This blog post is provided by Michiel Boom and tells the #StoryBehindThePaper for the paper “Postnatal growth rate varies with latitude in range-expanding geese – the role of plasticity and day length”, which was recently published in the Journal of Animal Ecology.

While some species are clearly struggling to adapt to this rapidly changing world, the barnacle goose seems to be prospering. Contrary to other species, this Arctic migrant has not shifted its breeding area to the north, but has expanded it to the south. While the original breeding areas are located in the Russian Arctic, barnacle geese now also breed in the Baltic region and along the North Sea coast. This exceptional situation offers researchers a unique natural experiment that can be used to answer questions about the adaptability of this species, as well as to study general processes that vary with latitude.

The main reason for the journey to the Arctic, for birds such as the barnacle goose, is reproductive success. Birds fly to the far north to take advantage of the vast amounts of food available there during the summer. Above the Arctic Circle, the birds also experience the benefits of 24 hours of daylight, which means they can forage all day long. The barnacle geese that breed at lower latitudes may have to make do with lower quality food and less daylight for foraging. However, breeding in the Arctic does not only have advantages. A migratory lifestyle also comes with time constraints, and Arctic birds have a relatively short time window to raise their young.

In our study, we looked at whether the goslings of barnacle geese breeding at different latitudes differ in growth rate. Growth rate is an important aspect of a bird’s life cycle, which can have an impact on future survival and breeding success. We found that goslings in the Arctic grew faster than goslings at lower latitudes, and found that these differences can largely be explained by differences in daylight experienced by the goslings. Therefore, it seems that the time goslings have to forage determines how fast they grow. Although goslings grow more slowly at lower latitudes, the adult birds of breeding populations at different latitudes do not differ in size. Despite the slower growth rate, the goslings at lower latitudes are able to reach the same size as geese in the Arctic, and thus have to grow longer. The time pressure experienced by Arctic geese seems to force them to grow quickly in order to leave the breeding area before winter sets in. Breeding in the Arctic therefore not only imposes the need to grow quickly, but also offers the opportunity to do so.

In a comparison between species breeding at different latitudes, we find a similar relationship between growth rate and latitude. In fact, the increase in growth rate with latitude that we find in the barnacle goose is comparable in magnitude to the increase in growth rate in several waders and waterfowl species.

The differences in growth rates between the northernmost and southernmost barnacle goose populations in our study were found to be so large, and have arisen in such relatively short time that it is highly unlikely that this is an evolutionary adaptation. Rather, our results indicate that this is a plastic response to the environment. A flexible growth rate can help birds cope with variation in food availability, but can also play an important role in colonizing new breeding areas, which might help in adapting to a changing environment.