where seabirds find food | 143do they have outward journeys with little feeding, followed by a period
of feasting on plenty, followed by a homeward return?
This chapter will delve into where seabirds actually feed, and why
they choose those feeding places, if indeed they do choose. In fact im-
plicit in much of the earlier chapters is a presumption that researchers
can pinpoint where seabirds actually feed, as opposed to where they
merely pass by. It is therefore timely to pause to mention the types of
evidence used to identify feeding areas. Without robust knowledge of
feeding areas, it is meaningless to ask questions about their characteris-
tics. (See Plate 7 in colour insert.)
If the bird enters a region where its mind turns from travel to food,
its path across the sea is likely to become more sinuous. This could be
particularly true after the successful capture of prey. Then, it makes
sense to continue searching in the same relatively small area; where
there’s one fish, there could well be another. Tracking data, be it from
satellites or GPS devices or geolocators can help identify these favoured
areas. Most simply, they are areas where the bird spends more time, as
compared to areas through which it quickly passes. Such areas can be
pinpointed.
But the details provided by satellite and GPS tracks also allow more
sophisticated identification of such favoured areas. In these areas, the
bird turns more, adopting area- restricted search. This is precisely analo-
gous to someone turning more, in response to cries of ‘You’re getting
warmer’, as she or he homes in on the thimble in a game of Hunt- the-
Thimble. The turning can be measured. So too can first- passage time,
the time required for a bird or indeed any animal to cross a circle with
a given radius. The more its route twists and turns, potentially revealing
a search for food, the longer will be that time.
Another approach to using tracking data to identify feeding areas
can be illustrated by shopping. Suppose, with a heavy heart, you spend
Saturday morning driving into town, first to fill up with petrol, a brief
pause, and then to complete the main weekly grocery shop, a longer
interval in the supermarket, before returning home. The rate at which
your straightline distance from home is changing will be at a minimum,
in fact virtually zero, during the period in the supermarket. Similarly
a bird’s rate of movement away from or towards the colony may reach a