Timing of arrival/emergence to the breeding grounds is under contrasting natural and sexual selection pressures. Because of differences in sex roles and physiology, the balance between these pressures on either sex may differ, leading to earlier male (protandry) or female (protogyny) arrival. We test several competing hypotheses for the evolution of protandry using migration data for 22 bird species, including for the first time several monochromatic ones where sexual selection is supposedly less intense. Across species, protandry positively covaried with sexual size dimorphism but not with dichromatism. Within species, there was weak evidence that males migrate earlier because, being larger, they are less susceptible to adverse conditions. Our results do not support the ‘rank advantage’ and the ‘differential susceptibility’ hypotheses, nor the ‘mate opportunity’ hypothesis, which predicts covariation of protandry with dichromatism. Conversely, they are compatible with ‘mate choice’ arguments, whereby females use condition‐dependent arrival date to assess mate quality.

A growing body of work shows that climate change is the cause of a number of directional shifts in the spring phenology of migratory birds. However, changes in autumn phenology are well studied and their consistency across species, as well as their link with population trends, remains uncertain. We investigate changes in the autumn migration dates of 11 species of soaring birds over the Strait of Gibraltar over a 16‐year period. Using models corrected for phylogeny, we assessed whether ecological and morphological characteristics, as well as population trends, account for interspecific shifts in migration times. We recorded different phenological changes in different periods of the migration season and suggest that these differences are due to age‐dependent responses. The variable best predicting advances in migration dates was population trend: species that did not advance their autumn migration dates were those showing a decline in their European breeding populations. We repeated our tests on a dataset representing the migration date of soaring birds across the Pyrenees Mountains and found that population trends at this site also predicted phenological shifts. Our results suggest that flexibility in migratory strategy and population trends may be related, such that different adaptive capacity in migration timing may be more relevant than other ecological traits in determining the conservation status of migratory birds in Europe and perhaps other regions.

Light-level geolocators are currently widely used to track the migration of smallsized birds, but their potentially detrimental effects on survival of highly aerial species have been poorly investigated so far. We recorded capture-recapture histories of 283 common swifts Apus apus and 107 pallid swifts Apus pallidus breeding in 14 colonies in Italy, Spain, Sweden and Switzerland that were equipped with 10 different types of geolocators ('geolocator birds'), and compared their survival with that of, respectively, 215 common and 101 pallid swifts not equipped with geolocators ('control birds'). Data were analysed using both GLMMs with return rate as a proxy for survival and mark-recapture models to estimate survival while accounting for recapture probability. In all the analyses, geolocator birds showed reduced apparent survival compared to controls. Geolocator weight was always lower than 3% of body mass, and did not affect survival per se. Geolocators with a light-stalk, which is used in some geolocator models to reduce light sensor shading by feathers, decreased apparent survival more than models without light-stalk. Apparent survival of geolocator birds significantly varied among sites, being much higher in northern Europe. Despite in our analyses we could only partly account for variable recapture probabilities among sites and for inter-annual variability in survival, our results generally showed that equipping swifts with geolocators decreased their survival prospects, but also that the magnitude of this effect may depend on species-specific traits. These conclusions are in line with those of other studies on aerial foragers. We suggest that future studies tracking the movements of aerial insectivorous birds should use devices designed to minimize drag.

Departure and stopover decisions are crucial for a successful migration. Such decisions are modulated by a complex interplay between endogenous (physiological state) and external factors, such as weather (e.g. wind) and geography (ecological barriers). In this study of the black redstart Phoenicurus ochruros, a short-distance migrant passerine, we investigate the effect of weather, as gauged by tailwind and crosswind conditions, rainfall, temperature, and barometric pressure, on departures from a stopover site in the central Mediterranean Sea, off the western coast of Italy (Ventotene island), during both spring and autumn migration. We found that stopover duration was longer in birds arriving with lower fat stores, and that birds departed with generally favourable weather conditions (favourable tailwinds, weak or no crosswinds, low rainfall, high temperatures, and high pressure). However, the effects of weather on departure decisions were stronger in autumn: this could be related to 1) a seasonal difference in selection pressures for early arrival at the goal areas, that are expected to be stronger in spring than in autumn or 2) a difference in the residual extent of sea crossing since, in autumn, birds are confronted with a much longer non-stop sea crossing (at least 300 km) than in spring (Â50 km). In spring we also found males to leave the study site under less favourable tailwinds than females, and adults to leave with more favourable tailwinds than young. Our findings indicate that departure decisions are flexible and differently affected by weather in different seasons, either because of seasonal effects or because of different distances to be covered before reaching the next stopover site. Moreover, our study suggests that sex-specific weather selectivity should be regarded among the proximate factors affecting differential spring migration of either sex.

No abstract