Dippers
Cinclus cinclus
In the River Teme Catchment
2009
Summary
Dippers were monitored at around 70 winter roost sites in the River Teme Catchment from 1987 to 2000. This Project resumed this monitoring in 2006, and has also implemented a nest box scheme. By the start of the 2009 breeding season, boxes had been installed at around 100 sites, and more than one-quarter of them have already been used.
Comparison of results obtained in 2006 - 2009 with those from the 1980s and 1990s show an initial overall decline in the number of Dippers, with a much greater decline on the lower reaches of the rivers than on the upper reaches, and a deterioration in the condition of the birds (measured by average body weight).
This is attributed to a loss of food as a result of reduced quality of the rivers, primarily due to pollution from, and silting up by, agricultural activities.
However, more Dippers were found in 2008 and 2009 than in any previous year, and this is attributed to an increase in the number of nest sites, and improved breeding success, in the upper reaches of the rivers as a result of the nest boxes.
Further long term monitoring of the Dipper population, and extending the nest box scheme, is recommended, to iron out any effect on the results from annual fluctuations, and the Environment Agency is recommended to analyse water sampling results from these river systems for the last 25 years, to ascertain if specific causes of the Dipper decline can be identified.
Photo © John Swift
Dippers
Cinclus cinclus
In the River Teme Catchment
Conservation of Threatened Birds
AONB Sustainable Development Fund Project
2009
Supported by Natural England and Severn Rivers Trust
Dippers feed almost exclusively on larvae that live on the stony beds of rapids and fast flowing streams, and are never far from such waters. The rivers in the South Shropshire hills, particularly the East and West Onny, the Clun (and its tributaries, the Unk and the Folly Brook), and other parts of the Teme catchment, are the County strongholds. Dippers stay here throughout the year, and might be seen either bobbing up and down on the rocks in the middle of the stream, or flying low over the water.
Breeding usually starts early, in late February or early March, and many nesting pairs will attempt to raise two broods. Though some Dippers nest in natural cavities along the riverbank, others build nests on ledges under bridges, and they take readily to nest boxes located directly above the flowing water, where predators are unable to reach them.
They are very territorial, so nests are evenly spaced on each stretch of river.
Because Dippers are restricted to, and dependent on, food from the river, they are relatively easy to monitor. Pairs nesting along poor quality (acidic or silted up) streams tend to lay their eggs later, lay smaller clutches, raise smaller broods, and raise only one brood. The average size of the territory, breeding success, productivity and survival rate are therefore all good indicators of the water quality.
As a result of the decline in the local population up until 2006, Dipper was added as a Target Species to the Shropshire Biodiversity Action Plan (BAP).
The Habitat Action Plan for Rivers and Streams in the Shropshire BAP also makes reference to Dippers as a key indicator species, and includes a number of actions to reduce the enrichment of the watercourses by agricultural activities (which cause diffuse pollution which in turn affects the Dippers’ food supply). The use of sheep dip / cypermethryn still continues to cause problems with invertebrates on certain smaller watercourses, and this can have a direct and dramatic effect on Dippers’ food sources.
The Dipper population in the River Teme catchment was monitored extensively in the late 1980s and 1990s, up until 2000. Concern about the apparent recent decline led to a reinstatement of this monitoring, together with action to improve breeding success, initially through the Upper Onny Wildlife Group, beginning in 2005, and subsequently through the Upper Clun Community Wildlife Group, from 2007 onwards.
In 2006-09 this Dipper Project was part-funded by the Shropshire Hills AONB Sustainable Development Fund and the Upper Onny Wildlife Group. The Project was extended in 2007, and Natural England’s River Teme Catchment Sensitive Farming Project contributed additional funding in 2007 and 2008, but was unable to continue funding in 2009. However, another division of Natural England contributed to the costs in 2009, and the Severn Rivers Trust has also provided financial support.
