Effects of stream acidity on non-breeding dippers cinclus cinclus in the south-central highlands of Scotland.код для вставкиСкачать
AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, VOL. 5, 25-35 (1995) Effects of stream acidity on non-breeding Dippers Cinclus cinclus in the south-central highlands of Scotland JOHN W. LOGIE Avian Ecology Unit, DBMS, University of Stirling, Stirling, FK9 4LA, UK ABSTRACT 1. Macroinvertebrate and Dipper Cinclus cinclus abundances and pH were assessed on four rivers in the south-central highlands of Scotland in early winter 1990 to examine the general applicability of Dipper/acidity relationships established during the breeding season (Ormerod et al., 1985; Vickery, 1991). 2. The most acid waters had the lowest invertebrate biomasses and Dipper densities. Birds on these rivers spent significantly more of the morning foraging and less time at rest, apparently reflecting reduced prey capture rates and hence the greater time required to replace overnight mass losses. 3. Analysis of published data on seasonal changes in Dipper population densities and individual energetic demands suggests that even accounting for the higher numbers of Dippers in all habitats during autumn and winter, energetic requirements both of individuals and populations are greatest during breeding, when the costs of self maintenance are coupled with the additional demands of chick rearing. 4. However, from the patterns found here and elsewhere during autumn it appears that the effects of acidity on Dipper populations are important generally during both the breeding and non-breeding seasons. INTRODUCTION Although the Dipper Cinclus cinclus is widely distributed over much of northern and western Britain (Gibbons et al., 1993), the susceptibility of many areas to surface water acidification (Battarbee, 1992, 1993), and evidence highlighting the effects of coniferous afforestation on water chemistry (Ormerod et a / . , 1989; Rundle et a / . , 1992; Ormerod et al., 1993) have led to concern over the status of Dipper populations in many upland catchments. Whereas other aquatic top predators and riparian passerines are able to utilize prey of terrestrial origin (Yalden, 1986; Mason and MacDonald, 1988; Vickery, 1991), Dippers are obligate lotic feeders, relying almost exclusively on aquatic invertebrates and fish (Ormerod, 1985; Ormerod et al., 1987). Many of the species important in the diet of Dippers are known to be scarce under acid conditions (Sutcliffe and Carrick, 1973; Harriman and Morrison, 1982) and breeding densities of Dippers have previously been linked to the availability of these prey items (Da Prato and Langslow, 1976; Price and Bock, 1983) and the acidity regimes influencing aquatic organisms (Ormerod et al., 1985; Ormerod et a/., 1986; Vickery, 1991). Since the importance of both catchment and riparian land use in determining the status of surface waters is now recognized (e.g. Ormerod e t a / ., 1989; Ormerod et a/., 1993), and European emissions abatement policies are currently being determined based on the susceptibility of aquatic communities to acidity effects (Nilsson and Grennfelt, 1988; Battarbee, 1992), a thorough understanding of the ecology of aquatic organisms in relation to water chemistry is required, at all CCC 1052-7613/95/010025-11 01995 by John Wiley & Sons, Ltd. Received 22 September 1993 Accepted 2 September 1994 26 J . W . LOCIE stages of their life cycle. Indeed, Boon (1992) states that ‘(river) conservation management is frequently ineffective because the requirements of important species or communities are not known’. It has been suggested that stream acidity will be of most importance for Dippers during the breeding season as acid waters may be unable to provide the calcium-rich prey needed during egg formation (Ormerod et al., 1991) or the large food items required for chick-rearing (Ormerod et al., 1985). However, acidity effects at other times have as yet received little attention. The aim of this work was to assess the general applicability of Dipper/acidity relationships established during breeding to populations in winter. METHODS pH and physical variables Four rivers were selected for study: the River Devon (NN 940 045-NN 994 044), River Knaik (NN 835 079NN 799 128), Duchray Water (NN 515 010-NN 442 004) and Keltie Water (NN 649 069-NN 639 139), all rising in the south-central highlands of Scotland (Figure 1). For comparative purposes only upper river sections were studied, reducing topographical differences between sites to a minimum. Water samples were collected during periods of intermediate flow at five sites along the length of each river, at approximately weekly intervals over a 3 week period. Samples were collected in sterile plastic bottles with each bottle being thoroughly washed in the