Strange roof fellows

About this time of the year some of the local mallard ducks start to act a bit strangely. They leave the local burn, where they happily spend most of the year, and start walking the streets and cavorting in our front gardens. Some even take to roosting on the roof-ridges of our houses, normally strictly the domain of the local gulls, starlings and jackdaws. I'm sure that sex is somehow involved in this mysterious behaviour!