The Project consists of three complementary activities:-
This Report presents the Results for 2009, and also incorporates the results from 2006 onwards for ease of comparison. Where appropriate it compares these recent results to those from the late 1980s and 1990s.
The Report is a public document, and the contents should be disseminated as widely as possible.
The work in 2009 repeated surveys carried out at many of the same sites, by the same surveyor (A.V. Cross), since 2006, and in the late 1980s and 1990s.
During 2009 survey work was conducted at 92 past and present roosting sites for Dippers under road and footbridges over the Rivers Corve, Teme, Onny, Clun and Redlake and the Quinney Brook. Surveys were carried out on the nights of 29-30 September, and 1-2, 7, 12-13 and 16-17 October.
Bridges were visited in the hours of darkness and were inspected with a torch to see how many birds were roosting underneath them. Birds commonly roost on girders or in holes, drainpipes and other cavities below the bridges, including inside old Dipper nests and nestboxes.
The white breast shows well on a roosting bird and counts are a true measure of the numbers present.
Once a count had been obtained an attempt was made (under a BTO ringing licence) to catch as many of the Dippers as possible in order to ring or examine any ring already present. Catching is relatively simple as the birds sit tight and can be lifted off by hand or netted in a small hand-net. After ringing/examining the birds are replaced back under the bridge and the majority settle back down. A small percentage fly off and presumably then roost in bank side trees or under riverbanks.
A total of 92 sites were visited, five more than last year. All these five additional sites were visited for the first time in 2009. The number of sites has increased in recent years as the installation of nestboxes allows some bridges to be utilised as roost sites which were previously unusable. Birds were found at 55 of the 92 bridges visited with a total count of 145 birds – mean 1.58 Dippers / bridge checked or 2.64 Dippers/occupied roost. Apart from a decline in 2007, attributed to the extremely wet weather which created swollen rivers which made it difficult for Dippers to find food and raise young, these figures have shown a steady increase since this project started. Comparison of the 2009 figures with recent years is given in Table 1.
Table 1. Bridges Checked and Dippers Found
In 2006 and 2007, when these figures were plotted against figures obtained in the 1980s and 1990s and fitted with linear trendlines, the trendlines clearly showed a marked decline both in the number of Dippers per bridge and the number of Dippers present at occupied sites. The rate of decline in the number of Dippers per bridge checked was much steeper as many former bridge roost sites had been abandoned completely.
However, the total number of Dippers found in 2008 (129) was the highest ever up until that year, and the number in 2009 (145) was even higher. Although more roost sites are visited now, only 28 of the birds found were at new sites (i.e. 28 sites visited for the first time in 2006 or later, particularly to check if the new nest boxes were being used as roosts). The number found at old sites (117) equals the highest number found previously, in 1988. This is also an increase on 2008, when the 109 found was the second highest number recorded at sites visited prior to 2006.
The increase is attributed to improved breeding success in 2008, partly due to calm river conditions in the breeding season, and in 2009, when a relatively large number of broods of 5 were found. Perhaps more importantly, the nest box scheme has also led to increased breeding success, as a result of creating additional nest sites, and bigger brood sizes. This is set out later in this Report.
The results summarised in Table 1 are shown in Figure 1.
Figure 1. Decline and Subsequent Increase in Number of Dippers at Bridge Roost Sites in the Teme, Onny, Clun & Corve Catchments
When this monitoring re-started, it was felt whilst undertaking the fieldwork that the bridges on the higher reaches of the rivers had maintained numbers quite well whilst the numbers found under bridges on the lower reaches seemed to have dropped much more. To investigate this further, the rivers were divided into lower and upper sections and the data for total number of birds found under all bridges on these sections over the past 20 years were plotted.
The divisions made are, on the Clun at Clun bridge, on the Onny below Horderley, and on the Teme below Knighton.
These results are shown in Figure 2.
