Timing is everything for some breeding birds. They must hatch their young in time to exploit a brief springtime abundance of food. Now a study shows for the first time that some birds adjust their efforts according to lessons they learned the previous year. The finding implies that such birds might be able to accommodate some environmental changes spurred by global warming, but scientists caution that such adjustments may be limited.
As oak trees leaf out in European woodlands each spring, caterpillars hatch and devour fresh young foliage. The 2-week burst of caterpillars provides blue tits, small birds akin to chickadees, with the food they need to satisfy a nest full of clamoring little mouths. Birds likely use a host of cues, such as day length, to sense that spring is in the air. In addition, some researchers have suggested, birds breeding too late or too early one year might learn from their mistake and adjust their timing the next year.
Fabrizio Grieco, Arie van Noordwijk, and Marcel Visser of the Center for Terrestrial Ecology in Heteren, the Netherlands, monitored pairs of blue tits as they bred in nest boxes for two consecutive years. The researchers took advantage of the birds' tendency to nest later than the natural caterpillar peak during their first year of breeding. Thus, most birds bred late in the first year, then advanced their breeding to match the caterpillar peak in the second. But pairs given both caterpillars and mealworms did not move up their breeding time in the second year, the trio reports in the 5 April issue of Science. In fact, the blue tits delayed it, apparently because the first year's supplemental feeding led them to expect that food abundance would peak later.
The birds' ability to adjust their reproductive timing implies a certain degree of resistance to the ill effects of climate change, says ecophysiologist Donald Thomas of the University of Sherbrooke, Quebec. But he points out that "major climate change over decades probably will overcome the birds' ability to learn."