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Studying the causes and consequences of route selection in animal migration is important for understanding the evolution of migratory systems and how they may be affected by environmental factors at various spatial and temporal scales. One key decision during migration is whether to cross “high transport cost” areas, or to circumvent them. Soaring birds may face this choice when encountering waterbodies where convective updrafts are weak or scarce. Crossing these waterbodies requires flying using energetically costly flapping flight, while circumventing them over land permits energetically cheap soaring.We tested how several atmospheric factors (e.g., wind, thermal uplift) and geographic, seasonal and state-related factors (sex and age) affected route selection in migrating white storks (Ciconia ciconia). We used 196 GPS tracks of 70 individuals either crossing or circumventing the north-easternmost section of the Mediterranean Sea, over Iskenderun Bay in southern Turkey.We found that westward and southward winds promoted a cross-bay journey in spring and autumn, respectively, acting as tailwinds. Also, overall weaker winds promoted a sea crossing in spring. Sea crossing was associated with flapping flight and higher values of Overall Dynamic Body Acceleration (ODBA) and resulted in higher ground speed than travel over land.The combined environmental conditions and the effects of route selection on movement-related energy costs and speed were likely responsible for an increase in the time spent flying and distance travelled of migrating storks that decided to cross the bay during spring. Notably, daily travel distances of spring migrants crossing the bay were 60 kilometres longer than those of land-detouring birds, allowing them to reach their destination faster but likely incurring a higher energetic flight cost. No such benefit was found during autumn.Our findings confirm that atmospheric conditions can strongly affect bird route selection. Consequently, migration timing, speed and movement-related energy expenditure differed considerably between the two migratory seasons and the two route choices, highlighting a time-energy trade-off in the migration of white storks.

Billions of migrating birds travel between their breeding and over-wintering areas twice a year, encountering various environmental conditions along their migration routes. Wind is of utmost importance for birds as wind speed and direction may strongly affect the birds’ flight speed and metabolism. Specifically, tailwinds were found to initiate flight and facilitate higher migration intensity and faster migratory movement while lowering the energetic cost of flight. Using radar, field observations and local meteorological measurements, we studied spring migrating raptors in a migration bottleneck in Italy near the Strait of Messina, between Sicily and Calabria. We explored the effects of wind on flight speed and the intensity of migration in soaring migrants. We found that the birds’ ground speed was positively affected by tailwind speed, and thus, tailwind likely allowed the birds to reach their destination faster. In addition, bird airspeed decreased under increasing tailwind speed, presumably lowering the energetic cost of flight. These findings are in line with predictions of optimal migration flight theory, yet, tailwind had an unexpected negative effect on migration intensity. We suggest that tailwind conditions induced a change in route selection by the migrants at a regional scale, causing a local decline in migration intensity. This change involves the undertaking of cross sea flight rather than overland detour. Furthermore, we found a modular response to crosswinds as birds compensated for winds blowing towards the sea and drifted when winds blew towards land. Our findings suggest that migrating raptors respond to en route wind conditions and coastline geography by adjusting several features of their flight in a manner that will increase their travel speed, reduce the energetic cost of flight, and permit a safe journey. Soaring birds exploit tailwinds to move faster and presumably to reduce the energetic cost of flight during migration. Using radar, we tested how bird flight is affected by tailwind and further predicted that the due to the advantages of using tailwinds, as increased number of birds migrating over the study area. We found that radar-tracked migrating birds flew faster with tailwinds over the study area, but despite this advantage, surprisingly low number of birds migrated when tailwinds prevailed. Furthermore, birds seem to drift with winds blowing from the sea to the land, but not in the opposite direction, suggesting that birds try to avoid drifting over the sea where it is riskier to fly. Hence, our study highlights the complexity of migrant flight behavior in relation to the wind and suggests a flexible response to different wind conditions and coastline geography at multiple spatiotemporal scales.

Human disturbances are increasingly becoming a conservation concern for many populations of colonial seabirds. Colonially reproducing species are particularly vulnerable to localised disturbances because detrimental elements can simultaneously affect the entire population. Studies of petrels and shearwaters have shown that light pollution, in particular, can be harmful for both fledglings and adults, but little is known of the way such anthropogenic elements affect the quality of parental care at the nest. Chick provisioning in petrels and shearwaters occurs exclusively at night and is also negatively correlated with the amount of moonlight. We tested the hypothesis that high-intensity light and sound disturbances will disrupt nest attendance and thus affect weight gain in chicks but that the magnitude of such effects would be modulated by moonlight conditions. We measured the effect of two outdoor disco events on overnight weight gain in 26 chicks of Scopoli’s shearwaters (Calonectris diomedea) from a breeding colony on Linosa Island. The two disco events occurred under contrasting moonlight conditions (moonless vs moonlight). Chicks situated closer to the disturbance gained significantly less weight compared to conspecifics from nests further away but the effect was only evident on the moonless night.Our results suggest that light and sound disturbances can have a negative effect on parental care in C. diomedea but moonlight might moderate the bird’s perception and thus the magnitude of the disturbance. However, while occasional disturbances may impact short-term weight gain in C. diomedea chicks, such effects are not perceivable at fledging when measured as differences in the weight or the date at which they left the nest.