It can be seen in the case of all three major Rivers (the Onny, Clun and Teme) that the number of birds on the lower reaches has indeed dropped considerably, whereas numbers on the upper reaches have increased. This trend continued in 2009 for the Upper Teme and Upper Onny. Numbers on the Upper Clun were not as high as 2008, but they were still higher than in 2006 or any previous year. Numbers on the River Redlake up until 2008 had declined similarly to those on the lower reaches of the other rivers despite, on face value, being more similar in river morphology to the upper reaches of
these rivers. However, numbers showed a welcome increase in 2009, and reached similar levels to 20 years ago.
Numbers on the lower reaches of the rivers increased somewhat as well, but not sufficiently to reverse the previous downward trend.
Of the total of 145 birds observed roosting, 114 were caught for examination. The Age and Sex of the birds, together with the ratio of Males to Females, and of Adults to First-year birds, is shown in Table 2 below, together with the comparable figures for previous years.
The slight population bias towards males found in 2006 was partially repeated in 2008, and more strongly repeated in 2009, but it is not reflected in the 2007 or “All Previous Years” figures.
The 2006 Report stated that the ratio of adults to first-years has remained the same throughout the study, suggesting that there has been little change in the breeding success and productivity of those birds which survive to breed, or the survival rate of newly-fledged young birds. However in 2007 this ratio was considerably less (1.08:1), compared to 1.23:1 in earlier years, indicating either an increase in mortality of adults, or an increase in newly fledged birds, due to increased brood size, or an improved juvenile survival rate, or increased opportunities for adults to be able to breed (or a combination of all these factors). The ratio in 2008 reflected the same pattern, and in 2009 the proportion of young birds substantially exceeded the adults for the first time.
Table 2. Age and Sex Ratios of Dippers Caught at Winter Roost Sites
It will be seen from Part 2 of this Report, summarised in Table 5, that the average brood size of birds ringed in the nest was also lower in 2007. This was attributed to the abnormal extremely wet weather in May and June 2007, which affected breeding success. However, the average brood size found in 2008 was higher than in any previous year, and that in 2009 was the second highest, so it is not unexpected that the ratio of adults to first-year birds shows a corresponding decrease.
While there is little change in the ratio for 2008 compared to 2007, it is still considerably less than the 2006 or the “Up to 2000” ratio. The high proportion of young birds in 2008, and the even higher proportion in 2009, is attributed to increased brood size, and an increasing number of breeding pairs, due to the nest box scheme (see Part 3 of this Report)
Figure 2. Dippers Counted At Winter Roost Sites 1987 – 2008, By River Section
In 2009, 64 of the 114 different birds caught were already wearing rings, including 12 individuals (11 males and one female) ringed earlier in the year as nestlings. The average distance moved by these 12 newly-fledged birds was 3.38 kilometres. Only the one female moved further than five kilometres. The 11 males moved an average of 2.07 kilometres, while the female moved 17.81kilometres.
In 2008, 68 of the 122 different birds caught were already wearing rings, including 15 individuals (nine males and six females) ringed earlier in the year as nestlings. The average distance moved by these 15 newly-fledged birds was 4.34 kilometres. Three males and three females moved further than five kilometres. The nine males moved an average of 3.57 kilometres, while the six females moved an average of 8.02 kilometres.
In 2007, 37 of the birds caught were already wearing rings, including seven ringed earlier in the year as nestlings. The average distance moved by these 7 newly-fledged birds was 23.28 kilometres. The one male moved 15.98 kilometres, while the six females moved an average of 24.5 kilometres.
Forty-four of the birds caught in 2006 were already wearing rings including 13 ringed earlier in the year as nestlings. The average distance moved by these 13 newly-fledged birds was 7.55 kilometres. The seven males moved an average of 3.00 kilometres, while the six females moved an average of 12.85 kilometres.
The Movements of Recently Fledged Dippers are summarised in Table 3.
