From "Willows: The genus Salix" by Christopher Newsholme (p. 10):
It is significant that perfect specimens of the honey bee (apis mellifera) have been found embedded in amber dating back 90 million years, well within the Cretaceous period. The pollination of willows, with a few exceptions, is still carried out by honey bees and, to a much lesser extent, by other insects.Also, according to Newsholme (p. 18),
From an ecological point of view Salix plantations encourage a wide range of birds, including willow warblers, wrens, wagtails, and practically all members of the tit family. Insectivorous birds are attracted by the dense leaf canopy that harbours the caterpillars and aphids upon which they thrive. The genus also provides a rich source of nectar and pollen for bees very early in the year when supplies of these essential ingredients are in short supply. Commencing in January, the flowering periods of different Salix species overlap one another throughout the spring.