Migration of the Skylark Alauda arvensis Along the Southern French Atlantic Coast

Mist-nettings of Eurasian skylarks (Alauda arvensis) were conducted throughout the day near the coastline in south-western France during the fall migratory period. The main objective of this work was to document some aspects of body condition (mass and fat stores) in skylarks. Between 1997 and 2004, a total of 8903 birds were captured in October-November. The median capture dates of fall migrations for females and males were October 24 and October 26, respectively. About 58.4% and 34.0% of birds were females and males respectively, the remaining 7.6% corresponding to unsexed birds. Skylarks arrived to our study site with low fat stores, 42% of the individuals showing no visible fat deposits. Rates of mass changes were found to be positively related to the time of day: individuals captured during the night before 07:00 were 3-4% heavier than individuals captured during daylight between 07:00 and 20:00. On the daytime, the average rate of body mass gain was about 5% for both sexes, suggesting that migratory birds were not only able to gain energy for their regular metabolism, but they were also able to store fat for migration. According to allometric equations to estimate the energetic cost of flight, flight ranges for the majority of birds were estimated to be less than 180 - 240 km for females and lower than 160 - 220 for males. As migratory stopover sites are used for refuelling by many migrant land birds, our data suggest that most of the skylarks might exhibit a feed-by-day, fly-by night strategy in our region, highlighting the need for preserving stopover sites, such as those encountered in south-western France, for successful migration. Considering that many migrant birds such as skylarks feed heavily on seeds, changes in agricultural practices might affect survival capabilities of skylarks during their fall migration and in winter.