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Woodlark populations suppressed by high level ofpredation06/07/2009 05:44:16
Woodlarks are threatened by an increase in predators such as stoats and kestrels. Credit Derek Belsey/Cliff Reddick/BTO. July 2009. Woodlark populations have faced an increasing threat from predators over the last 35 years, according to new research led by the University of East Anglia (UEA). In the study, researchers from UEA and RSPB examined the influence of local weather conditions on reproductive success, timing of breeding and survival in a population of woodlarks in the Breckland area of Norfolk and Suffolk. Woodlarks were one of only two species recently moved from the ‘Red List' of birds of conservation concern to the ‘Amber List' because their total numbers have increased in the UK. However, this new research suggests that populations would have done even better if there were fewer predators. Weather effects The findings show that woodlark can have longer breeding seasons when the weather is warm and start nesting earlier. They also showed that high rainfall can be damaging, affecting the number of eggs laid and the survival of chicks. However, these effects were not consistent over the 35 years and were minor compared to the increased rate of nest predation. The study was led by Lucy Wright, a former PhD researcher and Paul Dolman a senior lecturer in ecology in the School of Environmental Sciences at UEA. Lucy Wright is now a research ecologist for the British Trust for Ornithology. It is thought to be one of the very few studies of its kind in the world. Nest predation doubled "For a ground nesting songbird of conservation concern, any benefit of warmer earlier springs from global climate change has been negligible, and much less important in the long term population trend, than a large increase in the rate of nest predation," said Dr Wright. "We found that between 1971 and 2004 nest predation doubled and breeding productivity halved. There are many more predators in the countryside now than there used to be and more of a mix we think - not just foxes, but possibly hedgehogs, stoats and kestrels, as well as scavengers such crows." So...?Wildlife Extra thinks that this may be a good sign. If predator levels were being suppressed by 'game management' and other artificial causes, the fact that they are recoveringis bound to have an effect on other species, but also suggests that woodlark numbers may have been artificially high in 1971 due to lack of natural predation. Unless the dreaded mink are to blame. Dr Paul Dolman replies, click here. Co-author Dr Paul Dolman, said the effects in Norfolk might be more noticeable because of changes in land management and use over the years, but added that an increase in predators could be seen across much of the UK.  Woodlark nests may be threatened by crows, foxes and hedgehogs. Photo credit P.Dolman. Infra-red cameras to identify predators The identity of the predators remained a mystery in the original study, but Paul Dolman is now leading further work using infra-red nest cameras to identify which species are responsible for predating the nests of woodlark, and another ground nesting bird, the nightjar. Also involved in the study were Bill Sutherland (now at the University of Cambridge), Rhys Green (University of Cambridge and RSPB), Chris Bowden (RSPB) and John Mallord (another PhD researcher from UEA, now working with the RSPB). The research was funded by the Natural Environment Research Council, with some of the data collection funded by the Forestry Commission and RSPB. The study, Importance of climatic and environmental change in the demography of a multi-brooded passerine, the woodlark Lullula arborea, is published online in the British Ecological Society's Journal of Animal Ecology.
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Read the comments about this article and leave your own comment
I'm sorry that Mark Fisher thinks that our study is trying to suggest that predators should be controlled. Actually, if you read the paper in full, we do not suggest that at any point.
Although the media are very interested in the predation angle, nest predation is just one of a whole range of factors that affect woodlark numbers (as well as other ground nesting birds), and as Mark rightly points out this is a perfectly natural process. Mark is also correct to point out that very many other studies have showed that weather affects the timing of breeding, breeding success and over-winter survival of many bird species. However very few studies have investigated the relative importance of all these different effects in causing changes in the numbers of ground-nesting birds over a long period of time (35 years in this case). Most long-term studies of this kind focus on nest-box species like tits or pied flycatchers, rather than ground nesting birds in natural nests.
