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lördag 16 februari 2013

Under-water and over-water foraging behaviour of a deep diving seabird

Common guillemots (N. Am. common murres, Sv. sillgrisslor, Uria aalge) are remarkable in their abilities to live at the interface between land, sea, and sky. During the breeding period, in pairs they raise a single chick on a rocky ledge, with each parent making many foraging trips out to sea to catch fish to feed their chick. A typical foraging trip may consist of a flight of over 10 km, followed by sequences of exceptionally deep dives to over 50 m water depths, followed by a flight back to the colony carrying a single fish in their bill for their hungry chick! Following this behaviour in detail has been challenging. Devices must withstand the great water pressure found deep down. Device weights and dimensions must be kept low to avoid impairing the guillemots' flight and diving too greatly. Then to observe these remarkably both aerial and aquatic birds, one must both track their geographical positions and their vertical movements underwater.
Fig. 1. GPS tagged guillemot brooding its chick (not visible) with partner to left.

In a new study (Evans et al. 2013) just published in Marine Ecology Progress Series, we give a detailed account of the foraging behaviour of guillemots breeding at the Swedish Baltic Sea island of Stora Karlsö, located off the west coast of Gotland. We used small GPS loggers in conjunction with time-depth-recorders (TDRs), allowing the tracking of both the guillemots' foraging flights and their remarkable dives.
We found a strong temporal pattern in activity (see figure). Foraging trips overnight were of longer duration than daytime foraging trips, this corresponded with crepuscular diving activity, with higher diving frequency around dawn and dusk, and reduced dive depths at these times. This likely corresponding to prey availability, with the birds' main prey species, herring (sv. sill, Clupea harengus) and sprat (sv. skarpsill, Sprattus sprattus) displaying a diel vertical migration, moving to surface waters at night. The longer duration overnight trips may allow the guillemots to fly out to a good foraging site in the evening, with some 'self-provisioning' (feeding for the adult) in the evening, followed at dawn by foraging for fish to return to the chick.

Fig. 2. An example trip from Figure 2 in the paper. By combining GPS and time-depth-recorders (TDR) we gained very detailed insights into the guillemots' foraging behaviour. (A) GPS track with different activities coloured (orange -flight, blue - water surface resting, purple - diving periods), (B) Diving, (C) Wet or dry state of TDR tag, (D) distance from nest, (E) GPS recorded speed.
Flight behaviour appeared to be strongly affected by winds, with outward flights having much greater ground-speeds (the speed that the murre moves relative to the land surface, cf. airspeed, the speed relative to the air) than the return inward flights. This pattern most likely owing to outward flights being aided by tail winds and inward flights by head winds. Most of these flights were along the same axis as the wind direction (i.e. either head- or tail-winds, but not side-winds), this may reflect a strategy to reduce wind-drift. For over the sea detection of wind drift is difficult, with the lack of fixed land marks. As with head- or tail-winds there will likely be lateral drift.
Finally foraging intensity, as measured by number of dives per a trip, and the number of dives per a diving bout (dives occur in sequences with short inter-dive interval), measure for Stora Karlsö guillemots was lower than that found in studies at other colonies, suggesting good foraging conditions.

This study was a collaboration between CAnMove researchers, Tom Evans and Susanne Åkesson, and researchers at the Stockholm Resilience Centre of Stockholm University; Olof Olsson and Martina Kadin.

Evans TJ, Kadin M, Olsson O, Åkesson S (2013) Foraging behaviour of common murres in the Baltic Sea, recorded by simultaneous attachment of GPS and time-depth recorder devices. Mar Ecol Prog Ser 475:277–289.

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