Table 3. Movements of Recently Fledged Dippers
It is known that the females of several species move further from the natal site than males. This is believed to be a natural selection mechanism to promote genetic diversity (i.e. reduce the risk of in-breeding).
The movements data has been submitted to the AONB Office for mapping.
All birds captured were weighed and the mean weight is shown in Table 4, together with the mean weight for similar age/sex groupings in 2008, 2007, 2006, and in all years before 2000 combined.
The 2006 Report stated “All sexes/age classes have therefore shown a decrease in mean body mass of between 3.3 % and 2 %, suggestive of decreased food supply”. This decrease was even more pronounced in 2007, for all except Adult males. For them the mean weight went up slightly in 2007, compared to 2006, but it was still considerably lower than the “Up to 2000” figure.
Table 4. Mean Weight of Dippers Caught at Winter Roost Sites
In 2009, all except First-year females were the lowest mean weight ever recorded.
The annual change is shown in Figure 3.
Figure 3. Annual Variation in Mean Weight.
The deterioration in the condition of the Dippers,
as measured by mean body weight, therefore appears to be getting still worse.
The 114 birds caught for examination, described in Part 1 of this Report and summarised in Table 1, were all ringed.
Because of the importance of this Dipper Project, an increased effort was made to find nests and ring nestlings in 2008 and 2009. The increased provision of nest boxes (see next section of this Report), together with monitoring of the boxes so the timing of ringing visits was more efficient, contributed to the large increase in nests visited. A total of an Adult Male, and Female, together with 35 broods containing 145 nestlings, were ringed at 33 different nest sites (a first and a second brood were ringed at two of these sites). The average Brood Size was 4.14.
Data for 2009, and comparison with previous years, in shown in Table 5.
The mean brood size found in 2009 was not quite as high as in 2008, but it was still higher than that found in 2006, and in the early period, and reversed the sharp decline found in 2007.
Table 5. Dippers Ringed at Nest Sites
The annual variation in different brood sizes is set out in Table 6, and is shown as Annual Percentages in Figure 4.
Table 6. Annual Variation of Brood Sizes at Ringed Nests
It can be seen that the incidence of large broods (five and six nestlings) declined in 2006 and 2007 compared with the earlier years, but recovered in 2008, perhaps due to the natural tendency of many species to increase productivity following years of poor breeding success. However, the incidence of larger broods was even higher in 2009, and the proportion of broods with 5 young (almost half) was the highest found so far.
The impact of the nest box scheme, described later in Section 3 of this Report, has almost certainly been the driving force in this increased brood size, and the increase in productivity generally.
It should be noted that ringing effort has always been rather variable and fitted in around work on other species. Little data has been collected on the frequency and timing of second broods.
Figure 4. Annual Variation in Brood Size at Ringed Dipper Nests (Percentages)
Dippers take readily to nest boxes. Each nest box must be located directly above the flowing water, in a position where predators are unable to reach it. Installation of boxes therefore increases the number of available nest sites (and potentially the number of possible territories, if suitable stretches of river would otherwise have no suitable nest site), and reduces the level of predation. Installation of boxes should therefore improve breeding success, and potentially increase the population. Regular inspection of the boxes also facilitates monitoring of the population and productivity, and helps determine the range of the species (i.e. which parts of the rivers are inhabited, and which are not).
Eighty-two bridges were surveyed by John Swift on the Rivers Onny and Clun, and their tributaries, in 2005-07. Sixty-five specially designed nest boxes for Dippers were made and installed at 55 different sites, mainly under these bridges. In 2007-08 a further 28 boxes were installed, 10 in the Upper Clun, and 18 on the Lower Clun as far as Clunbury. These latter boxes are intended to find out how far downstream the Dippers’ current range extends.
In 2008 – 09 a total of 24 boxes had been installed by the end of the calendar year 2008:-
Some further boxes were installed prior to the 2009 breeding season, and in total over 120 boxes have been installed at almost 100 locations in the Teme catchment since 2005 (not counting replacements). All of the boxes installed beforehand were monitored during the 2009 breeding season.
