Emily Kench on the RSPB on the ever-busy coal tit.

A colleague once described a coal tit to me as a 'blue tit that had fallen down a chimney', with olive green upperparts that look like a blue tit sprinkled with dust, and a once duck-egg head dunked in a bucket of coal.

This rich black head is how the coal it adopted its name. Large and bulbous in proportion to its body, thickly coated in coal, with two snowy contoured cheeks, and a crisp white Mohawk painted from the nape of the neck to the tip of the crown. The white stripe is one of the coal tit's most distinguishing features, setting it apart from its relatives.

The size of the coal tit is another telling sign, it is the shortest in length of our tit species, unless you were to dock the tail off a long-tailed tit. Both the stripe and size are essential in identifying coal tits as they congregate in our gardens feeding in mixed tit flocks, particularly in the winter. More eyes enable these compact birds to find food and look out for predators.

A coal tit won't hang around though. All you'll get is a fleeting glance, as it pings onto the feeder, and dashes straight off again – nut in beak. Coal tits stash away their food. They hide it in nooks and crannies, caching it for a later date. This behaviour might seem strange, but coal tits are smaller than their relatives, with lower fat reserves, so they store food when it's plentiful and retrieve it when needed.

Unfortunately, coal tits don't always find their hidden treasures – their memory isn't quite as good as their hiding places, and it is possible that you might find forgotten sunflower seeds germinating in the most unlikely places.

In summer, though, coal tits take advantage of an abundance of insects. They thrive in coniferous woodland, with slender bills that enable them to probe insects between intricately knitted spruce and pine needles. With the increase in commercial soft-wood plantations in recent decades, we have seen an increase in our coal tit populations, as the perfect location to build a love nest and successfully feed young becomes more accessible.

This could explain why we have seen a surge in garden coal tit numbers over the last couple of months. The BTO found in a recent survey that coal tits have been seen in 70% of gardens this winter, and normally they are only seen in about 40% - lots of juicy insects in lots of coniferous trees during the summer might have resulted in a bumper breeding season.

However, there could be other explanations. The particularly cold winter we're experiencing might be driving coal tits to our feeders, there are less seeds on trees than during milder winters in the wider countryside, so our gardens are the perfect rescue. It's also been very cold on the continent for the first time in quite a few years. Coal tits might have headed over to the UK for warmer climes (believe it or not!). By moving somewhere warmer, these tiny birds can save on energy, they'll be wasting less time trying to keep their heat, and can spend more time feeding.

The Big Garden Birdwatch on the last weekend of January will offer us the perfect opportunity to see if the coal tit influx has continued throughout the season, and monitor coal tit, and other garden bird population trends.

Look out for coal tits in your garden in the Big Garden Birdwatch, register for free at rspb.org.uk/birdwatch, or text BIRD to 70030. You will receive a free bird identification chart, an RSPB shop voucher, and advice on attracting more wildlife to your garden.