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What percentage of birds don't manage to find a mate?

Sent in by Chris Wayman, Alsager, Staffordshire

Good question. There is no definitive answer to this; the percentage is significant, but it varies greatly from one year to another, and from one species to another.

Before a garden bird can contemplate a mate, he needs a territory. Having found a suitable territory, he will then need to defend it against other males, at the same time as attract a mate. He does both jobs by singing. To another male, the song is a war chant - 'stay away, I am big and powerful, this is my patch' is the message. On the other hand, to a female the same song is the sweetest love song he hopes she cannot resist.

There are only so many territories that an area can hold, and there are inevitably males that fail to secure a territory. Similarly, there are females that fail to find an available male with a territory. There are always these surplus birds around, more on years when previous year's nesting success was high and lots of birds survived over winter, fewer on years when past weather has caused a population dip.

What happens to the unpaired birds?

So what do these birds do with their time? Birds have a very strong urge to obtain a territory and a mate, and to breed. The unmated birds spend a lot of their time in the leftover spaces between territories, where they do all the things that birds do outside the breeding season, without being harassed by territorial birds. Still, they do stray into occupied territories, sometimes deliberately to test the resolve of the territory holders, since they are always on lookout for any opportunity to gain a territory and have a chance to breed. Many of the quarrels you see between birds as you watch them in your garden during the spring and summer involve these interlopers.

Vacancies in territories can appear at any time as a result of predation by a cat or a sparrowhawk, collisions with a window or a passing car, other injuries or disease just to name a few. As soon as such a gap appears, an unmated bird is quick to fill the space. Inevitably, there are always birds that fail in their quest. They must bide their time and hope to survive to another, more successful, breeding season.

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