Phénologie des oiseaux et des baies dans un petit bois isolé en zone de grande culture dans le Gâtinais (France)

Abstract

Summary.— Phenology of birds and berries of an isolated small forest remnant in an agricultural landscape in the Gâtinais (France).— The importance of berries and other small fruits as food for birds was investigated from 1993 to 1997 in the Bois Boucherot, a 46-ha forest remnant north of Orléans (France). With 62 species, 35 of which certain or probable breeders, the avifauna of the Bois Boucherot can be considered as rich. During the course of one year, the number of bird contacts on our 5-km sampling itinerary reaches a maximum in May and June and decreases strongly in July. It reaches a minimum in January and February, when the main berry-eating species (Blackbird, thrushes, Robin, Wood Pigeon) leave more or less the wood. Both number of species and number of individuals contacted per fortnight were constantly higher in 1995-96, characterized by a very abundant berry yield, than during the other seasons. Seed dispersers dominate the berry-eating bird guild, with about two-thirds of the species. In spring and summer, small species (Sylviids, Robin and Nightingale) represent 41 % of the dispersers, but only 8 % in winter, Bergman’s Rule being probably one explanation for this fact. Spring and summer berry species ripen one after the other from early May to the end of August. Spring and summer berries with a soft pulp are easily eaten by small species, and their high water contents make them attractive at a place where watering points are scarce and suitable for predators to hunt. These berries are eaten by the birds in succession as they ripe. Conversely, autumn berries are all ripe in late August and early September, and almost disappear in January. They are nutritionally rich and hold firmly on twigs, but most of them are no longer attractive for birds after having been frozen. They are consumed in whatever order, not taking into account their energetic value. Winter berries, represented only by ivy, disappear in January and February. Birds only use them for want of other food sources. There are no berries from late March to the first third of May. We have here a keen bottleneck, outside of which birds have a good supply of berries during most of the year, and specially so during the time they reproduce and prepare to migration. The conservation of bird diversity in the Bois Boucherot is thus conditioned by conservation of the diversity of berry-producing plants.