As well as a 24-hour clock for feel

As well as a 24-hour clock for feeling alert and tired at the right times, animals living in more temperate places also need a longer-term internal calendar so they can effectively 'predict' the future and anticipate seasonal conditions. Known as photoperiodism, it's the ability of plants and animals to measure day length to determine the time of year. Without this knowledge, animals wouldn't know the best times to carve out territories, mate, have their young, or even migrate. Having a baby, for instance, in midwinter when there's little food around would, in most cases, be pretty disastrous. Likewise, male songbirds would be wasting a lot of energy if they put all their effort into singing when females weren't interested in breeding.