How do garden birds know who is who?

It is the one question my mum always asks me about birds; she is fascinated by the thought of how birds recognise each other, given that they all look the same.

The question is a live one at the moment for me because I actually have two Great Tits currently visiting my garden that don’t look the same as all the others. One has a washed out back, a whitish tail above, white flecks on its head and creamy underparts instead of custard yellow.

The other is a real oddity in that the white cheek patch which in most birds is so large and distinct is tiny. Instead, the black extends across almost the whole head and shoulders. I have never seen a Great Tit like it. It, too, has wahed out underparts, so I suspect they are from the same brood.

What it means is that when I see either of them, I notice them as individuals, which is so unusual with garden birds. They are not just generic Great Tits – they are rather like familiar friends. I wouldn’t be surprised if they soon get names! I’ll try and ensure it is not just White-tail and Black-head.

So, regarding my mum’s question, how is it that all the millions of birds that ‘look the same’ can tell each other apart? Well, we don’t know for absolute sure, but we have a good idea. The thinking is that they do look different. And they sound different. Ever so slightly, but enough.

You only have to think of the Frozen Planet sequences of penguins coming back from weeks at sea waddling through giant creches of youngsters and appearing to find their own offspring. Sound appears to play a big part there. There is also evidence that Gannets, which to my eyes all look identical, can pick out their mate in flight, no sound involved

We also know that normal Great Tits have different thicknesses and shapes of the black stripe down their belly, while Robins have slightly different orange bib shapes

And the thing is when you’re a Robin, for example, you are designed to be acutely conscious of other Robins. It is written into your very being to be tuned in to the tiniest differences. You only have to think about how we can pick out a family member or loved one in a crowd, even from the back, or by the sound of their voice, even if they only say one word. Well, if you're a Robin, that's the same.

It might not only be plumage and song. It is quite possible that birds have little personal mannerisms – a deeper bob, a faster wing flick, a great nerviness.

But for now I’ll just enjoy White-tail and Black-head (here with a Blue Tit chum) . Oh dear, the names seem to have stuck already.