Northern gannet in flight. Photo credit: David Grémillet

Northern gannet in flight. Photo credit: David Grémillet

Soaring above the ocean, gannet parents embark on extensive foraging odysseys to find food for their young. For the parents, the voyage is an exercise in balancing the energy that they expend locating food against the rewards of returning with a full belly for their chicks. ‘They try to expend as little energy as possible’, explains Françoise Amélineau, from the CNRS Centre d'Ecologie Fonctionnelle et Evolutive in France. According to Amélineau and her colleagues, it has long been presumed that foraging birds expend less energy commuting to their foraging grounds than they do when wheeling above a shoal of fish. However, no one had directly measured the birds' energy expenditure in the wild because it was technically too challenging: that was until a new method that estimated energy expenditure based on the birds' acceleration was developed. Amélineau and her colleagues decided to attach accelerometers combined with GPS trackers and depth metres to gannets at their nesting site off the northern coast of France to find out which portions of a gannet foraging expedition cost the birds dear (p. 876).

During several trips to the Rouzic Island nature reserve in consecutive breeding seasons, Clara Péron, Amélie Lescröel, Pascal Provost and David Grémillet successfully tagged almost 50 birds before the animals headed north to their fishing grounds in The English Channel. ‘It is quite difficult to catch the gannets, especially the second time, because they have already been captured once so they are more afraid’, explains Amélineau. Fortunately, the team successfully downloaded data from 35 of the retrieved loggers, ready to find out which part of the expedition had been most arduous.

However, before they tackled the analysis, the team realised that there was another factor that they must consider. ‘We thought maybe the wind was an important factor affecting the cost of movement,’ explains Amélineau. Downloading wind speed and direction data from the Laboratory of Oceanography From Space portal for the area covered by the birds, the team was able to include the impact that the prevailing wind had on the gannets' voyage.

Converting each bird's acceleration plot into an estimate of the amount of energy expended at each stage of the journey and comparing it with the path taken at the time, the team could see that the birds used more energy as they twisted and turned over a foraging site than they did during the direct journeys there and back. And when the scientists factored in the impact of the wind direction, they found that the birds used more energy when flying into a head wind than when assisted by a tail wind.

However, the team were surprised to find that instead of taking advantage of a tail wind to assist during their heavily laden return, the Rouzic Island gannets were assisted by the wind on the outbound leg as they headed north, leaving them to battle the elements on their return. Explaining that theory suggests that birds prefer to return home using a tail wind, the team points out that the Rouzic Island gannets are forced to head north from their island home by the location of their prey, pitching them against the prevailing wind on the journey home.

So, Amélineau and colleagues have confirmed that taking tortuous routes over foraging grounds is more costly than flying along direct paths. She adds that it is important to understand the energetic costs of foraging for food to better understand an animal's ecology and the potential impact that climate change might have on them.

Windscape and tortuosity shape the flight costs of northern gannets
J. Exp. Biol.