By Ruth Dunn
Lots of seabird species spend large portions of their lives at sea. Despite this, we know relatively little about where many seabirds go, what they get up to and how they survive being thrown around by cold waves, under long periods of darkness, for weeks on end. Instead, the majority of our knowledge of seabird ecology is currently informed by what we have been able to observe from within seabird colonies during their breeding seasons.
For decades ecologists have joined the seabirds that we study in spending our summers at their breeding colonies, binoculars looped around our necks and notebooks in our hands. During one such annual trip, my colleagues from the UK Centre for Ecology and Hydrology sat on top of the cliffs of the Isle of May. Here, they attached small data loggers to rings around the legs of some of the common guillemots that breed on this Scottish island. We left the loggers for a year and retrieved them when birds that survived the annual cycle returned to the Isle of May the following breeding season.
During the year of data collection, many of the Isle of May's guillemot population did not survive to return to the island, with many others failing to breed successfully during the subsequent breeding season. We were therefore particularly interested in how the birds that did survive to return to the island, equipped with their data loggers, managed to do so.
The loggers recorded detailed data on the temperature and water depth experienced by each individual bird. I have spent a lot of time over the last year working with these data to estimate how much time each guillemot spent at the colony, flying, diving, swimming and resting on the water each day. Next, I calculated the daily energy expenditure of the guillemots throughout the entire annual cycle – something that has never been done before for this species.
We had thought that during the winter months, life might be tough for a North Sea guillemot. They experience reduced day lengths, cooler temperatures and the prospect of storms interrupting their foraging. Despite these potential constraints, we found that the guillemots that we gathered data from had adjusted their behaviour to ensure that their daily energy expenditure was relatively steady throughout the annual cycle. We identified different mechanisms that might have helped them to do this.
Firstly, whilst albatrosses are able to efficiently soar and glide with the wind, the high-speed flapping flight that guillemots perform to stay airborne is very energetically costly for them. Because of this we found that the guillemots reduced how much they flew during the majority of the annual cycle.
Additionally, whilst their small wings don’t help their flying capabilities, they are well adapted for underwater-flapping. This means that, like penguins, guillemots are excellent divers. Using their wings to propel them, it is thought that guillemots visually seek out the fish that they capture to eat whilst diving. Despite this, we found that they dived a lot during the night, potentially adapting their behaviour to mirror that of the availability of prey.
Guillemot energy expenditure did increase as the breeding season neared and was also high during the breeding season itself, as the birds spent progressively more time at the Isle of May. Heightened energy expenditure was largely driven by increases in flight activity, with guillemots needing to regularly fly between the colony and their marine feeding grounds.
Whilst I am unlikely to be returning to a seabird colony this summer, I am sparing a thought for the birds that will be, by adjusting their behaviour and managing their energy expenditure in order to survive the annual cycle.
This work has recently been pubilshed Open Access in Scientific Reports. Access it here:
Ruth E Dunn, Sarah Wanless, Francis Daunt, Mike P Harris & Jonathan A Green (2020). A year in the life of a North Atlantic seabird: behavioural and energetic adjustments during the annual cycle. Scientific Reports. 10, 5993. https://doi.org/10.1038/s41598-020-62842-x.