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Seasonal geophysical cycles strongly influence the activity of life on Earth because they affect environmental conditions like temperature, precipitation and day length. An increase in daylight availability during summer is especially enhanced when animals migrate along a latitudinal gradient. Yet, the question of how day length (i.e. daylight availability) influences the activity patterns of long-distance, latitudinal migrants is still unclear. Here, we ask whether migration provides benefits to long-distance migrants by enabling them to increase their diurnal movement activities due to an increase in daylight availability. To answer this question, we tested whether four vastly different species of long-distance migratory birds-two arctic migrants and two mid-latitude migrants-can capitalise on day length changes by adjusting their daily activity. We quantified the relationship between daily activity (measured using accelerometer data) and day length, and estimated each species' daily activity patterns. In addition, we evaluated the role of day length as an ultimate driver of bird migration. All four species exhibited longer activity periods during days with more daylight hours, showing a strong positive relationship between total daily activity and day length. The slope of this relationship varied between the different species, with activity increasing 1.5-fold on average when migrating from wintering to breeding grounds. Underlying mechanisms of these relationships reveal two distinct patterns of daily activity. Flying foragers showed increasing activity patterns, that is, their daytime activities rose uniformly up to solar noon and decreased until dusk, thereby exhibiting a season-specific activity slope. In contrast, ground foragers showed a constant activity pattern, whereby they immediately increased their activity to a certain level and maintained this level throughout the day. Our study reveals that long days allow birds to prolong their activity and increase their total daily activity. These findings highlight that daylight availability could be an additional ultimate cause of bird migration and act as a selective agent for the evolution of migration.Los ciclos geofísicos estacionales influyen fuertemente la actividad de la vida en la Tierra ya que afectan diversas condiciones ambientales como la temperatura, la precipitación y la duración del día. El aumento de la disponibilidad de luz solar durante el verano favorece especialmente a las especies que migran a lo largo de un gradiente latitudinal. Sin embargo, el efecto de la duración del día (es decir, la disponibilidad de luz solar) en los patrones de actividad de las especies que migran latitudinalmente largas distancias, aún no es claro. Aquí nos preguntamos si un aumento de la disponibilidad de luz solar representa un beneficio para los animales que migran largas distancias, al prolongar el periodo de actividad diurna. Para responder a esta pregunta, investigamos si cuatro especies diferentes de aves migratorias de larga distancia, dos migrantes árticos y dos migrantes de latitudes medias, pueden ajustar su actividad diaria y aprovechar los cambios en la duración del día. Cuantificamos la relación entre la actividad diaria (medida con datos de acelerómetro) y la duración del día, y estimamos los patrones de actividad de cada especie. Además, evaluamos el papel de la duración del día como mecanismo último subyacente a la migración de las aves. Las cuatro especies mostraron períodos de actividad más largos durante los días con más horas de luz, mostrando una fuerte relación positiva entre la actividad diaria total y la duración del día. Esta relación varió entre las especies evaluadas. La actividad aumentó en promedio 1.5 veces durante la migración desde las áreas de invernada a las áreas de reproducción. Los mecanismos subyacentes a estas relaciones revelan dos patrones de actividad diaria. Las especies que forrajean en vuelo mostraron un aumento en su patrón de actividad. En este caso, la actividad diurna aumentó uniformemente hasta el mediodía y disminuyó hasta el atardecer, mostrando una pendiente de actividad específica para la estación. De otro lado, las especies que forrajean en tierra mostraron un patrón de actividad constante. Según este patrón, la actividad diurna aumenta hasta un determinado nivel, a partir del cual se mantiene durante el resto del día. Nuestro estudio revela que el aumento en la longitud del día le permite a las aves prolongar su actividad e incrementar su actividad diaria total. Estos resultados señalan que la disponibilidad de luz diurna podría ser otro mecanismo último subyacente a la migración de las aves y puede actuar como un factor de selección en la evolución de la migración.

