Blackcaps are small songbirds that breed in Central Europe. Fifty years ago, most blackcaps went south for the winter to Spain or Portugal. But since the 1960s more and more blackcaps have started wintering in Britain and Ireland.
By 2003, blackcaps were seen perched at one in every three backyard bird feeders in Britain! But why did so many blackcaps change their winter vacation location?
The direction birds migrate is coded in their genes. Each population of birds has an average direction they will go, but individuals can vary as much as forty degrees from the average.
Although historically, the average direction for blackcaps was towards Portugal, a few birds' genes pointed them in a slightly different direction and they ended up in Britain.
But for the numbers of blackcaps wintering in Britain to change so dramatically, there must be some significant advantage to fly north rather than south.
Perhaps the benefit is simply from the shorter, less taxing flight to Britain. Or maybe it's because Britain has many bird lovers that stock backyard birdfeeders.
This Nest Is Taken!
But researchers from Germany and the UK believe the key difference is the shorter winter days in Britain. Shorter days affect migration and breeding behavior, so British birds return to summer breeding grounds about ten days earlier than birds that went south.
This ten-day head start lets British migrants claim the best territories and start breeding sooner. Females paired with British males laid more eggs and hatched more chicks than those mated to southern birds.
Each year, blackcaps with genes orienting them towards Britain pass on those genes to more chicks. And so the number of blackcaps with the disposition to fly to Britain steadily increases.