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Bird Populations in a North Yorkshire Woodland

Gordon W Follows and Ron Gash

This paper was presented as a poster at the spring meeting of the
Yorkshire Naturalists’ Union in Harrogate, 26th February 2000

[Note that clicking on the graphs will import bigger, more detailed versions, mostly c.13K.]

Ever since the early 1970’s, the breeding biology of a number of hole nesting birds found in the oak woodlands of both Duncombe Park, Helmsley and Sleightholmedale near Kirbymoorside, has been studied and the findings published (2), (3). Following the publication of these two papers, we have continued to monitor the population of those birds using nest-boxes at Sleightholmedale (SE 664 891), in particularly recording population size, timing of the breeding season and productivity of each species, and observations have been published as informal annual reports. The study area is located near Sleightholmedale Lodge in the valley of the Hodge Beck, one of the southerly flowing streams draining the North Yorkshire Moors National Park. Most of the indigenous, deciduous woodland along the slopes of the valley has long since been felled and replaced by conifer. However, Mell Bank Wood remains deciduous with English Oak Quercus robur and Ash Fraxinus excelsior the dominant species whilst Alder Alnus glutinosa is particularly abundant along the banks of the river. As might be expected, such habitat is suitable for both Pied Flycatcher Ficedula hypoleuca and Wood Warbler Phylloscopus sibilatrix as well as Redstart Phoenicurus phoenicurus, Nuthatch Sitta europea and those other woodland species commonly found in North Yorkshire.

As was detailed in (3), there are approximately one hundred boxes in the study area with just over one-third in Mell Bank Wood; these are at an altitude of 99–107 metres asl whilst the remainder are located along one kilometre of river bank between 69 and 84 metres asl. All boxes are of the standard tit type with an overhanging roof and a 3 cm diameter entrance hole, and for ease of inspection are situated 2 metres above ground level. During the 26 years of the study, significant changes have been noted in the status of certain species and although the population of all species using the boxes fluctuates from year to year – as was well illustrated in (3) – we have noted that in recent years there has been a continuing decline in the numbers of Pied Flycatchers, the reasons for which are not fully understood at present. This paper details these findings and discusses possible reasons for the changes.

Although at least ten species have used the nest-boxes over the years, only three are sufficiently common to allow the data to be used to assess population trends. This is illustrated in the chart below, which is a summation of all the data collected between 1974 and 1999.


Pied Flycatchers are widespread in the forested areas of upland western Britain, being particularly common in Wales, in western England from Devon northwards to Cumbria, and in parts of south-west Scotland. The birds breeding within the North Yorkshire Moors, however, are an isolated population confined to just a handful of 10-kilometer squares and, with the exception of isolated pairs in south-east England, is the easternmost population within the United Kingdom (1). The size of the Pied Flycatcher population using the nest-boxes during the years 1974 to 1999 inclusive is detailed in the diagram below, which depicts not only the total population, but also the number which used nest-boxes in Mell Bank Wood. As mentioned above, the population does fluctuate from year to year – 9 pairs being a typical minimum and 23 the maximum – with between a third and a half normally located in Mell Bank Wood. However, from 1992 the population appears to have been in decline with less than four pairs in each of the last five years and only one of these in Mell Bank Wood.


It is interesting to compare the above with similar data for the Blue Tit Parus caeruleus and Great Tit Parus major populations which also use the nest-boxes. Again, the tit population fluctuates quite dramatically from year to year, with the severity of the late winter weather having been shown to have a major influence upon the size of the breeding population the following spring (3). However, in spite of these quite dramatic fluctuations, there is no indication of a similar long-term decline.


During the course of this study, Pied Flycatchers have shown a slight extension in range, particularly in south-west Wales and north-west England, although in North Yorkshire there have been signs of a slight decline (1). Part of this national expansion could be due to the wider availability of nest-boxes, which is also reflected in the number of nestlings ringed which increased from 4,064 in 1976 to 22,217 in 1994 (4), (5). Elsewhere in Yorkshire a small number breed in the west, in the valleys of Wharfedale for example, whilst in North Yorkshire a few birds can be found in other south-draining valleys and odd pairs breed annually in Kildale and Eskdale. However at Helmsley, where there has also been a long-term nest-box study, a similar decline has been observed which appears to mirror observations at Sleightholmedale (6); a graphical comparison of the two populations is given below.


Other factors which could influence the size of the population include the activities of both avian and terrestrial predators which in some years can be quite significant; Great Spotted Woodpecker Dendrocopos major, Weasel Mustela nivalis and humans have all been implicated. However, this is erratic in occurrence – as illustrated below – albeit when it does occur the outcome can be quite dramatic as all boxes in a particular area can be affected.