The relevant parts of this work have been carried out under the auspices of the Upper Onny Wildlife Group since 2005, and the Upper Clun Community Wildlife Group since 2007, and the work is also described in the respective Annual Reports of the two Community Wildlife Groups.
Subsequently two boxes were installed in 2005, and the Upper Onny nest box scheme continued since then. Virtually every bridge in the area which is marked on the OS Map now has a Dipper nest box, and several bridges have two. A total of 40 boxes had been installed under 26 bridges prior to the 2008 breeding season (12 on the West Onny, 17 on the East Onny, two on Darnford Brook and nine on Criftin Brook). The possibility of installing boxes at other locations, for instance under fallen trees which span suitable rivers, or on private bridges in the area that are not shown on Ordnance Survey maps, will continue to be explored.
All the boxes installed prior to 2009 were made of wood. Single boxes are one foot long, with an eight inch square entrance. Double boxes are twice that length, with an entrance at each end. However, several of these boxes were washed away each year when the rivers flooded during the winter.
As a result, a new design was trialed in 2009. A one-foot length of eight inch round flexible plastic pipe was used, and a plastic plant pot was forced into one end to enclose it. The intention is that, when the water level rises, the plant pot is washed away, but the pipe remains in place, so the whole box does not need to be replaced, but only a new plant pot needs to be inserted into the remaining pipe.
Several boxes of this type were installed prior to the 2009 breeding season, and at least two of them were used.
This design will be used in future on bridges prone to flooding.
Monitoring of the boxes in the Upper Onny and the Upper Clun areas, and known natural sites, continued in 2009
The number of pairs with nests that were found, together with the number in boxes, and the number of successful pairs, in shown in Table 7.
Table 7. Nests in Boxes
N.B. Table 7 lists the number of pairs found, not nests (i.e. a pair is counted once only, irrespective of whether the nest was completed, or whether nests failed or succeeded, and whether or not two broods were attempted or raised)
Initial survey work in the Upper Onny area in 2005 found seven Dipper nests. In three cases where the nest in each of two adjacent territories was found, the average nearest neighbour distance extended 1.4 kilometres along the river. More nest boxes, and more systematic monitoring since then, has shown that the river will support a higher breeding density than that. In 2007, seven nests were found along around 9 kilometres of river along Darnford Brook and the East Onny (including five in 2 kilometres, a neighbour distance of only 500m), and there were eight on this stretch in 2008 and seven in 2009. In all three years there were three on 7.5 kilometres of the West Onny (including two nests 1.3 kilometres apart), but only one on Criftin Brook (and only in two of the three years – none were found there in 2009. The twelfth nest in 2007 was at Eaton, just past the confluence of the West and East Onnys, but in 2008 there were nests at two different sites at Horderley, and one of these was occupied again in 2009. It is likely that the scheme has increased the Dipper population in the area, although some if not all of the pairs in boxes will have moved from less satisfactory natural sites.
On the upper Clun, there were 20 nesting attempts in 2007 (i.e. including completed nests with eggs – the 13 referred to above - and seven other nests which were started but not completed), and 17 in 2008 (eggs were laid in at least 14 of the 17 nests). In 2007 there were eight nests on 7 kilometres of river in Newcastle and upstream from there (average neighbour distance = 1 kilometre) and seven in 2008, two more nests in Newcastle on the Folly Brook, and two more further up the Folly Brook in both 2007 and 2008 (total of four in around 4 kilometres on Folly Brook). No nests were found between Newcastle and Clun, but there were two in Clun itself (one outside the UCCWG area) in 2007, three on the Unk in 2007 and two in 2008, and one on the Mardu at Whitcott Keysett in both years, but another three on the Mardu in 2008. In 2009, the territorial spacing was similar - nests of 14 pairs were found, eight on the Clun itself, four on the Folly Brook, one on the Unk and one on Mardu Brook
Inspection of a map of these nest sites on the Upper Onny and Upper Clun shows regular spacing, but several gaps. There are boxes in some of these gaps, but it is not yet known whether Dippers are using other (natural) nest sites on these stretches of river, or they are really absent; and if so, whether this is due to absence of suitable feeding habitat. Now the number of nest boxes is close to the maximum possible, further work will be done to clarify this, and identify where Dippers are absent. The average neighbour distance on the different sections of river where Dippers are found will also indicate the relative quality of the habitat, which may reflect natural variations, but may also indicate sections of river which have been rendered unsuitable by farming activity, or other man-made influences.
