Obviously bees don't follow the calendar, but instead their activities are dependent on the weather and available food sources. Therefore it is easier to talk about the bees' year by referring to what is flowering. For the bees, the onset of spring is marked by warm weather and the flowering of alder (Alnus), hazelnut (Corylus), and willow (Salix). In some years, however, alder and hazelnut flower too early for the bees to benefit from it. In contrast, the flowering of willows mostly coincides with the first days of temperatures above +10 °C, and different species of willows continue blooming for several weeks. During these days, the bees are collecting large amounts of pollen and even some nectar from the catkins. There are many species of willows with slightly different flowering peaks, allowing the bees to visit this valuable food source over a long period. During midsummer the main nectar flow usually comes from raspberry, clover, and oil rapeseed. In some areas heather is also a very important food source.