There has also been an increasing tendency for Tree Wasps Dolichovespula sylvestris to utilise nest-boxes for breeding. However, even in the worst year (1995) when some 23 boxes were occupied by wasps, there were still 50 which remained unoccupied, which suggests that the Pied Flycatcher population is not limited by a shortage of nest sites.


It is also interesting to compare the above Pied Flycatcher trends with those of the Wood Warbler Phylloscopus sibilatrix,, another summer visitor which is also habitat specific, often favouring closed canopy oak woodland with little or no understorey. In fact, in favoured habitats it is often the winter grazing of sheep, which prevents the undergrowth from becoming too dense for this species (1).


The numbers for Mell Bank Wood were monitored by recording the territories of singing males during May and early June, and show a twenty year decline culminating in local extinction since 1992. This tends to support the view that a change in habitat may have been partly responsible for the decline of the Pied Flycatcher population in Mell Bank Wood, Sleightholmedale. Certainly, with selective tree felling and the subsequent rapid growth of saplings, the understorey in the wood has changed markedly in recent years, so that the ‘free space’ below the canopy which is so favoured by feeding flycatchers has been much reduced; the presence of saplings will also have made access to the nest-boxes more difficult. On the other hand, there are probably other factors which could have influenced the decline in the numbers of birds breeding both along the Hodge Beck and in Duncombe Park. As an isolated population on the edge of the range, the North Yorkshire population of Pied Flycatchers will always be vulnerable to any change in the population as a whole, which could result for example, from increased mortality either on migration or in winter quarters; in this context it is particularly interesting to hear of a recent unexplained decline in the Forest of Dean population (7) which is a traditional stronghold for the Pied Flycatcher in England.

During the winters of 1995/6 and 1996/7, a limited amount of coppicing was undertaken in Mell Bank Wood to clear some of the understorey. Unfortunately, this did not have any obvious beneficial effect on the Pied Flycatcher population, and so during the winter of 1998/9, more dramatic measures were taken in order to try to better understand if local changes in habitat were responsible for the decline. During that winter, ca.50% of the boxes were removed and relocated some 4 metres above the ground and relatively clear of any secondary growth. This involved all those boxes in the southern portion of the study area, both in Mell Bank Wood and along the Hodge Beck; the remaining boxes (in Mell Bank Wood north and along the Hodge Beck upstream of the ford) were left in the same position some 2 metres above the ground to act as controls for the experiment. Results are depicted in the two charts below which suggest that occupation of the low boxes was little changed over 1998; however, occupation of those boxes which had been raised was substantially if not significantly greater, unfortunately not by Pied Flycatchers but by Blue Tits. This preference for higher boxes by Blue Tits supports the findings of an earlier Welsh study (8).



So what do we conclude? It seems quite possible that changes in habitat within Mell Bank Wood have played a part in the local extinction of the Wood Warbler as a breeding bird at Sleightholmedale. However, with Pied Flycatchers the situation is perhaps more complex; certainly, changes in habitat will have had some effect on the population of those hole nesting species using the boxes, but with signs that the local decline has been mirrored elsewhere in Yorkshire and perhaps nationally, we probably need to look further afield to understand what the future holds for the local population of Pied Flycatchers.


To the late Mrs Gordon Foster and Mrs Rosanna James of Sleightholmedale Lodge for permission to site boxes at Sleightholmedale and for their encouragement and interest during the course of this long term study; and to Wilf Norman for supplying and permission to use the Duncombe Park data.


  1. The New Atlas of Breeding Birds in Britain and Ireland: 1988 – 1991. BTO, SOC, IWC.
  2. Follows, G.W. (1982) Some Aspects of the Breeding Biology of the Pied Flycatcher (Ficedula hypoleuca) in a North Yorkshire Woodland. Naturalist 107: 31–35.
  3. Follows, G.W. (1990) A Nest-box Study in North Yorkshire. Naturalist 115 113–122.
  4. Robert Spencer and Robert Hudson. (1978) Report on Bird-Ringing for 1976. Ringing & Migration 1 4.
  5. J.A.Clark, S.Y.Adams, W.J.Peach and J.R.Simons (1996). Report on Bird Ringing in Britain and Ireland for 1994. Ringing & Migration 17 (1) May.
  6. Personal communication ex W. Norman.
  7. F. Lander Pied Flycatcher – a cause for concern. Ringers’ Bulletin 10 No.2 p9 (1999)
  8. F. Slater Nest-box use in relation to height and wood treatment Ringers’ Bulletin 9 No.10 p74 (1998)


Gordon Follows has written an Appendix to this article summarising the 2000 figures: Sleightholmedale Nest Box Study - 2000 Summary.

Copyright © Gordon W Follows and Ron Gash, December 1999

Brooklands, The Grove
North Yorkshire
TS15 0HD


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