LIFE is tough for songbirds just now.
They need extra body fat, heating fuel, to get them through the long cold winter nights. A bird may increase its weight by 10% during the day, burn off the extra fat in the hours of darkness and be back to normal weight by morning. Finding enough food for such an enterprise is challenging. Berries, fruit and nuts are disappearing fast. Frozen cold ground is hard and impenetrable; rooting for worms and creepy-crawlies is a labour-intensive business. A snowfall can hide what little food there is, open water may freeze and birds may die of thirst. There are few daylight hours in which to find it.
Predation risk and the cost of being fat, by Gosler, Greenwood and Perrins, appeared in Nature in 1995. It claimed the weights of great tits in Britain increased significantly during the period when birds of prey had been wiped out by pesticides. With no predators to target them, birds could afford to become fat and sluggish. When the pesticides were banned, the predatory birds returned and the great tits became lighter again. Fat ones had become ‘sitting ducks’ for sparrowhawks. They had to slim down and make themselves manoeuvrable to survive.
Chancing on a late crop of berries or a well stocked garden feeder, a bird may be tempted to ‘make hay while the sun shines’ and gorge itself. Some do. A waxwing in Norfolk, for instance, ate 390 berries, twice its own weight, in two and a half hours. But such behaviour, as the great tit weight changes showed, may be unwise. So what should small birds do? If they eat every morsel they find, they risk ending up in the talons of a hawk. Passing up feeding opportunities, on the other hand, could lead to hypothermia during the night. Could there be a third option? A paper just published in the Royal Society’s journal Biology Letters claims there is.
The birds of Wytham Woods in Oxfordshire are among the most studied in the world. They have been ringed and monitored continuously for over half a century. Damien Farine and Stephen Lang fitted passive identification tags (PITs) to tits and nuthatches there. These devices, also used with pets and livestock, enable a bird’s identity to be recoded electronically. Next, the researchers chose 18 potential feeding stations, at least 150m from each other, at Whytham. Each morning, feeders and PIT reading devices were placed at three of the locations. In the afternoon, three further locations were selected. The identities of the birds arriving at the stations were logged. Two weeks later, the experiment was repeated but with the morning and afternoon locations reversed.
A curious pattern emerged. Birds which discovered a feeding station in the morning seldom stayed for long but moved on, showing up at other stations. In the afternoon however, there were fewer ‘new’ birds arriving at the stations; those visiting tended to be ones which had been there in the morning. There was ‘a clear burst of relocations, approximately two hours before dusk’, the paper notes.
The researchers believe that, in the morning, birds eat only enough for their immediate needs. By doing so, they remain light and agile when searching for sources of food, reducing their risk of being taken by a hawk. In the afternoon, however, they must take on extra food to sustain them through the night, so they return to the locations they know from their morning reconnaissance that food will be available. Knowing where to go in the evening reduces travel distances and the amount of flying to be done when a bird is heavy and vulnerable to a predator. Thrifty shoppers visit all the local supermarkets to discover where items are cheapest and return later to avail of the best offers. It seems that winter birds do the same.
* The early bird gets the worm: foraging strategies of wild songbirds lead to the early discovery of food sources, Damien Farine and Stephen Lang, Biology Letters 9. 2013.
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