Remarkably similar migration patterns between different red-backed shrike populations suggest that migration rather than breeding area phenology determines the annual cycle

The regular fluctuation of resources across the Globe guides movements of migratory animals. To ensure sufficient reproductive output and maintain viable population sizes, migratory animals should match arrival at breeding areas with local peaks in resource availability. It is generally assumed that breeding phenology dictates the timing of the annual cycle, but this is poorly studied. Here, we use light-level geolocator tracking data to compare the annual spatiotemporal migration patterns of a long-distance migratory songbird, the red-backed shrike, Lanius collurio, breeding at widely different latitudes within Europe. We find that populations use remarkably similar migration routes and are highly synchronized in time. Additional tracks from populations breeding at the edges of the European range support these similar migration patterns. When comparing timing of breeding and vegetation phenology, as a measure of resource availability across populations, we find that arrival and timing of breeding corresponds to the peak in vegetation greenness at northern latitudes. At lower latitudes birds arrive simultaneously with the more northerly breeding populations, but after the local greenness peak, suggesting that breeding area phenology does not determine the migratory schedule. Rather, timing of migration in red-backed shrikes may be constrained by events in other parts of the annual cycle.</p

Sex-specific nestling body mass in relation to brood sex composition in the Eurasian Sparrowhawk Accipiter nisus

The trade-off between brood size and offspring quality, as predicted by life history theory, has been extensively studied in birds. However, in sexually size-dimorphic birds, where the larger sex requires more investment from the parents, the potential additional trade-off between brood sex ratio and offspring quality has received less attention. Effects of brood sex composition on nestling fitness can have implications for optimal sex allocation strategies. A harmful effect of a higher proportion of the larger sex on either one of the sexes predicts selection towards parents producing the less harmful, smaller sex. In addition to Fisherian equal allocation this should lead to considerable sex ratio biases in extremely sexually size-dimorphic birds. We tested this in the Eurasian Sparrowhawk Accipiter nisus, where female nestlings require about 40% more parental investment than male nestlings. Sex-specific nestling body mass, calculated as the relative difference with the average body mass, corrected for age, for both sexes, was used as an indicator of nestling quality. With a sample size of 120 broods, containing 483 nestlings, we found that individual nestlings were of lower body mass when they hatched in larger and female dominated broods. This effect was particularly pronounced in female nestlings. Under these conditions, sex ratio theory predicts a male bias in the population. However, this is not supported by empirical evidence from literature.

Remarkably similar migration patterns between different red-backed shrike populations suggest that migration rather than breeding area phenology determines the annual cycle

The regular fluctuation of resources across the Globe guides movements of migratory animals. To ensure sufficient reproductive output and maintain viable population sizes, migratory animals should match arrival at breeding areas with local peaks in resource availability. It is generally assumed that breeding phenology dictates the timing of the annual cycle, but this is poorly studied. Here, we use light-level geolocator tracking data to compare the annual spatiotemporal migration patterns of a long-distance migratory songbird, the red-backed shrike, Lanius collurio, breeding at widely different latitudes within Europe. We find that populations use remarkably similar migration routes and are highly synchronized in time. Additional tracks from populations breeding at the edges of the European range support these similar migration patterns. When comparing timing of breeding and vegetation phenology, as a measure of resource availability across populations, we find that arrival and timing of breeding corresponds to the peak in vegetation greenness at northern latitudes. At lower latitudes birds arrive simultaneously with the more northerly breeding populations, but after the local greenness peak, suggesting that breeding area phenology does not determine the migratory schedule. Rather, timing of migration in red-backed shrikes may be constrained by events in other parts of the annual cycle

Data from: Remarkably similar migration patterns between different red-backed shrike populations suggest that migration rather than breeding area phenology determines the annual cycle

Pedersen L, Onrubia A, Vardanis Y, Barboutis C, Waasdorp S, van Helvert M, Geertsma M, Ekberg P, Willemoes M, Strandberg R, Matsyna E, Matsyna A, Klaassen RHG, Alerstam T, Thorup K, Tøttrup AP. 2020. Remarkably similar migration patterns between different red-backed shrike populations suggest that migration rather than breeding area phenology determines the annual cycle. J Avian Biol. doi:10.1111/jav.02475,The regular fluctuation of resources across the Globe guides movements of migratory animals. To ensure sufficient reproductive output and maintain viable population sizes, migratory animals should match arrival at breeding areas with local peaks in resource availability. It is generally assumed that breeding phenology dictates the timing of the annual cycle, but this is poorly studied. Here, we use light‐level geolocator tracking data to compare the annual spatiotemporal migration patterns of a long‐distance migratory songbird, the red‐backed shrike, Lanius collurio, breeding at widely different latitudes within Europe. We find that populations use remarkably similar migration routes and are highly synchronized in time. Additional tracks from populations breeding at the edges of the European range support these similar migration patterns. When comparing timing of breeding and vegetation phenology, as a measure of resource availability across populations, we find that arrival and timing of breeding corresponds to the peak in vegetation greenness at northern latitudes. At lower latitudes birds arrive simultaneously with the more northerly breeding populations, but after the local greenness peak, suggesting that breeding area phenology does not determine the migratory schedule. Rather, timing of migration in red‐backed shrikes may be constrained by events in other parts of the annual cycle.