In 2009 the total number of Dippers found at winter roost sites was considerably higher than the number found in 2008, which was already the highest since monitoring started in 1987. The average brood size found in 2008 (4.39) was the highest found, and that in 2009 (4.14) was only slightly lower, and higher than in any previous year except 2008.
While low rainfall in the spring of 2008 undoubtedly helped adults raise large broods, as finding food in the unswollen rivers would have been easier than usual, analysis of the number of birds in the 34 ringed broods suggests that the nest box scheme has also played a part in the population increase.
In 2008, of 33 ringed complete broods (average brood size = 4.39), 10 were in nest boxes. The average brood size of these 10 nests was 4.60, compared with 4.26 in the 23 other nests.
The nest boxes are mainly in the Upper Onny and Upper Clun areas, and all those used were in these two areas. In the Upper Onny, five broods raised in boxes averaged 4.60 compared with 4.20 for the other five broods. In the Upper Clun, the five broods raised in boxes included in the analysis averaged 4.60, compared with 4.33 for the other three broods.
In 2009, of 35 ringed broods, (average brood size = 4.14), 14 were in nest boxes. The average brood size of these 14 nests was 4.00, compared with 4.24 in the 21 other nests. Six of the broods in boxes were on the Redlake or Lower Clun, where nest boxes were available for the first time.
However, boxes provide more secure nest sites, and are often preferentially selected by the Dippers. The boxes have only been available for four breeding seasons at most, and mainly in only two parts of the whole area, but in that time five pairs have moved from previously known nest sites into boxes in the Upper Onny and Upper Clun. This includes a pair that regularly built a nest on a girder that was too narrow to support it, so the nest usually fell off into the water, and another pair whose nest was regularly predated by rats. Neither of these two sites produced any fledged young in the two years prior to installation of boxes, but both pairs have succeeded in raising young since, as a result of the secure nest sites provided by the boxes.
More importantly, by 2008, seven pairs in the Upper Onny and Upper Clun had moved into boxes on bridges that were previously unsuitable (there was no ledge or hole where a nest could be constructed). Some of these were almost certainly additions to the total breeding population, as the boxes allowed new territories to be occupied in stretches of suitable river that had no available natural nest site.
In 2009, two of the pairs whose broods were ringed nested in boxes on the Redlake, and two others built nests in boxes on the Lower Clun. Of the former, one nest was in a box under a bridge that was otherwise unsuitable, and the other was under a bridge that was apparently suitable, but which had not been used previously. On the Redlake, one previously known pair moved into a box, but another previously unknown pair nested under a bridge that was unsuitable before the box was installed.
Although some of these pairs may have moved from natural sites, recollections from many years of monitoring Dippers suggest that the level of nest success in natural sites on riverbanks, or amongst boulders in the streams, is much lower than those under bridges, as they are more vulnerable to flooding or predation.
It must be stressed that these more marginal sites are more difficult to find, and most of the nests included in the current study, apart from those in new boxes, were still in relatively secure sites under bridges. The average brood size actually found outside nestboxes in 2008 and 2009 is therefore likely to be much higher than the overall average for the area. The boxes enable many more pairs to occupy secure nest sites, though there is no quantified data to calculate the actual increase in breeding pairs or improved breeding success as a result of this.
