Dr Malcolm Burgess explains the value of long-term nest box monitoring primarily at Yarner Wood

In 1955, soon after Yarner Wood was designated as a National Nature Reserve, 50 nestboxes were put up in a small area near today's office. These were for an experiment investigating whether Blue tits had any height preference for nesting, with two boxes put on trees at different heights to test this. The addition of nestboxes to Yarner Wood had an unintended consequence. In the first year a pair of Pied flycatchers bred in one of them. Pied flycatchers had not been recorded as a breeding bird at Yarner Wood before, with only a few breeding records in Devon by that time. Slowly the population of Pied flycatchers grew, although even twenty years later there still were only around 12 pairs. It wasn't until the 1980s, by which time nestboxes were found in other Dartmoor oak woods, that the population increased rapidly. The population peaked in 1990 after which numbers were reduced, but today the population is back to 1990s levels in part due to a further increase in nestbox provision. The Yarner Wood population today is around 55-65 pairs, with a further 30 pairs in the Bovey Valley Woodlands, where boxes were put up in the late 1970s.

Pied flycatchers

Every spring the nestboxes are monitored weekly and the contents recorded. Reserve staff undertook this for the first 50 years. Today a dedicated volunteer group does this with the assistance of Bryan Thorne (Reserve Warden) owing to the number which need to be checked (237). Nest building stages are observed and recorded first, then counts of eggs and chicks and finally nest outcomes. This enables us to know the timing of breeding which is important in climate change studies, and clutch size and how many chicks fledge which we can relate to factors that influence these such as weather.

Dr. Malcolm Burgess checking bird nestboxes at Yarner Wood

Since 1955 all the young and many adult Pied flycatchers have been ringed by licensed bird ringers. Ringing with uniquely numbered rings enables us to follow individuals throughout their life - this works very well for Pied flycatchers since the young usually return to breed locally and so we follow all their breeding attempts as adults. One chick we ringed returned to Yarner and bred for 9 consecutive years and we know it bred with 8 different males, laid 62 eggs and fledged 46 young. Since 2009 researchers from the University of Exeter, PiedFly.Net and the RSPB Centre for Conservation Science have undertaken the catching and ringing, and have added to the standard nest box monitoring for particular projects. For example, since 2009 we have monitored caterpillar and flying insect abundance (i.e. bird food) through the breeding season. The scientists have also tracked Pied flycatchers with miniature tracking devises, revealing their migration routes and behaviours for the first time. https://bou.org.uk/blog-burgess-tracking-pied-flycatchers

Blue tit; Bird in the hand

Monitoring

The nest monitoring undertaken at Yarner Wood part of East Dartmoor NNR is one of the longest running such schemes in Europe. Many thousands of Blue tit, Great tit and Pied flycatcher nests, and smaller numbers of Redstart, Nuthatch, Coal tit and Marsh tit nests have been monitored.

Coal tit

Nestboxes have high occupation in oak woods like Yarner Wood with a history of coppice management - the resulting abandoned oak coppice means there are fewer veteran trees and nest holes. Over the 67 years of monitoring we can see how populations have changed. Notable is an increase in Blue tit numbers - no doubt due to higher survival from warmer winters and supplementary feeding, plus nestboxes increasing the availability of nest sites. We see that as Blue tits became more abundant, clutch sizes have got smaller - probably because higher densities result in pairs holding smaller breeding territories through the increased competition (in science speak this is called density dependence).

Great tit; Nuthatch

Humans became more aware of climate change in the 1980s, but the study of how this affects birds relies upon long-term datasets like ours. Such long running datasets are rare. Climate change has resulted in spring starting earlier compared to the 1950s, and the nestbox monitoring shows that the date that egg laying starts is two weeks earlier now than in the 1950s. This is adaptation, although the migratory Pied flycatcher maybe less able to adapt because they need to arrive to Dartmoor earlier to maintain a match between the timing of breeding with the short peaks in insect abundance. Our work has been at the forefront of research in this field.

Research

Malcolm has digitally preserved all the long-term data, in so doing making it available to researchers across Europe studying a wide range of topics. The East Dartmoor NNR population represents the UK in these range-wide studies. Climate change related research has been the main beneficiary, focusing on both Blue tit and Pied flycatcher. Research has also investigated the evolution of plumage colour (we measured colour in different feathers) and the evolution of song (we did some experiments that involved recording chick begging in response to different calls). Recently we contributed to a genetic study that shows Pied flycatchers first colonised the UK between the last glacial and the current interglacial period ~20-10 thousand years ago after an historic change in climate. University of Exeter PhD student Fraser Bell has conducted his PhD with data collected on the reserve. Fraser has published tracking work describing the migration behaviour and route, and investigated how this is shaped by weather patterns. These tracks from the reserve’s flycatchers took Fraser and a team of RSPB scientists to Liberia where our flycatchers winter, investigating winter habitat use. Samples were collected in Liberia to investigate malaria infection, which along with hundreds of samples collected over 10 years from breeding birds at East Dartmoor shows how infection is more prevalent in some years than others, and that birds wintering in wet habitats have higher infection rates.

We have also used the nestbox monitoring information to inform management of the reserve. Woodland management has been recorded in detail for each of 136 'compartments' of Yarner Wood, which we used to investigate how woodland management affected the hole-nesting bird populations and breeding. Much less than we expected as it turned out, as weather is so much more important. This actually led to doing less woodland management aimed at birds, and so the thinning programme is reduced and now done with aims that benefit other taxa such as lichens.

The nestbox monitoring has contributed so much to our understanding of the reserve hole-nesting bird population, but also the wider science work which has furthered our knowledge of Pied flycatcher ecology and evolution. This work continues - who knows what other questions will be asked of this long-term dataset in the future!

Written by Albert Knott and Dr Malcolm Burgess

Malcolm Burgess has had a long-term relationship with Yarner Wood and the rest of East Dartmoor National Nature Reserve starting off as a volunteer and now works for the RSPB Centre for Conservation Science, and is an Honorary Research Fellow at the University of Exeter, and set up PiedFly.Net.

Bryan Thorne: Credit for pair of Pied flycatchers image

John Sheppard Credit for male and female Pied flycatcher images

Robert Petley-Jones: Credit for Great tit, Nuthatch, Coal tit, Blue tit images

Andy HamblyCredit for Bird in the hand image