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Winter survival strategies in seabirds

Internationalisation Fellowship | 17/01/2017

Photo: Carsten Egevang Photography

Winter survival strategies in seabirds

The aquatic environment is changing continuously. These changes are being accelerated by the combined effects of climate, environmental change and anthropogenic influences, and are affecting trophic interactions by altering primary productivity, species abundance and species phenology. Seabirds in the auk family inhabit Polar regions during breeding, non-breeding, or both periods. Compared to more temporal regions, polar regions are facing rapid climate changes and increasing anthropogenic changes, making it increasingly important to acquire a thorough understanding of seabird ecology such as survival strategies, foraging behaviour, spatial distribution, predator-prey interactions and intra- and interspecific relationships.

With recent advances in technology, it is now possible to gain unprecedented insight into seabird behaviour at sea. The aim of this study, supported by the Carlsberg Foundation, was to investigate how diving birds cope with the harsh weather and low food availability they experience during the winter, by mapping their migration routes and examining their survival strategies. I primarily focused on auks (in this case, the thick-billed murre). Auks are some of the most aquatically adapted species of birds in the northern hemisphere. Furthermore, being one of the most ecologically important species in the Arctic, this knowledge will have profound public relevance, and aid in the prioritisation of ecosystem management as well as conservation strategies.

Surviving the winter

Aquatic environments in the temperate, boreal and arctic climate zones are highly seasonal in terms of biological productivity. Many aquatic organisms (whose internal temperatures vary widely) survive the winter, when food availability is low, by reducing their activity levels and/or performing vertical migrations to deep cold waters where energy consumption is low. This causes potential problems for predators (with stable body temperatures), which require constant access to nutritious prey to survive. Top predators, such as diving birds, are particularly vulnerable to periods of low prey availability, as they are relatively small and unable to store significant amounts of fat. Among diving birds, many species resolve this by migrating to warmer waters, where prey items are more active and thus more accessible.

Jannie Linnebjerg explains: “It has remained a bit of a mystery how the large numbers of seabirds and other aquatic birds breeding at high latitudes manage to access sufficient prey during the winter season and thus maintain their body condition. In this part of the project, we tried to answer this question by examining year-round individual movement behaviour and energy use of thick-billed murres breeding in the high Arctic. More specifically, we looked into individual patterns of activity exhibited by thick-billed murres breeding in Greenland, and how the energy used differ with respect to the stage of the breeding/migration cycle, and how this is linked to the distribution of available prey (Fig 1)”.

Figure 1: The thick-billed murre dives much deeper during the winter than during the summer. This is a response to the distribution of the prey. The deepest dive recorded in this study was on the 26th of February (date (in red) and time on the x-axis), where one bird dived down to 191 meters (y-axis).

The best migration strategy

Migration is very costly in terms of time and energy. Birds have therefore developed different strategies to minimize the cost of their migratory journey. The most common migration strategy consists of repeated cycles, where a few days of foraging (stopover sites) are followed by flight (until the destination is reached). However, if food is readily available on the migration path, the most favourable strategy in terms of both time and energy is the fly-and-forage strategy.

“Movement by flight is the most costly form of locomotion for the thick-billed murres, but several other factors also influence the migration of the thick-billed murre”, Jannie Linnebjerg explains.

Males have their movement restricted by having to take care of the flightless and slow swimming chick. Thus they only have two options after the breeding season has ended. They can stay in the local area until the chick is independent and able to fly or they can swim-migrate with the chick.

Females on the other hand have three options. They can stay close to the colony until the moult is over, they can swim-migrate or they can fly away from the breeding area to another favourable area to moult.

Thick-billed murres have a very unique breeding strategy (shared with common murres and razorbills) in which chicks leave the colony accompanied by the male parent when about one quarter of adult size and unable to fly. The duration of parental care for thick-billed murres after leaving the colony has until now been poorly known, but findings from this project estimate the post-fledging parental care period to be 37 ± 2 days.[1] 

Another important factor affecting the onset of migration of murres is moult. The adult birds are flightless during the entire moulting period (~2 months), which takes place soon after the breeding season, and for males this occur during parental care at sea. Because males do all the parental care at sea, males and females have different constraints. The main aim of this part of the projected was to find out how these different restrictions affected male and female movement behaviour and thus how they manage migration (Fig 2).

Figure 2. The map shows the migration route and overwintering areas of one female thick-billed murre. The non-breeding period has been divided into different categories (depicted by the different colours). The table shows how much time the bird spent flying, resting and foraging during these different periods.

Relevance, impact and cultural aspects of the project

The number of seabirds around the world has declined dramatically during recent decades, and a high percentage of the world’s pelagic seabird populations are threatened. The major threats at sea are commercial fisheries (via competition and bycatch), pollution and hunting, but also climate change altering the availability of prey. 

In the Arctic, the thick-billed murre, one of the most abundant, ecologically important and popular seabird species to hunt, is in serious decline. Data from a new study [2] on the migration patterns of thick-billed murres in the North Atlantic (where my involvement was supported by the Carlsberg Foundation) has shown a clear relationship between the over-wintering areas and population trends. The populations overwintering in Greenlandic waters or in areas east of Greenland have shown a dramatic decline at the breeding colonies (Svalbard, Iceland and large parts of Greenland). On the contrary, breeding populations in Canada and Northwest Greenland are stable, and, interestingly, these birds overwinter primarily in Canadian waters. 

This pattern suggests that conditions in the different wintering areas play a crucial role in the population decline. This information is of great interest to the management agencies concerned with conservation and/or the use of living resources, as it can aid when planning and implementing management of this species.

The grant from the Carlsberg Foundation has allowed me to spend two years at the Centre for Animal Movement Research (CAnMove) in Lund, Sweden. The aim of CAnMove is to facilitate trans-disciplinary research and promote basic and applied science to study the evolutionary causes and consequences of animal movement across scales. Working amongst the world’s leading animal movement ecologists has truly allowed me to take my research career to the next level.

References

[1] Elliot KH, Linnebjerg JF, Burke C, Gaston AJ, Mosbech A, Frederiksen M and Merkel FR (in revision) Growth, not mortality, drives parental care strategy in auks. American Naturalist.

[2] Frederiksen M, Descamps S, Erikstad KE, Gaston AJ, Gilchrist HG, Johansen KL, Kolbeinsson Y, Linnebjerg JF, Mallory ML, McFarlane Tranquilla LA, Merkel FR, Montevecchi WA, Mosbech A, Reiertsen TK, Robertson GJ, Steen H, Strøm H & Thórarinsson TL (2016) Migratory connectivity of a declining seabird on an ocean basin scale: conservation implications. Biological Conservation 200:26-35.