river before collection of the final sample. pH measurements were made in the laboratory on the day of collection using a Corning-Eel pH meter (Model 7). The slopes and altitudinal ranges of the river sections were obtained directly from 1 :25 000 0s Maps. Macroinvertebrate sampling To minimize differences attributable to short-term seasonal variations, all invertebrate samples were collected within a period of 22 days in October 1990. Each length of river was divided into five equal sections from which an area of riffle suitable for feeding by Dippers was chosen. Three randomly selected sites (0.0929m2) were then enclosed within the raised frame of a Surber sampler (Hynes, 1970) and all large stones were lifted and cleaned by hand before being discarded downstream. The remaining enclosed area was then thoroughly disturbed allowing any invertebrates present, including those removed from the larger stones, to be washed into a collecting net (mesh size 350pm). Samples were preserved in 70% ethanol River E m Figure I . The study sites in the south-central highlands of Scotland. EFFECTS OF STREAM ACIDITY O N DIPPERS 27 in the field for later analysis. In the laboratory all invertebrates were sorted by family and size, with individuals being counted and grouped into the size classes < 5 mm, 6-10 mm and 2 11 mm before being dried to constant mass at approximately 40°C. All molluscan species were removed from their shells prior to drying and weighing. ‘Available biomass’ was calculated to include all individuals greater than 5 mm in length. Ormerod (1985) recorded elminthid larvae (Coleoptera) within the faeces of Dippers in numbers considerably scarcer than expected from their relative abundance as assessed by kick-sampling, suggesting that this prey may not be routinely captured by Dippers. Since the usual maximum size achieved by Elminthidae is approximately 5.5 mm, for this study all individuals greater than 5 mm in length were, therefore, arbitrarily assumed suitable prey for Dippers. All groups were included since Dippers are thought not to show strong prey selection outside the breeding season (Ormerod and Tyler, 1986; Smith and Ormerod, 1986). Analysis of the macroinvertebrate data was undertaken using one-way analysis of variance with log,, ( x + I ) transformations being applied in all cases. Comparisons were made between rivers for total biomass, available biomass (all families combined and families considered separately in each case) and available prey items (all families combined). Significant differences were identified using Tukey’s Honestly Significant Difference test. Dipper density As late autumn is known to be a period of general population stability (Shaw, 1979; Bryant et al., unpublished data), and following moult Dippers are generally conspicuous, visual censuses may be used to assess their comparative status on different river systems. The Dipper population of each river was assessed directly, with each river section being walked twice (on different days) and the numbers of Dippers recorded. Tributaries entering the main rivers were checked for approximately 150 m to record birds that may have moved briefly from the main river courses. Care was taken that the movements of birds were also noted to avoid recording individual birds more than once. Time-activity logging The daily activity budgets of birds on the Rivers Devon, Knaik and Keltie were sampled using methods similar to those described by Bryant and Tatner (1988). Activity data were collected during all daytime hours using a telescope at distances of 30-80 m, and recorded on a micro-processor based data-logger. To avoid bias, care was taken not to record any individual bird more than once in a day or at similar times in following days. To achieve this, the starting point for activity recording was changed daily and movement along the river bank was always in a single direction to avoid repeatedly encountering any individual. Changing river conditions and time of year are known to affect the foraging behaviour of Dippers (Da Prato, 1981; Bryant and Tatner, 1988; O’Halloran et al., 1990); consequently all observations were made over a 10 day period of cool, clear weather with low discharges and by visiting the Devon and Knaik on alternate days. Differences in behaviour are therefore considered not to have been influenced by changes in river flow or weather conditions, although the short period of data collection may mean that the results presented are not representative of other seasons or conditions. For analysis, the activities were grouped under one of two main headings. Bryant and Tatner (1988) concluded that the general form of foraging adopted by Dippers was primarily governed by water depth and may, therefore, be expected to change within and between riffles. For this