The big question that I think our work addresses is whether climate change, or other changes in the environment have the biggest impact on woodlarks' population change. Actually we found that during the last 35 years climate change had a significant, but small, impact on woodlark population change, while other changes in the environment (one of which was nest predation rates) had a much bigger impact. This is important because many studies focus on the impacts of climate change without considering how other changes to the environment are affecting bird populations. It is important that we continue to study the impacts of ALL types of changes in the environment on wildlife populations if we are to understand how best to conserve them. The best way to do this is to encourage the long-term monitoring of populations so that data are always available to investigate any future changes in bird numbers. I was very lucky during my PhD to have access to a 35-year high-quality dataset collected by Ron Hoblyn and other volunteers. There must be very few datasets in the world of this quality on ground-nesting bird species; we need more like this so we can better understand the various environmental factors affecting other species in other areas.
Woodlarks were almost extinct in Norfolk and north Suffolk (our study area) by 1970, but between 1970 and 2000 their numbers increased to around 600 pairs because a lot of new habitat was created by the Forestry Commission, Wildlife Trusts and other conservation organisations. However since 2000 the numbers have declined. The decline is caused partly by increased nest predation and partly by an decline in the number of birds that survive the winter. This change in over-winter survival is not related to weather, so there must be some other factor causing reduced survival, but we don't know what that is and it may be difficult to find out because, at the moment, we don't know exactly where most of our woodlarks spend the winter.
Woodlarks have evolved to cope with a reasonably high rate of nest predation; they are multi-brooded and will make many repeated nesting attempts during the season. This means that around two-thirds of their nests can be predated without causing the population to decline, so clearly nest predation is a perfectly natural process. However, at the moment more than two-thirds of nests are being predated and this is what is causing concern.
As Paul points out the high rate of predation is probably because of artificial factors like road-kill, urbanisation, changes to farming, and introduced prey species like rabbits that make the area more suitable for predators. It could also be that woodlark nests are more exposed because of introduced herbivores like muntjac and fallow deer that reduce cover. There is a whole range of possibilities and we don't fully understand the mechanisms causing this at the moment. We also don't know what species are predating woodlark nests (although ongoing studies with nest cameras may shed some light on this), and we are certainly not suggesting any type of control.
Posted by: Lucy Wright | 07 Jul 2009 10:42:57
I think Dr Dolman's response pulls the rug from underneath this research, even if it were ever actually standing on any form of floor covering in the first place. This just repeats so many other studies that show bird breeding of ground nesters is affected by weather (duh!) and it also rachets up yet again the animosity towards predation of a misnamed (wood?lark) "song bird". I wonder if Dr Dolman considers predation itself to be unnatural? It would perhaps be asking too much of all doctoral research (which this is based on) to be original or ask any of the bigger questions, but what is its point? To add weight to the continuing slaughter of native mammals (the predators) under the guise of nature conservation?
Posted by: Mark Fisher | 06 Jul 2009 12:23:13
Dr Dolman replies:
As all aspects of landscape and nature in Britain have been extensively modified by humans, and the abundance of species we see reflects this with some winners and some losers, speaking of artificially high or low numbers of anything is misleading, nothing in Britain is natural.
In the past, game management and predator control was damaging to populations of some birds of prey and predatory mammals, for example Polecat is now returning to England and Buzzard has recently greatly increased its range.
But increases in predator numbers have reduced numbers of woodlark, so it is probably fair to say that in the past woodlark benefited from game management. Numbers in 1971 were low because very little habitat was available. Once habitat became available through forest management, woodlark numbers in Breckland increased. If they had been exposed to today’s levels of predation, they would probably not have been able to increase as well as they did. But although reduced game management has allowed predators to increase in numbers, this is not replacing something artificial with something natural – predator numbers are also affected by road-kill, carrion, farming, food in urban dustbins, the large amounts of unnatural prey like rabbits, and the absence of larger predator species that have been driven extinct and so can no longer control the ones that are left.
Posted by: Powell Ettinger | 06 Jul 2009 10:23:31