Despite the fact that we enjoyed a couple of summer-like days earlier this week, there is no escaping the fact that autumn is upon us. The telltale signs are there; browning of the bracken, the scent of mushrooms in the woods and a change in bird behaviour. For many birds, the time has arrived to start out on an autumn migration that will take them south from our shores, into Europe and beyond into the dark heart of Africa. In most species, this is not a journey upon which they can embark without first making some important preparations. Flight requires fuel and small birds, like migrant warblers, must take on board the fuel necessary for at least part of the journey, perhaps stopping en route to top up their reserves. Only those that can feed on the wing, such as swallows and swifts, can avoid having to lay down large reserves of fat in advance.
Many of our small migratory birds will add to their fat reserves very quickly, perhaps adding between 10 and 13pc of their body weight each day during this period of fattening. Some of the increase comes from eating more food, with birds making the most of autumn’s bounty. At the same time, many species make better use of the food they ingest, either through changes in their metabolism or by selecting foods that are easy to digest. Many fruits are ideal; low in fibre, abundant at this time of year and easy to digest, they also tend to be rich in sugars (which help in the deposition of fat). Hedgerow shrubs heavy with berries will be well-attended by warblers and thrushes over the coming weeks. Other species, such as the sedge warbler, feed selectively on aphids, a behaviour that may be related to the relatively high sugar content of these invertebrates.
Bird ringers catching birds at this time of the year often comment on just how fat some of these small birds can become. While a species like a garden warbler would normally weigh about 17g, just prior to migration it may reach 34g, raising the question of how it manages to get off the ground. This highlights the trade-off that a migrating bird has to make. The more fuel it takes on board, the heavier it becomes and the slower it flies (making it a target for predators). There must be an ideal fuel load for the journey, something that will depend on how far the bird has to go, whether it can refuel en route and how likely it is to meet a predator. It is a good job that things are less complicated when we go on holiday!