reason, a general heading of ‘Feeding’ was used and included all activities recorded as foraging, swimming and diving (see Bryant and Tatner, 1988 for descriptions of activities). ‘Stationary activities’ included resting, preening and singing. Although commonly associated with foraging, flight movements were often initiated by 28 J . W. LOGIE Table I . Dipper densities and selected habitat features of four rivers in central Scotland during early winter, 1990. River Section length (km) 10.0 9.5 9.5 6.9 Duchray Water Keltie Water River Knaik River Devon Stream pH (mean L- SD) 4.13 6.11 6.62 7.07 (0.23) (0.33) (0.67) (0.31) Dipper density birds 10 km-' (mean k SD) Mean gradient (m km-I) 0.0 & 0.0 2.1 L-0.0 11.6+ 1.2 35.4 & 2.0 6.5 12.Sa 8.4 8.4 Altitudinal range (m) 30-95 75-267 110-190 205-260 "Excludes Bracklinn Waterfalls. Dipper densities based on two census counts, p H from 15 water samples. the presence of, or disturbance from, other birds and in many cases the motivation for flight was unclear. For the purposes of analysis, therefore, flying was considered separately. Comparisons of time allocations between rivers were made for similar activities at similar times using the Mann-Whitney 'U'test. Comparisons of changes between daily time periods were undertaken on each river separately using the Kruskal-Wallis test. Only observations exceeding 60 s are included, with each receiving similar weighting to avoid bias. The data for all birds are treated together as differences between sexes and adultdjuveniles are minimal (O'Halloran et af., 1990). RESULTS The mean pH values for the rivers sampled fell within the range of 7.07 to 4.13, with the River Devon appearing circumneutral, the Knaik slightly acid and the Keltie and Duchray Waters highly acidic (Table 1). Macroinvertebrate biomass All invertebrate samples were dominated by plecopterans and trichopterans, with ephemeropterans also contributing substantially to the invertebrate biomass of the Keltie, Devon and Knaik (Figure 2). h % 1500 - Duchray Keltie Knaik Devon 1250 5 2 750E a E P T O E P T O Bhemeroptera Elecoptera E P T O '&ichoptera E P T O Others Figure 2. The macroinvertebrate biomass of four rivers in south-central Scotland during early winter 1990. Biomass figures represent dry weights derived from 15 samples (five sites). Standard deviations given for total biomass only. 29 EFFECTS OF STREAM ACIDITY ON DIPPERS Gammarus spp. accounted for the bulk of the remaining biomass on the circumneutral Knaik and Devon, whereas on the more acidic streams dipteran larvae appeared proportionately more abundant. A trend of decreasing stream biomass and invertebrate abundance was associated with increasing acidity (Figure 2). The River Devon had a significantly greater total biomass than all other rivers (F3,20= 78.46, p < 0.01) with the Knaik, Keltie and Duchray holding only 36'70, 1 1 % and 8% of the Devon total respectively. The River Knaik also held a significantly greater total biomass than both the Duchray and Keltie (F3.20 = 62.42, p<O.Ol). The Duchray and Keltie could not be separated statistically although the less acid Keltie held 29% greater total biomass and 32% larger available biomass than the Duchray Water. Of an estimated mean total of 421.9 individuals m-2 (all families combined) on Duchray Water, only 45.5%, principally plecopterans, trichopterans, and dipterans, together accounting for 48.5% of the total biomass, were of an available size. Fifty-one per cent of the total biomass of the 601.1 invertebrates m-2 of Keltie Water was available to Dippers although this was contributed by only 19.6% of all individuals, due to the numerically predominant families-the ephemeropterans Ecdyonuridae and Baetidae and the plecopteran Leutridae-together making up 71 070 of all individuals but accounting for only 1 1 (70 of available biomass. The Knaik held a significantly greater available biomass (F3,20= 42.49, ~ ~ 0 . 0than 1) the two more acid rivers with 82% of the total biomass and a mean of 1438 invertebrates m-2 (61.4% of all individuals) being in an available form. On the River Devon, available biomass was significantly greater than for all other rivers (p<O.OI). A mean of 1960.2 individuals m-2 (57%) was in an available form, accounting for 79.6% of total biomass. All significant differences between rivers in total and available biomass were also significant for corresponding prey item counts (F3,20= 22.05, p < 0.01 (total prey items); F3,zo=51.71, ~ ~ 0 . (available 0 1 prey items)). Dipper density The trend in invertebrate biomass was paralleled in the abundance of Dippers on the river sections studied, with the Devon holding the greatest density of birds, followed by the Knaik and the Keltie (Table 1, Figure 3). No Dippers were recorded on Duchray Water. For the Devon, where the assumed 'true' population size was known from intensive mist-netting and roost-catching, the recorded population was equivalent to 79.4% of all ringed birds. This is likely to be a minimum percentage, however, since birds captured in the area at roost were known to forage away from the section studied. In addition to the full censuses, the presence of birds was also noted for all rivers on each visit to collect water and during an additional complete walk of each river section while collecting invertebrate samples. m n 'E - 3500 - 3000: 2500 - Available Biomass Dipper Dcnsity b h -30 2000' -20 B - .