Migration of red-backed shrike populations (data from Pedersen et al. 2020)-tracks

The regular fluctuation of resources across the Globe guides movements of migratory animals. To ensure sufficient reproductive output and maintain viable population sizes, migratory animals should match arrival at breeding areas with local peaks in resource availability. It is generally assumed that breeding phenology dictates the timing of the annual cycle, but this is poorly studied. Here, we use light‐level geolocator tracking data to compare the annual spatiotemporal migration patterns of a long‐distance migratory songbird, the red‐backed shrike, Lanius collurio, breeding at widely different latitudes within Europe. We find that populations use remarkably similar migration routes and are highly synchronized in time. Additional tracks from populations breeding at the edges of the European range support these similar migration patterns. When comparing timing of breeding and vegetation phenology, as a measure of resource availability across populations, we find that arrival and timing of breeding corresponds to the peak in vegetation greenness at northern latitudes. At lower latitudes birds arrive simultaneously with the more northerly breeding populations, but after the local greenness peak, suggesting that breeding area phenology does not determine the migratory schedule. Rather, timing of migration in red‐backed shrikes may be constrained by events in other parts of the annual cycle

Migration of red-backed shrike populations (data from Pedersen et al. 2020)-twilights

The regular fluctuation of resources across the Globe guides movements of migratory animals. To ensure sufficient reproductive output and maintain viable population sizes, migratory animals should match arrival at breeding areas with local peaks in resource availability. It is generally assumed that breeding phenology dictates the timing of the annual cycle, but this is poorly studied. Here, we use light‐level geolocator tracking data to compare the annual spatiotemporal migration patterns of a long‐distance migratory songbird, the red‐backed shrike, Lanius collurio, breeding at widely different latitudes within Europe. We find that populations use remarkably similar migration routes and are highly synchronized in time. Additional tracks from populations breeding at the edges of the European range support these similar migration patterns. When comparing timing of breeding and vegetation phenology, as a measure of resource availability across populations, we find that arrival and timing of breeding corresponds to the peak in vegetation greenness at northern latitudes. At lower latitudes birds arrive simultaneously with the more northerly breeding populations, but after the local greenness peak, suggesting that breeding area phenology does not determine the migratory schedule. Rather, timing of migration in red‐backed shrikes may be constrained by events in other parts of the annual cycle

Migration of red-backed shrike populations (data from Pedersen et al. 2020)-light-levels.csv

The regular fluctuation of resources across the Globe guides movements of migratory animals. To ensure sufficient reproductive output and maintain viable population sizes, migratory animals should match arrival at breeding areas with local peaks in resource availability. It is generally assumed that breeding phenology dictates the timing of the annual cycle, but this is poorly studied. Here, we use light‐level geolocator tracking data to compare the annual spatiotemporal migration patterns of a long‐distance migratory songbird, the red‐backed shrike, Lanius collurio, breeding at widely different latitudes within Europe. We find that populations use remarkably similar migration routes and are highly synchronized in time. Additional tracks from populations breeding at the edges of the European range support these similar migration patterns. When comparing timing of breeding and vegetation phenology, as a measure of resource availability across populations, we find that arrival and timing of breeding corresponds to the peak in vegetation greenness at northern latitudes. At lower latitudes birds arrive simultaneously with the more northerly breeding populations, but after the local greenness peak, suggesting that breeding area phenology does not determine the migratory schedule. Rather, timing of migration in red‐backed shrikes may be constrained by events in other parts of the annual cycle

Migration of red-backed shrike populations (data from Pedersen et al. 2020)-reference-data

The regular fluctuation of resources across the Globe guides movements of migratory animals. To ensure sufficient reproductive output and maintain viable population sizes, migratory animals should match arrival at breeding areas with local peaks in resource availability. It is generally assumed that breeding phenology dictates the timing of the annual cycle, but this is poorly studied. Here, we use light‐level geolocator tracking data to compare the annual spatiotemporal migration patterns of a long‐distance migratory songbird, the red‐backed shrike, Lanius collurio, breeding at widely different latitudes within Europe. We find that populations use remarkably similar migration routes and are highly synchronized in time. Additional tracks from populations breeding at the edges of the European range support these similar migration patterns. When comparing timing of breeding and vegetation phenology, as a measure of resource availability across populations, we find that arrival and timing of breeding corresponds to the peak in vegetation greenness at northern latitudes. At lower latitudes birds arrive simultaneously with the more northerly breeding populations, but after the local greenness peak, suggesting that breeding area phenology does not determine the migratory schedule. Rather, timing of migration in red‐backed shrikes may be constrained by events in other parts of the annual cycle