altered environmental conditions will also modify material properties of animals. For example, ocean acidification, particularly when coupled with increased temperatures, compromises calcified shells and skeletons of marine invertebrates and byssal threads of mussels. These biomechanical consequences can lead to population declines and disintegration of habitats. Integrating biomechanical research with ecology is instrumental in predicting the future responses of natural systems to climate change and the consequences for ecosystem services such as fisheries and ecotourism.