Going for a song?
Dick Warner wonders what the future holds for the song thrush.
Blackbirds and song thrushes are popular and common birds in Irish gardens. Actually blackbirds out-number song thrushes by about five to one in this country, so they are much more common. Every winter there is a survey of Irish garden birds carried out by volunteers and coordinated by BirdWatch Ireland.
The results for this past winter were not available at the time of writing but checking on the winter of 2006/07 I found that blackbirds occurred in 97.9% of the gardens surveyed making them the third most frequently occurring species. Song thrushes were found in 69% and were fourteenth on the list.
There is some evidence to suggest that blackbird numbers are increasing and fears that song thrushes may be on the decline. There is a certain amount of guesswork in this because the comprehensive scientific data is a bit out of date, though a new census of breeding birds is under way at present and eventually this should provide the answer. But observers have noticed that blackbirds are spreading out from lush lowland habitats and colonising uplands, barren areas in the west and even offshore islands. This does suggest a population expansion.
I haven't been able to find any statistical data to conclusively prove that song thrushes are declining in Ireland. I mentioned that in 2006/07 they were the fourteenth most frequently recorded birds in the Irish Garden Bird Survey. Looking back over the figures in 05/06 they were at number thirteen, in 04/05 back at fourteen and in 05/04 at number nine.
But observers have noticed that blackbirds are spreading out from lush lowland habitats and colonising uplands, barren areas in the west and even offshore islands. This does suggest a population expansion.
This might suggest a decline but the real worry is based on data from Britain where the monitoring is more intense. Their figures show a massive decline starting in the mid-1970s and continuing today. They are so worried that they have recently added the song thrush to their Red List of birds of serious conservation concern.
The two species are very closely related and have similar preferences for habitat, diet and nest sites. There are a few differences. Song thrushes, for example, eat snails while blackbirds don't and, though song thrushes will sometimes eat fruit, including apples, they don't have the same passion for it that blackbirds do.
Another difference is that blackbirds are slightly larger and very much more aggressive. This leads to the suspicion that the increase in blackbird numbers and decline in song thrushes might be connected. The smaller and meeker bird might be losing out to competition from its more aggressive relative.
But when I went back to the British figures and studied them a bit more closely I found an odd anomaly. They showed a 73% decline among song thrushes in farmland but only a 49% decline in woodland. You would imagine that the competitive pressures would be similar in both habitats.
On the other hand in the 1970s farming in Britain, and in Ireland, suddenly became much more intensive. It begins to look as though we should be blaming the farmers not the blackbirds. Several things happened at this time. There was a decline in traditional mixed farming, a switch from hay to silage and from spring cereals to winter ones and, of course, there was an increase in the use of chemicals in agriculture. Any or all of these could have contributed to a decline in song thrush numbers. Other factors, in urban areas, may have included the use of garden slug pellets and increased numbers of domestic cats.