At this time of year Red Kites become much more vocal, calling to each other with a shrill "wee-ooh" sound. Many will have paired up already, and pretty soon their attention will be on nest building, or nest rebuilding.
They can be seen adding sticks to a previous years nest as early as January. More usually March is the time when nest building begins in earnest, a few weeks before the eggs are laid.
The nests of Red Kites are quite haphazard structures. They're about 60cm in diameter, and often built in the fork of a tree 10m or 15m above the ground.
Most nests are constructed with sticks and lined with wool or grass, but sometimes Red Kites like to pimp their nests up a bit with other materials scavenged from their locality. Nest inspections in the past have revealed socks, a soft toy, an England flag, gloves and a pair of pants. In the photo above you can see a Red Kite about to add the decomposing wing from another bird. Nice.