Survival varies seasonally in a migratory bird: Linkages between breeding and non-breeding periods.

Migratory species form an important component of biodiversity; they link ecosystems across the globe, but are increasingly threatened by global environmental change. Understanding and mitigating threats requires knowledge of how demographic processes operate throughout the annual cycle, but this can be difficult to achieve when breeding and non-breeding grounds are widely separated. Our goal is to quantify the importance of variability in survival during the breeding and non-breeding seasons in determining variation in annual survival using a single population and, more broadly, the extent to which annual survival across species reflects variation in probability of surviving the migratory period. We use a 25-year dataset in which individuals of a long-distance migratory bird, the alpine swift Tachymarptis melba, were captured towards the beginning and end of each breeding season to estimate age- and season-specific survival probabilities and incorporate explicit estimation of the correlations in survival between age-classes and seasons. Monthly survival was higher during the breeding period than during the rest of the year and strongly affected by conditions in the breeding season; effects that remained apparent in the following non-breeding season, but not subsequently. Recruitment of juveniles was dependent on the timing of breeding, being higher if egg-laying commenced before the median date, and substantially lower if not. Across migratory bird species, variation in annual survival largely reflects variation in the probability of surviving the migratory period. Using a double-capture approach, even within a single season, provides valuable insights into the demography of migratory species, which will help understand the extent and impacts of the threats they face in a changing world.

What makes Alpine swift ascend at twilight? Novel geolocators reveal year-round flight behaviour.

ABSTRACT: Studying individual flight behaviour throughout the year is indispensable to understand the ecology of a bird species. Recent development in technology allows now to track flight behaviour of small long-distance bird migrants throughout its annual cycle. The specific flight behaviour of twilight ascents in birds has been documented in a few studies, but only during a short period of the year, and never quantified on the individual level. It has been suggested that twilight ascents might be a role in orientation and navigation. Previous studies had reported the behaviour only near the breeding site and during migration. We investigated year-round flight behaviour of 34 individual Alpine swifts (Apus melba) of four different populations in relation to twilight ascents. We recorded twilight ascents all around the year and found a twofold higher frequency in ascents during the non-breeding residence phase in Africa compared to all other phases of the year. Dawn ascents were twice as common as dusk ascents and occurred mainly when atmospheric conditions remained stable over a 24-h period. We found no conclusive support that twilight ascents are essential for recalibration of compass cues and landmarks. Data on the wing flapping intensity revealed that high activity at twilight occurred more regularly than the ascents. We therefore conclude that alpine swift generally increase flight activity-also horizontal flight-during the twilight period and we suppose that this increased flight activity, including ascents, might be part of social interactions between individuals. SIGNIFICANCE STATEMENT: Year-round flight altitude tracking with a light-weight multi-sensor tag reveals that Alpine swifts ascend several hundred meters high at twilight regularly. The reason for this behaviour remains unclear and the low-light conditions at this time of the day preclude foraging as a possibility. The frequency and altitude of twilight ascents were highest during the non-breeding period, intermediate during migration and low for active breeders during the breeding phase. We discuss our findings in the context of existing hypotheses on twilight ascent and we propose an additional hypothesis which links twilight ascent with social interaction between flock members. Our study highlights how flight behaviour of individuals of a migratory bird species can be studied even during the sparsely documented non-breeding period.

First evidence of a 200-day non-stop flight in a bird.

Being airborne is considered to be energetically more costly as compared with being on the ground or in water. Birds migrating or foraging while airborne are thought to spend some time resting on the ground or water to recover from these energetically demanding activities. However, for several decades ornithologists have claimed that some swifts may stay airborne for almost their whole lifetime. Here we present the first unequivocal evidence that an individual bird of the Alpine swift (Tachymarptis melba) can stay airborne for migration, foraging and roosting over a period of more than 6 months. To date, such long-lasting locomotive activities had been reported only for animals living in the sea. Even for an aerodynamically optimized bird, like the Alpine swift, flying requires a considerable amount of energy for continuous locomotive control. Our data imply that all vital physiological processes, including sleep, can be perpetuated during flight.