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.

Previous laboratory studies have demonstrated the role of photoperiod in cueing the migration timing of small land birds; however, how migration timing of young birds in wild environments develops in relation to these cues have rarely been investigated. Such investigations can make important contributions to our developing understanding of the phenotypic plasticity of migration timing to new conditions with climate change, where changes in the timing of nesting may expose juvenile birds to different photoperiods. We investigated the impact of manipulating photoperiod during nestling development in a long-distance migratory songbird on the timing of post-breeding movements in the wild. Using programmable lighting installed in the nest-boxes of purple martins ( Progne subis ), we exposed developing nestlings, from hatch to fledge date, to an extended photoperiod that matched the day length of the summer solstice in Manitoba, Canada. We found that birds with a simulated, earlier photoperiod had a longer nesting period and later fledge and autumn departure dates than control group birds. This study demonstrates the phenotypic plasticity of first-year birds to the ontogenetic effect of their hatch date in the formation of the timing of their first post-breeding movements. Further, we discuss how these results have implications for the potential use of assisted evolution approaches to alter migration timing to match new conditions with climate change.

The Hyrcanian forests of northern Iran support a great diversity of bird species. Yet, the ecological relationships of cavity-nesting birds (CNBs), the main components of forest bird communities in this area, remain unknown. We examined the importance of ecological factors in 90 study plots during the breeding season to predict the probability distribution and habitat suitability of primary cavity nesters (PCNs), secondary cavity nesters (SCNs) and weak primary cavity nesters (WPCNs). Ecological factors included forest structure, habitat complexity, geographical attributes and dead tree features. The significant habitat parameters determining the presence-absence of each CNB species were also determined. PCN richness and abundance were most influenced by the density of larger trees, snags (standing dead trees) and logs (fallen dead trees). The densities of snags, logs and larger trees and the extent of the shrub layer influenced distributions of SCNs. The Eurasian nuthatch (Sitta europaea) was the only WPCN identified, and its distribution was strongly affected by altitude and by larger snags and trees in complex habitats. Our results showed that CNB species could be classified in three groups: (1) species dependent on mature forests, (2) species dependent on younger forests and (3) species adaptable to different forest stages. Overall, snags and logs played vital roles in the habitat preferences of CNB communities as well as in forest structure and complexity.

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