There has been a song thrush singing from the top of a tall tree in my garden for the past two months. Also known as "mavis" or "throstle", the song thrush has been a popular springtime songster for many hundreds of years. This harbinger of spring keeps up the vocal display for several hours without a break - regularly over five hours! So what is the purpose of this showy display?
It is all about advertising. The female song thrushes are attracted to a singing male and are able to judge his worth by the strength and complexity of his song. If she is suitably impressed they will pair up and start building a nest. The female is also stimulated by the song to start the egg producing cycle and this coordination of breeding condition increases the chances of producing fertile eggs.
This outpouring of notes, in a clear bell-like tone is typical of song thrushes who use a large repertoire of phrases which are repeated several times before a new phrase is used. These phrases are not only innate in the bird from hatching; each bird learns new phrases from other thrushes in its neighbourhood as well as sometimes adopting man-made sounds. This rich range of sounds makes the thrush more appealing as a prospective mate to any passing female, which is one reason for putting all that energy into its song.
Another reason for the male to put on such vocal gymnastics is to proclaim the territory as being occupied and other males of the same species will not be tolerated. This ensures that there is enough space between territories to reduce border disputes and also provides for ample food supplies for the resident pair and their offspring.
However, by making itself so noticeable at the top of a tree, the male thrush runs the risk of being predated by a bird of prey. Although at one time thrushes were more numerous than blackbirds, they have suffered a large decline in population over the last 40 years. Despite this, research by the British Trust for Ornithology has shown that the decline is not due to predation of birds by sparrowhawks or of eggs by corvids (crows and rooks), but more likely to be habitat loss.
The causes are not clear but one reason could be that without woodland and nearby grassland to nest and forage in, thrushes are finding it harder to maintain their numbers even though they can have two broods each year. Another reason may be the use of pesticide in gardens, where the war on snails and slugs has resulted in collateral damage to thrushes.
Having grassland near their nesting sites is important as thrushes eat a wide range of invertebrates (mainly earthworms) and snails. The evidence for the song thrushes large appetite for snails can be seen in piles of broken shells round a prominent stone - the thrush's "anvil" and on peaceful summer days, you can hear them pounding the shell to get the soft snail body out.
After the breeding season is over, song thrushes seem to disappear but it is really just because they are quite solitary birds and so they are harder to find than others that form large flocks to forage in autumn and winter. However in particularly harsh winters song thrushes from Scotland and northern England may well migrate to Ireland, and at the same time many hundreds of thrushes from the Netherlands seek sanctuary in southern Britain for the same reason.
If you aren't lucky enough to have a song thrush reminding you of spring, then listen out for the rest of the dawn chorus as you wake in the mornings and revel in the sound-scape of the natural world.