In most bird species, fattening for the first migration does not begin until after the young are fully grown and able to feed themselves efficiently. In some bird species, however, the young fatten while in the nest, courtesy of their parents, and migrate independently within a few days after fledging. They include many seabirds, notably various petrels (Procellariiformes), in which the nestlings reach peak weights that greatly exceed the adult weight. Thereafter, they receive little or no additional food from their parents and lose weight until they leave the nest. The young continue to grow their feathers, but are still extremely fat at the time of their first flight. Although this fattening of seabird nestlings has been known for many years, it has usually been regarded as an insurance to tide the bird over difficult times until it learns to feed itself. However, in many such species the young set off within a few days after fledging on long journeys for which fuel reserves are almost certainly needed.
Take the Manx Shearwater Puffinus puffinus as an example. At their peak, the young weigh around 20% more than the adults, and are extremely fat, from which point they are fed at a much reduced rate. After about 10 further days, the young leave for the sea and thereafter have no further contact with their parents (Harris 1966). Their fat reserves may not only support them during times of hardship and migration, but may also allow their parents to divert food to building their own body reserves and so to migrate earlier than otherwise possible. Some 36 hours after having been ringed on Skokholm Island off Wales, some young shearwaters were recovered several hundred kilometres to the south, in the Bay of Biscay. Within six weeks, many others were recovered in the seas off Brazil, more than 9000 km across the Atlantic Ocean. One was found, an estimated three days after it had died, on the coast of Brazil 16 days after being ringed at the nest. Even if it had left immediately, it made the 9600 km journey at a rate of at least 740 km per day (Perrins et al. 1973). Another young bird was recovered in the Canary Islands, close to the Great Circle route from Skokholm to Brazil, six days after it was ringed on Skokholm, an average of at least 460 km per day. These flights almost certainly required substantial fuel reserves, for they left little time for feeding by such inexperienced birds. The young would gain obvious advantage in being able to fly directly to the wintering area without having to spend time in a perhaps fruitless search for food on the way. Judging from the estimated fat contents of the young, and the likely energy costs of their migration, 'it seems just possible that fat birds could make the 9,000 to 10,000 km flight on their reserves' (Perrins et al. 1973). This is not to imply that the birds would ignore any potential food encountered, but merely that the birds leave as fat as possible in order to make the journey if necessary without further supplies.
All species of petrels that have been studied achieve peak weights that are higher than adult weights, and lose some weight to fledging, but many still leave at above the usual adult weight (Warham 1990). This applies to non-migratory as well as migratory species, but in both groups the young typically travel long distances from the colonies soon after fledging. Their substantial body reserves presumably help to fuel these movements, whether classed as migration or dispersal. Fat reserves would in any case add buoyancy, so could presumably hinder any ability of the young to obtain food by diving for many days following fledging (in those species that feed by diving). In Wilson's Storm Petrel Oceanites oceanicus, chicks in the weight recession phase were found to lose body fat and water (Obst & Nagy 1993), but in the Northern Fulmar Fulmarus glacialis and Cape Gannet Morus capensis the weight loss was apparently due to water alone (Navarro 1992, Phillips & Hamer 1999). In the Northern Fulmar, fledglings had enough fat to stave off starvation for at least 18 days (Phillips & Hamer 1999).
Like petrels, young Northern Gannets Morus bassanus become very obese before fledging, reaching around 30% heavier than adults, and carrying around 1 kg of fat. The young leave the nest before their flight feathers are fully grown, but can glide down to the sea, reaching distances of 3-5 km from the colony (Nelson 1978, Wanless & Okill 1994). Once on the water, they seem unable to take off again, at least for several days, by which time they have lost further weight, and their flight feathers are better grown. They swim long distances in this period; and once they become airborne they can travel even longer distances, and at some later stage begin to hunt for themselves, using the aerial diving technique characteristic of the species. Six young gannets that were caught on the sea in the flightless period were recovered 10-16 days later at distances of 394-2483 km (Wanless & Okill 1994). They had mean rates of travel of 15.1-155.2 km per day, similar to those of many landbirds on migration (Chapter 8). Again, the birds were unlikely to have covered the longer of these distances without substantial fuel reserves.
The young of some landbird species may also migrate immediately they leave the nest, and without further parental support. An example is the Common Swift Apus apus in which the young also fly at weights that often exceed those of adults (Lack 1956). One individual was recovered in Madrid three days after it left the nest in Oxford, a journey of 1300 km in three days (C. M. Perrins, in Wernham et al. 2002). Immediate post-fledging migration could occur in other landbird species, as well as in other seabirds, but further research is needed. It is clear, however, that migratory fat deposition occurs not just in full-grown birds, but in some is an integral component of nestling growth.
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