Guest bloggers: Ben Hatchwell and Pip Gullet, University of Sheffield.
After two weeks of diligent incubation, the eggs are ready to hatch. The newborn chicks are tiny, blind bags of skin and bone, and it's going to take some busy hunting by both parents to fill them out.
In just two weeks each chick will be heavier than its parents, fully feathered, and almost ready to fly. For mum and dad, who work dawn 'til dusk to satisfy the gaping mouths of their hungry brood, two weeks may not seem such a short time. However, help may be at hand.
If another pair of long-tailed tits nearby has failed to breed (probably because a hungry predator has swung by their nest), one or both failed parents may start helping to raise their neighbours' chicks instead!
It is this truly remarkable phenomenon that sets long-tailed tits apart from every other British bird. About half of all pairs are assisted in this way by as many as eight 'helpers' (more commonly one or two). Each helper works almost as hard as the parents in feeding the hungry brood.
Why feed someone else's chicks when you could save your energy to survive the year ahead and breed again the following year? Blood is thicker than water, and kinship holds the key. If you can't pass on your genes to the next generation by producing your own offspring, the next best thing is to pass them on by increasing the success of your relatives.
Helpers are usually close relatives of the brood they care for, most often an uncle. Helped chicks are fatter chicks, and fatter chicks are more likely to survive their first year of life and become parents themselves the next spring.
The private life of long-tailed tits
This extraordinary behaviour raises all sorts of fascinating questions. With such a brilliantly detailed 20-year dataset, we can tackle mysteries such as how do long tailed tits recognise their relatives, how do they avoid inbreeding in this seemingly incestuous scenario, why are females rarely helpers, and why are more species not cooperative?
Answering these and many other questions will keep us busy in the Rivelin Valley for many years to come. It's been a pleasure to share it.