The elusive and unique swimming migration of young guillemots

Submitted by Michi on 18 October 2023.

Chick and father Brünnich’s guillemots departing the breeding colony

This blogpost is based on the article by Merkel and Strøm about what happens with young guillemots when they leave the cliff and swim out to sea with their fathers.

In seabirds, we can observe a wide variety of parental care strategies, with the young of some species leaving the nest shortly after hatching, while others stay in the nest until they are full size and able to fly. Three species of auk (common guillemot Uria aalge, Brünnich’s guillemot Uria lomvia and razorbill Alca torda) on the other hand have a unique third breeding strategy somewhere in between these. Their young leave the colony flightless, at only a quarter of adult body size, accompanied by the father and later become independent out at sea. This gives rise to the famous natural history phenomenon of tiny chicks leaping off steep cliffs and down sometimes hundreds of meters to swim out to sea with their fathers.

        The breeding cliffs of the study colony on Bjørnøya.

This second part of the rearing period out at sea has been a mystery for the longest time as observational data are hard to come by in the open ocean. In this study we tested several hypotheses about this elusive period, often referred to as a swimming migration as the fathers with their young, that are unable to fly, have to swim. We equipped 34 chicks of common guillemots and Brünnich’s guillemots breeding at Bjørnøya, a major colony in the Svalbard archipelago in the European Arctic, with satellite transmitters to track their swimming migration after they left the colony.

    Common and Brünnich’s Guillemots on the cliffs at the study site on Bjørnøya.

All young, presumably accompanied by their fathers, swam actively, rather than passively drifting away from the colony with currents, as previously hypothesised. They swam fastest during the first two days after leaving the colony. This coincided with the only time they actively crossed a major current and the time needed to leave the area of prey depletion around the colony. They otherwise took advantage of available currents, while still swimming actively during the remainder of their migration towards previously known autumn areas used by adult birds of the same species. These regions correspond to areas used by adult birds during their moulting period (when they renew their flight feathers), rather than being specific nursery areas only used by young (as previously hypothesised).

The Brünnich’s Guillemot’s moved about 250km away from the colony in around 14 days after their jump off the cliffs, whilst the Common Guillemots moved up to 650km in a swimming migration that took around 29 days. The young birds moved at a speed of around 1km per hour, and the Common Guillemots covered an average of 22km per day. Migration to these autumn areas took only a fraction of previously reported fledging periods out at sea (suggested in previous studies). This indicates that not only the swimming migration, but also the known adult autumn staging areas constitute in effect breeding areas.

       Map illustrating the journey of the Brünnich’s (in blue) and common guillemots (in red), from the breeding colony on Bjørnøya.

This work has important implications for our understanding of population dynamics within the Alcini subfamily and the management of these species under multiple threats, while providing the foundation to investigate swimming migrations across their entire species range.

There is a nice video of a Guillemot chick jumping from a cliff in Scotland that shows the incredible behaviour that leads into the swimming migration at the centre of the study by Merkel and Strøm.

TEXT BY BEN MERKEL AND PHOTOS BY HALLVARD STRØM.

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