Aggressive behavioural interactions between animals are widespread in nature, with ecological and evolutionary consequences of such interactions reported for both individuals giving and receiving aggression. Yet despite the importance of aggressive interactions in determining social dominance and conferring fitness benefits to successful individuals, we lack a general understanding of the conditions that influence the variation in agonistic behaviours among individuals and species. We conducted a global meta-analysis of published time–activity budgets, using a data set comprising 555 values from 88 studies, to determine variation in the time that waterbirds engaged in aggressive interactions. The mean ± SD percentage of time devoted to aggression was 2.0 ± 3.6%, with a range of 0.0–35.0%. We used our data set to test four predictions regarding avian aggression, based on the findings of earlier site-based studies. We predicted that the time spent on aggression would be influenced by four factors: (1) age class, (2) sex, (3) seasonal timing and (4) functional feeding group. A comparison of linear mixed-effects models using an information theoretic approach indicated that the proportion of time waterbirds spent engaged in aggressive interactions was best explained by the age class and sex of the focal waterbird species. More time was spent on aggression by males than females, and by adults than juveniles. We found no evidence that the time spent on aggressive behaviours varied across latitudes or body mass, with seasonal timing, sexual dimorphism, migration or breeding strategies, or between different functional feeding groups. Our findings highlight the high levels of variation in the time devoted to aggression across species, feeding groups, latitudes and seasons. Furthermore, our study demonstrates the utility of the numerous published time–activity budget studies that are available as a valuable source of data that can be used to answer broad-scale questions regarding animal behaviour.

BackgroundFor the conservation and management of migratory species that strongly decrease or increase due to anthropological impacts, a clear delineation of populations and quantification of possible mixing (migratory connectivity) is crucial. Usually, population exchange in migratory species is only studied in breeding or wintering sites, but we considered the whole annual cycle in order to determine important stages and sites for population mixing in an Arctic migrant.MethodsWe used 91 high resolution GPS tracks of Western Palearctic greater white-fronted geese (Anser A. albifrons) from the North Sea and Pannonic populations to extract details of where and when populations overlapped and exchange was possible. Overlap areas were calculated as dynamic Brownian bridges of stopover, nest and moulting sites.ResultsUtilisation areas of the two populations overlapped only somewhat during spring and autumn migration stopovers, but much during moult. During this stage, non-breeders and failed breeders of the North Sea population intermixed with geese from the Pannonic population in the Pyasina delta on Taimyr peninsula. The timing of use of overlap areas was highly consistent between populations, making exchange possible. Two of our tracked geese switched from the North Sea population flyway to the Pannonic flyway during moult on Taimyr peninsula or early during the subsequent autumn migration. Because we could follow one of them during the next year, where it stayed in the Pannonic flyway, we suggest that the exchange was long-term or permanent.ConclusionsWe have identified long-distance moult migration of failed or non-breeders as a key phenomenon creating overlap between two flyway populations of geese. This supports the notion of previously suggested population exchange and migratory connectivity, but outside of classically suggested wintering or breeding sites. Our results call for consideration of moult migration and population exchange in conservation and management of our greater white-fronted geese as well as other waterfowl populations.

According to migration theory and several empirical studies, long-distance migrants are more time-limited during spring migration and should therefore migrate faster in spring than in autumn. Competition for the best breeding sites is supposed to be the main driver, but timing of migration is often also influenced by environmental factors such as food availability and wind conditions. Using GPS tags, we tracked 65 greater white-fronted geese Anser albifrons migrating between western Europe and the Russian Arctic during spring and autumn migration over six different years. Contrary to theory, our birds took considerably longer for spring migration (83 days) than autumn migration (42 days). This difference in duration was mainly determined by time spent at stopovers. Timing and space use during migration suggest that the birds were using different strategies in the two seasons: In spring they spread out in a wide front to acquire extra energy stores in many successive stopover sites (to fuel capital breeding), which is in accordance with previous results that white-fronted geese follow the green wave of spring growth. In autumn they filled up their stores close to the breeding grounds and waited for supportive wind conditions to quickly move to their wintering grounds. Selection for supportive winds was stronger in autumn, when general wind conditions were less favourable than in spring, leading to similar flight speeds in the two seasons. In combination with less stopover time in autumn this led to faster autumn than spring migration. White-fronted geese thus differ from theory that spring migration is faster than autumn migration. We expect our findings of different decision rules between the two migratory seasons to apply more generally, in particular in large birds in which capital breeding is common, and in birds that meet other environmental conditions along their migration route in autumn than in spring. published