![]() ![]() Many individual tracking studies have documented increased departure probability with supporting winds, while others have shown no effect of wind, variable effects of wind, or increased departure probability with headwinds. Studying departure decisions made by individual birds has resulted in more variable conclusions about the role of weather when compared to radar studies. Radar is a powerful tool for describing broad-scale avian migration patterns, but it cannot be used to study the behavior of individual animals, and therefore how weather affects decision making at the level of the individual remains less certain. Finally, birds use celestial cues (i.e., sunset position, sunlight polarization patterns, stars) to orient and navigate, and they may therefore be more likely to depart on clear nights when such cues are visible. Rising atmospheric pressure generally predicts warm temperatures and clear skies at synoptic spatial scales in the days ahead, and birds may use changes in pressure to predict weather in the near future. Wind can dramatically increase or decrease flight speed and the energetic costs of flight. Precipitation immediately prior to departure increases thermoregulatory costs and may inhibit insect activity and successful foraging, and during migration itself, precipitation can cause disorientation. Studies of broad-scale migration patterns using radar have demonstrated that migration intensity often increases on nights without precipitation, with supporting winds, when atmospheric pressure is rising, and when skies are clear. Presumably these factors act in a hierarchical fashion, with sex and age narrowing the window for the beginning of migration prior to the effects of shorter-term changes in body condition and day-to-day variation in weather. Migration phenology is ultimately controlled by endogenous time-keeping mechanisms, but it can be proximately modified by intrinsic and extrinsic factors including sex, age, body condition, and weather. Because they spend so much of their lives in the aerosphere, flying animals such as birds are likely particularly impacted by atmospheric conditions, and weather has long been thought to alter avian behavior, including seasonal migration. In response to poor weather conditions, animals may seek shelter, move to lower elevations, alter the timing of daily activity patterns, change foraging strategies, delay breeding, or delay emergence from hibernation. To appropriately respond to weather conditions, animals have evolved a diverse array of behavioral adaptions. ![]() Weather is a ubiquitous factor in the daily lives of animals, and it can have both immediate and long-term impacts on reproduction and survival. 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. 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. By contrast, wind profit, precipitation, and cloud cover were each only informative predictors of departure probability in a single species. 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. ![]() We found that the probability of departure was related to changes in atmospheric pressure, almost completely regardless of species, season, or location. We use these data to determine how wind profit, atmospheric pressure, precipitation, and cloud cover affect probability of departure from breeding and wintering sites. 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. How weather affects individual decisions about the initiation of migratory flights, particularly at the beginning of migration, remains uncertain. 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. Weather can have both delayed and immediate impacts on animal populations, and species have evolved behavioral adaptions to respond to weather conditions. ![]()
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