Chicks are best in a well-dressed nest

A few months ago, I built a new riddling table (essentially a mobile soil sieve on legs) for our local community farm - the soil is very stony and it's a good way of making a fine tilth or lump-free compost. It wasn't going to be needed much over winter, so was stored under some polythene sheeting and next to a thick hedgerow to keep the weather off until required. However, when the cover was removed a few weeks ago, there was a surprise - a bird's nest complete with four pale blue, slightly speckly eggs. The cover was quickly replaced and a notice sent round to let everyone know that the table couldn't used for a while. Even without looking at the eggs it was clear that it belonged to a pair of song thrushes (Turdus philomelos) as the parents could soon be seen attending it. A week or two later, I revisited the nest to check its progress.

Song thrush nest with 3 nestlings

As you can see, there were 3 nestlings - still bald with their eyes closed. The nest is on a shallowly sloping  wooden platform (used to funnel sieved soil into a wheelbarrow) underneath the sieving tray - you can see one of its wheels at the back - with the whole structure covered in polythene - an excellent hideaway. Song thrushes usually build their nests in trees and shrubs, typically near the trunk around 1-4m up, though they can sometimes be found in buildings, on the ground, on stumps and fallen branches or in hollows among creepers. Whatever the precise location, they tend to be in shady, well-hidden places in or near woodland or hedgerows, and this artificial construction suited their needs very well.

Song thrush nestlings - some downiness has already appeared with a few feather shafts just beginning to develop

These were the only photos I took as I didn't want to visit the nest for longer than required to obtain enough information to make a useful record. Such records, if obtained carefully, provide valuable information about the breeding success of many species and in the UK, the BTO's Nest Record Scheme uses volunteers to follow the progress of nests, providing training where required. With the bad old days of egg collecting largely behind us (though some illegal collecting still occurs), a Code of Conduct needs to be followed when recording nests. Much of this is common sense (don't visit more often than necessary, limit the amount of disturbance caused and try to approach the nest when it is unattended) but some aspects are less obvious.

For example, it is widely believed that visiting a nest will increase the probability of the clutch failing. However, reviews of research into nesting success (Götmark 1992, Mayer-Gross et al. 1997) indicate that this is not the case, if the Code of Conduct guidelines are followed. Examples of some other less obvious actions that need to be taken are as follows (for a full list, see the Code):

• In case parent birds are watching, approach nests casually, as if by chance, rather than directly and deliberately - you are then likely to be seen as harmless (like a passing herbivore) rather than a potential nest predator.
• Adults are most sensitive to disturbance at the start of the breeding attempt, during egg laying and very early in the incubation period - it these times, they have invested less energy in the nesting attempt and have more time to lay a replacement clutch.
• Conversely, the parents become less sensitive of disturbance as the nesting attempt continues, but the chicks’ become more aware, and when partially feathered, the young of some species may instinctively scatter from the nest, a process known a ‘exploding’. This is adaptive when a real predator raids a nest is it gives a chance of survival for at least part of the brood, but once out of the nest the chicks are vulnerable to cold and to ground predators. Also, don't forget that chicks can only legally be handled by licensed bird ringers.

Here, the nest was unattended, the adults past the period of maximum senstivity and the chicks still too young to scatter - the perfect time to visit. After this, the adults could be seen bringing food to the nest and all three nestlings were seen to fledge. Once it was clear that the nest was no longer in use (young stay in the nest for 12-16 days), it was removed for closer inspection.

Song thrush nest after the chicks had fledged

The nest, built by the female (though both parents feed the young) is made of a woven circle of small twigs, leaves, grass, roots, moss and string surrounding a smooth cup of papier-mache made from rotten wood-pulp (sometimes mud is used). Close up, it really is an impressive structure - neatly woven and well camouflaged (or would be in a tree), and the inner cup thin, light and neatly smoothed. Although the young are very similar to those of the mistle thrush (T. viscivorus), as are the nests and breeding times, the nest structure is clearly different with the mistle thrush making a bulkier cup of plant material with soil mixed into it and lining it with finer grass.

If you want to know more about how to identify nests, eggs and nestlings, Harrison & Castell's 2002 guide is excellent, but do take care as noted above, and if possible join the Nest Record Scheme; information about breeding success is vital for well-informed conservation.

References

Götmark, F. (1992). The effects of investigator disturbance on nesting birds. Current Ornithology 9: 63-104.
Harrison, C. & Castell, P. (2002). Bird Nests, Eggs and Nestlings of of Britain & Europe with North Africa and the Middle East (rev. ed.). HarperCollins, London.
Mayer-Gross, H., Crick, H.Q.P. & Greenwood, J.J.D. (1997). The effect of observers visiting the nests of passerines: an experimental study. Bird Study 44: 53-65.