The prelaying exodus of petrels

A different kind of 'within-season' movement is undertaken by some pelagic seabirds. Once they have returned to breeding areas, re-occupied nest-sites and re-established pair bonds, many procellariiform species leave their breeding areas for periods of days or weeks to feed up for egg production and incubation. In the process, they may travel to foraging areas hundreds or thousands of kilometres from the nesting colonies. For example, Manx Shearwaters Puffinus puffinus from Skokholm Island off Wales fly southwest to the Bay of Biscay off Spain during this two-week period, a major foraging area some 700 km from the colony (Perrins & Brooke 1976), while White-chinned Petrels Procellaria aequinoctialis nesting on South Georgia fly 2000 km northwest to the Patagonian Shelf off central Argentina, which is also a major wintering area for the same birds (Phillips et al. 2006). We must assume that any body reserves brought to the colony by birds on first arrival have largely gone by the time of egg-laying, making it necessary to accumulate fresh reserves then, and that it is more economic to fly long distances to rich feeding areas than to attempt to accumulate reserves in the vicinity of the colonies at that time. Both sexes need reserves to get back to the colony; the female also to produce the single large egg, and the male to undertake the first major incubation stint.

During the rest of the breeding cycle, birds tend to feed closer to the colony. This is shown mainly by the duration of their return foraging trips and in some species by satellite-based radio-tracking (for White-chinned Petrel Procellaria aequinoctialis see Phillips et al. 2006).

In some procellariiform species, both sexes participate in the pre-laying exodus, travelling independently of one another, but females are often away for longer than males. In other species, the journeys are made mainly or only by the females. The period away varies from a few days to several weeks, depending on the species and the distances involved (Warham 1990). The females apparently form the egg while away and usually lay within a day or two after their return, whether the male is back or not. This confirms that fertilisation occurred in the pre-exodus period, up to several weeks earlier, the sperm being kept in special utero-vaginal storage glands. The birds from different nesting sites may head for different feeding areas, which can lie in various directions from the colony. Those feeding areas that have been identified lie in a customary migration or wintering area. Almost certainly, then, these rich feeding areas are learnt from previous wanderings. Nevertheless, it is amazing that birds may return from spring migration, remain a few weeks in their nesting places, and then migrate back to staging or wintering areas, before returning to lay an egg and proceed with the nesting attempt. Although widely known among petrels, a prolonged pre-laying exodus may occur chiefly among individuals nesting in areas remote from rich feeding grounds. Only later in the summer may food become abundant locally.

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