What's bothering birds in the South of England?

At least 4000 species of birds are long-distance migrants: they spend the spring and summer in locations like the UK, where they mate and rear their chicks, and then they fly off somewhere warmer for the winter. But for this to work successfully, the birds have to time their arrival in the spring so it coincides with a big surge in the caterpillars they need for feeding their young, which happens when the trees first burst into leaf. With the climate changing, though, some species are increasingly getting it wrong and arriving too late. So is this to blame for the drop we’re seeing in bird populations in the south of England? Or is something else going on? Chris Smith spoke to Malcolm Burgess from the RSPB and the University of Exeter…

Malcolm - We were trying to find out whether the extent of mismatch, that’s the mismatch between when the chicks are hungriest and the timing of the peak availability of their favourite food, which is caterpillars. We were interested in whether the extent of this mismatch differed between the north and the south of the UK. Our paper shows that spring is later the further north you go.

Chris - How did you actually do the study? What were the questions you were asking and how did you gather the information to nail this?

Malcolm - We’re looking a three levels really: we’re looking at the emergence of the oak leaves, when the caterpillars emerge and when they’re most abundant, and then we were looking at timing of breeding and the breeding success of the birds. We used citizen science datasets for the oak leafing data, we used records that people submit to the UK phenology network.

Chris - What’s that; people going out and asking is that oak in leaf yet?

Malcolm - That’s right. They’re noting down when they first see the buds burst of the oak trees right across the UK.

Chris - And the caterpillars and the birds?

Malcolm - The caterpillars is a bit harder. But I linked up to somebody called Ken Smith who’s come up with a very simple idea of collecting caterpillar droppings in a simple seed tray which we put underneath oak trees in woods across the UK all through the spring. Caterpillar poo is cylindrical in shape, so it means that we can separate it from everything else that falls in the traps, dry it and weigh it, and from that we an see very clearly actually, when the peak in the availability of caterpillars is.

Chris - What about the arrival of the birds; how did you log that?

Malcolm - We looked at both resident birds, which is blue tits and great tits which are in our woodlands all year round, and the migratory pied flycatcher. We measure their time of year breeding by monitoring their nests and, as part of the British Trust for Ornithology’s nest record scheme, many thousands of these nests are monitored each year, mostly again by citizen scientists. And we’re able to use this information from across the UK to quantify timing of egg laying, when the chicks hatch. And when they’re ten days old is when they require the most food and that’s the moment that needs to coincide with the peak availability of caterpillars.

Chris - Bringing all of this information together, what trends emerge; what did you find?

Malcolm - We found that spring was later in the north but, importantly, the extent of mismatch in any one year didn’t vary across the UK, so a population in the north of the UK is just as mismatched as the population in the south. That’s really important because declines of many insectivorous birds and many migratory birds isn’t uniform across the UK. The declines are greater in southern England and it’s been very often linked to this mismatch theory, but importantly we show that that isn’t actually the case, there are other things driving these declines.

Chris - Obviously, the south is experiencing quite considerable development. So do you think that it’s a population/development/agricultural impact, all these things working together and they’re affecting the environment and that’s what’s impacting the birds?

Malcolm - Those things will certainly all affect the birds, yes. Particularly thinking again about insectivorous birds, all those things are detrimental to availability of insects. I’m sure that many of us can remember when we were younger the amount of insects that would hit our car windscreens which really doesn’t happen anymore. So yes, any development and agricultural intensifications have been well shown to affect insectivorous and seed-eating farmland birds for example.