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How do birds know when it is time to build a nest?

Sent in by Adam Robinson, Northern Ireland

There are lots of factors which trigger the start of birds' breeding seasons. As with most vertebrates, hormones such as prolactin control changes inside the birds.

However, conditions in the environment are often the triggers for the hormonal changes to begin. One of the most well-known influences of bird breeding behaviour is day length (or 'photoperiod').

For birds which are active during daylight hours ('diurnal'), the lengthening of the days is a sign of the start of favourable conditions for breeding. Change in temperature can also be an important factor, as can abundance of food.

In the UK, spring is when most species show a distinct change in behaviour, spending much more time on activities related to breeding such as singing, territory defence and looking for nest sites.

The external triggers for this are noticeable all around us. The weather is getting milder, more food is available - in the form of fresh buds and insects - and the days get longer.

However, some birds breed much earlier or later in the season, when the weather may still be harsh and day length is decreasing. This may be linked to the abundance of a specific type of food that is abundant at that time.

For example, crossbills are opportunistic breeders that time their breeding to coincide with the abundance of pine cone seeds, even if that means starting breeding in December or January.

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