Table 8. Comparative Size of Broods in Nest Boxes (Ringed Broods Only)
Note that seven broods were ringed in 2009 in parts of the Catchment where no boxes have been installed yet, so these broods are excluded from Table 8.
It therefore appears that nestboxes have contributed to the increase in population that has been found, through increased brood size and breeding success as a result of providing more secure nest sites, and through providing new nest sites which allow new territories to be occupied.
However, it must be stressed that nest boxes will not in themselves allow the re-colonisation of the whole of the Dippers’ former range. The birds are very territorial, and each territory requires a food supply as well as a nest site. Action is necessary by the statutory agencies to improve the river quality to restore the previously available feeding sites, particularly in the lower reaches of the rivers.
It is important that the Project continues to monitor breeding success in nest boxes in future years, to confirm that their apparent benefit does not just a reflect an unusual pattern in 2008 , and they make a real contribution to achieving the Shropshire BAP target to increase the population.
Deans, P., Sankey, J., Smith, L., Tucker, J., Whittles, C., & Wright, C. 1992. An
Atlas of the Breeding Birds Of Shropshire. The Shropshire Ornithological Society.
(Referred to as The Atlas (1992) throughout)
Shropshire Biodiversity Action Plan, produced on behalf of the Shropshire Biodiversity Steering Group by Shropshire County Council November 2002, Revised and Updated November 2006 (including an Action Plan for Dippers)
Management Plan 2004-2009 and Management Plan 2009-14 for the Shropshire Hills Area Of Outstanding Natural Beauty, published by the Shropshire Hills AONB Office July 2004 & 2009
Cross, A.V. Monitoring the Dipper Population In The River Teme Catchment (Report of AONB SDF Project 2006 - 07)
Smith, L. Dippers In The River Teme Catchment (Report of AONB SDF Project) Annually since 2007
Smith, L. Lapwing, Curlew, & Other Wildlife In The Upper Onny Valley (Upper Onny Wildlife Group Survey Results & Report. Annually since 2004, and including Dippers since 2005
Smith, L. Upper Clun Community Wildlife Group Report Annually since 2007
Tony Cross carried out the survey work at 92 Winter Roost Sites in 2009, and ringed the Dippers there and at 33 nest sites, and provided most of the information (including the historical information) contained in this Report.
John Swift made and installed the Dipper nest boxes, and provided the remainder of the information contained in this Report.
The work is part of the Conservation of Threatened Birds in the Shropshire Hills AONB Project, co-ordinated by Leo Smith Ornithological Surveys and Consultancy.
This work was supported and funded by
This support is gratefully acknowledged.
Thanks are also due to:-
Leo Smith prepared this Report, which is printed on recycled paper
The Report and its content are public documents, and the results should be disseminated as widely as possible. Copies are being supplied to
Natural England
(All at Parkside Court, Hall Park Way, Telford)
Environment Agency
Shropshire Hills AONB Partnership
Upper Onny Wildlife Group
Upper Clun Community Wildlife Group
Severn Rivers Trust
Shropshire Wildlife Trust
Shropshire County Council
Royal Society For The Protection Of Birds
Shropshire Ornithological Society
Birds In Counties
Project Workers
It is hoped that people on the Distribution List will pass the Report on to other relevant members of their organisations.
Additional copies are available from
Leo Smith
8 Welsh Street Gardens
Bishop’s Castle
Shropshire
SY9 5BH
01588 638577
leo.smith@dsl.pipex.com
When the monitoring of Dippers restarted through this Project in 2006, there was little doubt that, in the catchments of the Rivers Teme, Clun, Onny, Corve and the Quinney Brook, which drain a large part of the South Shropshire Hills (as well as neighbouring Radnorshire and Herefordshire), there had been a steady decline in the number of Dippers roosting at traditional bridge winter roost sites over the previous 20 years or so.
