Breeding phenology determines evolutionary transitions in migratory behaviour in finches and allies

Demography of Sooty Fox Sparrows (Passerella unalaschcensis) following a shift from a migratory to resident life history

Identifying causes and consequences of variation in species life history has the potential to improve predictions about how climate and land-use change may affect the demography and distribution of species in future. Sooty Fox Sparrows (Passerella unalaschcensis (J.F. Gmelin, 1789); or commonly grouped within the Fox Sparrow, Passerella iliaca (Merrem, 1786)) were migrants that rarely bred in the Georgia Basin of British Columbia prior to ∼1950 but have since established resident populations. Data on 270 color-banded birds and 54 nests on Mandarte Island, British Columbia, allowed us to estimate demographic vital rates and population growth in one recently established population. Annual fecundity (F), estimated as the product of the number of broods initiated (1.5 ± 0.01; mean ± SD), clutch size (2.82 ± 0.44), and probability of survival to fledging (0.68 ± 0.02), exceeded values reported for migrants, supporting the hypothesis that residents invest more in reproduction, on average, than migrants within species. Estimating juvenile and adult overwinter survival (Sj = 0.32 ± 0.06 and Sa = 0.69 ± 0.05) next allowed us to simulate an expected distribution of population growth rates as λexp = Sa + (Sj × F), given parameter error. Our estimate of λexp (1.61 ± 0.57) implies expeditious population growth, consistent with the species’ recent colonization of the region.

Why migrate during the day: a comparative analysis of North American birds

Migration can take place primarily during the day or at night, or during both depending on the species. Why the timing of migration varies among species has been the object of much research but the underlying ecological processes are still unclear. Proximally, migration timing may be influenced by the timing of other activities or may be more prevalent in species that migrate over long distances. Adaptive scenarios emphasize the reduction in flight costs at night especially in smaller species and the advantages of travelling in groups during the day to locate staging sites more efficiently. I used phylogenetic independent contrasts to examine these hypotheses in all North American nesting birds. I uncovered 24 evolutionary transitions in migration timing, most of which involved a switch from nocturnal to diurnal migration. Few of these transitions involved a concomitant change in the timing of foraging habits or migration distance. However, species in diurnal clades were larger, travelled in larger flocks and were generally more sociable than their nocturnal counterparts. The results support the hypotheses that a reduction in flight costs and the ability to pool information from companions are associated with migration timing in North American bird species.