The use of nocturnal flights for barrier crossing in a diurnally migrating songbird

Background The migration patterns of land birds can generally be divided into those species that migrate principally during the day and those that migrate during the night. Some species may show individual plasticity in the use of day or night flight, particularly when crossing large, open-water or desert barriers. However, individual plasticity in circadian patterns of migratory flights in diurnally migrating songbirds has never been investigated. Methods We used high precision GPS tracking of a diurnal, migratory swallow, the purple martin (Progne subis), to determine whether individuals were flexible in their spring migration strategies to include some night flight, particularly at barrier crossing. Results Most (91%) of individuals made large (sometimes > 1000 km), open-water crossings of the Caribbean Sea and the Gulf of Mexico that included the use of night flight. 32% of all water crossings were initiated at night, demonstrating that night flight is not only used to complete large crossings but may confer other advantages for diurnal birds. Birds were not more likely to initiate crossings with supportive winds, however crossings were more likely when they reduced travel distances. Our results are consistent with diurnal birds using night flight to help achieve time- and energy-savings through ‘short cuts’ at barrier crossings, at times and locations when foraging opportunities are not available. Conclusions Overall, our results demonstrate the use of nocturnal flight and a high degree of individual plasticity in migration strategies on a circadian scale in a species generally considered to be a diurnal migrant. Nocturnal flights at barrier crossing may provide time and energy savings where foraging opportunities are low in an otherwise diurnal strategy. Future research should target how diel foraging and refueling strategies support nocturnal flights and barrier crossing in this and other diurnal species.

Autumn Migration of the Wood Pigeon, Columba palumbus, at Eastern Part of the Polish Baltic Coast

The Wood Pigeon, Columba palumbus, is a common diurnal migrant across most of the Europe. The visual observations were carried out during autumn work of the Operation Baltic ringing stations, mainly at Mierzeja Wiślana (54°21’N, 19°19’E) parallely to the ringing of birds. Observations of passing birds were performed 15 minutes per every hour from sunrise to sunset. The autumn observation period was long enough to cover whole pigeons migration at the southern coast of Baltic (14 Aug. – 11 Nov.). Altogether 119,019 Wood Pigeons were observed within 20 seasons of observations. The goals of this work is (1) to describe the course of the Wood Pigeon autumn migration at the Polish Baltic coast as to numbers in long-term aspect and the seasonality of migration, (2) to propose further development in the study on wave structure of the bird passage. The main analysis of this work-description of the seasonal dynamics of migration - was carried out with modified and widened method used for that kind of analysis in earlier publications. In the present paper the wave structure is described by the estimated border days between supposed waves taking under consideration different properties of the passage dynamics. Here are used three groups of parameters: (1) total numbers of birds observed daily during several years, inter-year variability (as SD between year number values), and the coefficient of variation (V), (2) number of migration peaks that occurred in that date within years of the study (there were used two classes of peaks – High Peaks and Peaks), (3) numbers of birds passing during recognized peak days. All estimations were done independently of others and only finally summarized using common ranking scheme. Such step by step procedure make estimation more and more reliable. Very high agreement between estimations based on three groups of parameters suggests strongly that the proposed estimation procedure is effective enough.