Despite fairly major changes in the bridge network in neighbouring areas during the 20 odd years of this study prior to 2006, little renovation work had been undertaken in this area, and only two bridges had been altered to such an extent that they became unsuitable. Several other bridges were actually improved as potential roost sites by renovation / maintenance work.
Observations, although not well documented, also pointed to an abandonment of some of the traditional nest sites on the lower reaches of the rivers, especially on the Rivers Clun and Corve, despite the sites themselves appearing to remain suitable. Loss of habitat in the lower reaches of the rivers was confirmed by analysis of the numbers found at roost sites in the upper and lower reaches of the rivers in 2006-08, which showed that substantial declines had occurred in the lower reaches of all the rivers in this study.
These observations point to the causal factor of the decline being something other than the 
availability of suitable nest / roost sites.
The impression derived when visiting the roost sites, especially on the lower reaches of the rivers, is of a river-bed which is now subject to a much greater growth of slimy algae than it was during the late 1980s and early 1990s. This is presumably due to nutrient enrichment from agricultural run-off. Some silting up also appears to have occurred. Hopefully routine Environment Agency water sampling has recorded the increase in nutrient loading, and silt, in these rivers.
Much concern has also been expressed about the possible effects of sheep-dip chemicals such as cypermethryn on aquatic invertebrates, which would further reduce Dippers’ food supply.
The reduction in mean body mass of all age and sex categories of Dipper caught during 2006-09, in comparison to those caught in earlier years, strongly suggests a decline in food supply
. The relatively poor condition in which they start the winter threatens their survival through to the next breeding season, and this has almost certainly contributed to the decline, and is of great concern. In addition, the number of fish observed in the torchbeam whilst searching under roost bridges also appears to have declined greatly during the same period.
However, the number of Dippers found at roost sites in 2008 and 2009 was higher than in any of the previous years, and this appears to represent the beginning of a partial reversal of the decline. There is clear evidence that the nest-box scheme has improved breeding success in the upper reaches of the rivers, but provision of additional potential nest sites will be of no help to birds in the lower stretches of river where there is no food.
It therefore appears that the nest-box scheme, coupled with favourable weather conditions in 
the breeding season in 2008 and 2009, has led to an increase in the population, but the poor 
condition of the rivers, particularly in the lower reaches, has led to a contraction of range, and 
reduced the condition of the surviving birds. 
Further monitoring of the Dipper population in these catchments is therefore necessary
, through a combination of continued roost counts and the ringing of birds present, together with extending the nest box scheme, as well as visiting nest sites and ringing the birds there too. This will facilitate a much more systematic study of the range, neighbour distances, breeding success and productivity. In particular, extending the nest box scheme into the lower reaches of the rivers will confirm whether or not these waters have become unsuitable.
All this information will help discover the causal factors in the recorded decline of Dippers in some parts of the catchment. Their population is a key indicator of the health of the aquatic ecosystem in these upland rivers, and addressing the factors responsible for their decline will help to restore these increasingly barren waters to their previously healthy state, in accordance with the targets in the Shropshire Biodiversity Action Plan.
This work should be repeated for several more years
to remove any random annual fluctuations in the counts, particularly insofar as it might affect the relative population trends in the upper and lower reaches of the rivers, the anomalous trend on the River Redlake, and the high counts in 2008 and 2009. A rigorous statistical analysis of the data should also be carried out, to clarify the apparent trends identified above.
In addition, the Environment Agency is recommended to analyse water sampling results from 
these river systems for the last 25 years
or so, to measure nutrient enrichment and pesticide concentrations from agricultural run-off, and silting up, and assess whether these or other factors are responsible for the overall decline in the Dipper population, the variation in the decline between the upper and lower reaches of the rivers, the apparently anomalous trend on the River Redlake, and the observed reduction in mean body weight.
Tony Cross
John Swift
Leo Smith
March 2010