The Dipper Population of Derbyshire, 1958-68
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The Dipper population of Derbyshire, 1958-68 Philip Shooter During the mid-1950's I spent a considerable amount of time walking by the many rivers and streams in the Peak District of north-west Derbyshire. As I descended from the rough moorland edges into the greener valleys, or through the grey limestone dales, it seemed to me that Dippers Cinclus cinclus were commoner in the area than had been generally supposed. Their sedentary nature, and the existing public access agreements covering almost all the streams in this National Park, led me to undertake a population survey. It was soon clear that only about 10% of the Dippers in Derbyshire bred outside the National Park, mostly in the Matlock area which was easily accessible, so the survey was extended to cover the whole county. DISTRIBUTION AND TOPOGRAPHY In 1955 the Derbyshire Ornithological Society listed all the parishes in which Dippers were known to have bred; with their help I then checked every possible river and stream again during 1958-60. The occupied stretches were much as expected and (with the exception of some small streams) these are shown in fig. 1. This also marks the division of the Peak District into two parts—the 'Dark Peak', mosdy moorland and rough upland pasture on Millstone Grit, and the 'White Peak', pasture-land on Carboniferous Limestone with steep-sided wooded dales. The rivers of the latter, some 300-400 feet below the general altitude of about 1,000 feet, have very few tributaries compared with those of the Dark Peak, and many are fed from underground; for this reason they are warmer, the Wye being about 4°F more than the Derwent at the same altitude. Almost all the Dipper records for Derbyshire have been to the north-west of a line joining Sheffield, Ambergate and Ashbourne, which are all about 300 feet above sea- level; south and east of this line the rivers appear to be too deep and slow-moving. POPULATION During 195 8-62 the Dipper population was more or less static at about 107 breeding pairs: the Derwent and its headwaters, with the Amber, held about 51, the Dove and streams around Ashbourne 22, the Wye and its tributaries 20, the Goyt-Etherow system 13, and the Hipper one. There were no hard winters during this period, but the prolonged drought in the summer of 1959 made the limestone rivers (particularly the Dove) almost dry by late summer, and up to six Dippers were then seen feeding within a few feet of each other. 158 rivers occupied by Dippers other rivers county boundary • •• gritstone-limestone boundary Fig. i. North-west Derbyshire, showing rivers and streams occupied by Dippers Cinclus cinclus. The principal watershed (altitude 1,600-2,088 feet) extends NNE from Buxton. The boundary between Millstone Grit (in the north and east) and Carboni ferous Limestone is indicated i6o Dipper population of Derbyshire The extremely severe winter of 1962/63 froze almost all the head waters of the gritstone streams for many weeks, reducing the population to 97 pairs in the summer of 1963. Numbers slowly recovered, but one or two of the ten territories left unoccupied in 1963 remained so until 1966. The winter of 1966/67 was much milder than usual, and during the spring of 1967 five pairs were found in completely new territories on gritstone in the south-east of the species* range, one on the Hipper, one on the Amber and three near Matlock. Until then the limestone population on the Dove and Wye had remained constant, but in 1967 several surplus pairs were attempting to set up new breeding sites between existing long-established territories, causing considerable disruption. The lack of tributaries left little room for expansion in the limestone area, whereas the many small tributaries of the gritstone streams are used at times by additional pairs (although many are hardly suitable for Dippers). In the event 112 pairs bred in 1967, but another cold winter reduced the population to its original level of 107 or 108 pairs in the summer of 1968. One nesting site was lost by the building of the new Errwood Reservoir in the Goyt Valley, the only one destroyed during the survey period. In the past, however, more than 15 miles of river have been flooded by the construction of reservoirs in the gritstone area, probably destroying between six and nine territories; these steep-sided reservoirs are very deep and rarely visited by Dippers. Birds of prey are now very scarce in the Peak District, apart from Kestrels Fako tinnunculus, and overhanging rocks seem to afford plenty of roosting places which are safe from ground predators; occasional clutches of eggs are also still taken, mainly by children, but none of these factors is thought to have any significant effect on the population. In the gritstone area the major influence appears to be the severity of the previous winter, while reduced breeding success resulting from excessive territorial competi tion probably plays a larger part on the warmer limestone rivers. TERRITORIES Dipper territories in the gritstone area range from five to more than 50 feet wide and vary a good deal in length, more or less compensating for variation in width, whereas those on the Wye and Dove are fairly uniform in both width (15-30 feet) and length. The only two stretches of river where a fair comparison can be made between gritstone and limestone are the Noe and Derwent from Edale to Baslow and the Wye between Buxton and Bakewell. Both are about 15 miles long and flow south-east; they are of similar altitude, width and depth, and have about the same number of weirs. The first (on gritstone) held 13 territories and the second (on limestone) held 14 during each year of the survey. These figures are, of course, for the main river only, excluding tribu taries. In each case, weirs account for about two miles of water which Dipper population of Derbyshire 161 Dippers do not normally use, so the average density on both stretches of river was about one territory per mile. A survey carried out during 1962-67 by George Wigglesworth and students at Matlock College of Further Education (unpublished) showed that the limestone rivers of the Peak District contain over three times as much potential Dipper food as those on gritstone at the same altitude. It is thought, however, that it is not the length or breadth of a stream, nor even the quantity of food in the water, which governs territory size, but the accessibility of the food. Dippers can dive and swim in quite deep water to obtain their food, but they very much prefer water only a few inches deep, and it seems that territory size is principally controlled by the area of such shallow water. This is very difficult to express quantitatively and varies with rainfall, but during the breeding season it appears to be about an acre. Applying this to the two lengths of river mentioned previously gives a minimum territory length of about a third of a mile. In practice, this area of favoured feeding-ground is often broken by several stretches of much deeper water, which presumably explains the average density of one territory per mile. The deep, clear, alkaline waters of the River Lathkill, seven miles long, support only two pairs of Dippers, but they probably contain more potential Dipper food per mile than any other river in Derbyshire. BREEDING More than 100 nest records were obtained during the survey from British Trust for Ornithology nest record cards and other sources. About 25% relate to nests over 800 feet above sea-level, which is about the altitude at which moorland tributaries join to form the main rivers. Here the breeding season is short; the earliest date of finding a nest with eggs was 29th April, a clutch of five, and the latest nest with eggs was a clutch of four on 2nd June; the largest clutch was five and the average 3.7, Below 800 feet, a clutch of five was found as early as 19th March and young were sometimes still in the nest at the end of June; the largest clutch was six and the average 4.3. Four territories, thought to represent a good cross-section, were kept under close observation during 1960-68. The first was on a tributary of the Derwent, among wooded pasture at around 550 feet above sea-level. It was unoccupied in 196} and 1964, but during the other seven breeding seasons eight clutches totalling 35 eggs were laid, and 26 or 27 young fledged; the largest clutch was five. The second was also among wooded pasture at the same altitude, but on a tributary of the Wye. It was occupied for all nine breeding seasons, but two clutches of eggs were taken and one nest with eggs disappeared. From twelve clutches, totalling 49 eggs, 27 or 28 young fledged; the largest clutch was again five. The third was on gritstone moorland at i6z Dipper population of Derbyshire 1,200 feet above sea-level. During 1960-68, from six clutches totalling 28 eggs, 15 young fledged, but the fate of one clutch was unknown; the largest clutch was four. In spring 196} only one bird was seen, but a half-built nest was found; in 1968 a nest was built, but apparently no eggs were laid. The fourth and last territory was on a limestone river alongside a village; this site was chosen to see whether Dipper nests suffer the same fate as many others near a village.