Birds all over the world are facing ?a scramble to keep up with spring,? says leading bird biologist Don Thomas of the University of Sherbrooke in Canada.

As global warming increases, spring comes earlier. The problem is that birds can?t adjust to these kinds of weather changes, since they are genetically programmed to breed at a particular time of year, when the insects and seeds they need are supposed to be most plentiful. When birds breed early, they have to expend a lot more energy getting enough food to feed their babies.

Thomas has been studying the blue tits of southern France. The birds already live ?close to the upper limit of their metabolic effort,? says Thomas. ?When conditions conspire to make breeding a little more costly in terms of energy, they suffer.?

Since their usual trees weren?t yet leafy enough last year, the blue tits built nests in evergreens. Blue tits produce their offspring when caterpillars are available in the greatest numbers, but caterpillars emerged on the evergreens several weeks later than on the other trees, which led to hungry chicks and frantic parents. They expended twice as much energy finding food, which led to many deaths from exhaustion. Only half as many tits turned up this year to resume breeding with their mates.

Thomas believes that ?natural selection will slowly shift the breeding date in birds to track the optimal date.? But no one knows whether bird evolution can keep pace with the increase in global warming. In the meantime, people who feed birds are reporting extraordinary intensity around their feeders, as birds scramble to supplement scarce natural forage.

Birds are thought to be an essential element in the maintenance of the human food chain. Without a flourishing population of birds, hordes of insects will overwhelm human agriculture, because the amount of pesticide needed to kill them will be so high that it will poison and taint the plants it is intended to protect.

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