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Foraging movements and migration in auks

During the chick rearing period, how do razorbills (Alca torda) provision their chicks; where do they forage, and how deep do they dive? Why do razorbills breed later than conspecific common guillemots (Uria aalge)? Also, how do data loggers affect behaviour in alcid species? These are some of the questions we hope to help answer with our study at the beautiful Baltic island of Stora Karlsö, in collaboration with researchers from the Baltic Seabird Project.

Combining data from guillemots and razorbills will allow a comparison of the foraging behavioural ecology between the two species. This may help answer an outstanding question of Baltic Sea ecology around Stora Karlsö; why do razorbills breed later than guillemots? It does not appear to be due to different diets, as previous studies at the island suggest both species feed on clupeid species, both herring (Clupea harengus)and predominantly sprat (Sprattus sprattus).

Diving and flight abilities

Flying auk
Flying razorbill. Photo Tom Evans
A potential answer is that the two species differ in their diving and flight abilities. A recent study suggested that since razorbills have lower wing loading than guillemots, they follow a different foraging strategy. Razorbills are travelling further, but diving less deeply while guillemots fly shorter distances due to their costly flight, but are able to dive more deeply and for a greater duration since their smaller wing area is beneficial fo diving. It will be interesting to see whether a similar difference in foraging strategies between the two species is apparent in the Baltic.

GPS trackers and time-depth-recorders

GPS tracker (let) and TDR (right).
GPS tracker (left) and TDR (right). Photo: Tom Evans

Using GPS trackers and TDR, time-depth-recorder devices, we plan to follow razorbills’ behaviour during the chick rearing period. These devices will provide both spatial data on where and when birds are foraging, and detailed information on diving activity, including diving durations and depths. Together this information will provide a detailed picture of spatio-temporal foraging ecology in the species.

Affect on behaviour?

Razorbills. Photo: Tom Evans
Razorbills. Photo: Tom Evans

An important consideration in any study of wild animals is; does the method of observation affect the animal’s behaviour? If one wishes to investigate the natural behaviour of a species it is important that any observation effects are small. Several studies have looked at whether devices attached to seabirds affect their behaviour, however most of these have used fairly insensitive measures, such as breeding success; few have looked in detail at behaviour, and none on alcid species.

On a nestbox. Photo:Tom Evans
On a nestbox. Photo: Tom Evans

In this study, as two devices are used, the TDR and the GPS, we plan to attach only TDR devices to one group, and TDR with GPS to the other. With the TDR devices recording both temperature and pressure data (for dive depth) it is possible to broadly recognise behaviour types; flight, water surface resting, diving, and presence at the nest. Thus it is possible to compare the proportion of time spent on these behaviours between the two groups, and also fine scale diving measures, such as maximum dive depths, and the number of dives made in a diving bout.

Device on razorbill
Photo: Tom Evans

Our plan is to deploy the TDR devices throughout the year; being small and attached to a leg ring this is possible. This will allow behaviour to be compared across the year. Especially interesting is the moult-period. For the first stage of the moult birds are flightless for several weeks, with reduced wing area from lost primary wing feathers. Measuring diving activity in this period could throw-light on trade-offs between diving and flight - fascinating as these birds are highly specialised in diving, able to dive to depths over 100 meters, and yet they can still fly; better than penguins!

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Centre for Animal Movement Research
Evolutionary Ecology, Department of Biology
Ecology building S-223 62 Lund Sweden