-x 1500- a A? 1OOo- -10 500- 0-0. 2- t - I = I . , . , 6 'CI ri 0 5 Figure 3. Dipper and macroinvertebrate abundances in relation to pH on four rivers in south-central Scotland during early winter, 1990. Biomass figures represent dry weights derived from 15 samples (five sites), Dipper densities from two census counts. 30 J. W . LOGlE River Ik v o n River Knaik 100 100 80 60 gb 40 40 & 20 20 0 0800-1000 H 1000-1200 Stationary Feeding 1200-1400 RYh3 1400-1600 1 0 Other 0800-1000 1OC&1200 Stationary 1200-1400 Feeding 0 Total time =219.45 minutes, Total time=244.35 minutes, n=57 observations (15 birds) n=67 observations (10 birds) 1400-1600 Flying Figure 4. The time-activity budgets of Dippers on the Rivers Knaik and Devon during a period of low water flow in early winter, 1990. The low number of birds recorded under acid conditions was confirmed in this way, since it is unlikely that birds could be missed after repeated visits to a study site. Time-activity budgets The mean time-activity budgets of Dippers on the Rivers Devon and Knaik are given in Figure 4. Insufficient data were available for Keltie Water and so this site is excluded. During daytime hours on both rivers, Dippers spent most time foraging. On the Knaik, however, significantly more time was spent foraging during the time periods 0800-1OOOh (94.5% of time on the River Knaik, 55.9% on the Devon; p<O.O15) and 1000-1200h (88.3% on the Knaik, 49.9% on the Devon; p<0.025), this being associated with less time spent at rest (0800-l000h, 4.55% on the Knaik, 39.05% on the Devon, p<O.Ol; 1000-1200 h, 10.8% on the Knaik, 43.4% on the Devon, pe0.02). Although Dippers on the Devon appeared to spend more of the afternoon foraging (43.1% and 61.1 Yo for time periods 1200-1400 h and 1400-1600 h respectively on the River Devon, 35.7% and 38.7% on the River Knaik), and less time resting (51.7% and 35.0% on the Devon, 62.2% and 61.0% on the Knaik) these differences were not significant. No differences were apparent for time spent in flight within or between river sections. DISCUSSION Previous studies have found strong evidence for prey-mediated Dipper densities (Da Prato and Langslow, 1976; Price and Bock, 1983; Ormerod et al., 1985; Ormerod et al., 1986. Vickery, 1991). Although most work on Dippers has focused on breeding birds, food availability is known to be a major factor determining the autumn population sizes of many passerine species, in many cases improving winter survival and/or inducing autumn immigration and settling in years of high food abundance (e.g. van Balen, 1980; Jansson et al., 1981; Kallander, 1981; Nilsson, 1987). Census results have suggested that Dipper/pH relationships evident in the breeding season may also be important among non-breeding populations, yet only one previous study has investigated autumn Dipper densities on acid rivers (Ormerod et al., 1988). The results presented here, obtained in an area remote from earlier studies, further support the EFFECTS OF STREAM ACIDITY ON DIPPERS 31 findings of Ormerod et al. ’s autumn study, suggesting that acidity relationships noted during breeding may also apply at other times. Altitude, gradient and riffle area (Robson, 1956; Shooter, 1970; Shaw, 1978; Marchant and Hyde, 1980) are all known to be important influences on local Dipper densities. The Devon, Knaik and Keltie flow at similar altitudes and gradients and these factors could not cause marked differences in Dipper abundances. Riffle area was not assessed quantitatively, although the rivers were apparently similar in this respect and it is known to be highly correlated with gradient if the effect of river width is removed (as was done through experimental design). The effects of pH, however, manifest through invertebrate communities, appear to be important in determining local Dipper densities (Figure 3). Considering the Devon and Knaik as circumneutral and the Keltie and Duchray as acid (the classification used by O’Halloran et al., 1990) the circumneutral waters held at least 3.35 x more ephemeropterans, 3.41 x more plecopterans, 3.06 x more trichopterans, 3.45 x more total biomass (all families) and 5.57 x more available biomass (all families), and this is entirely consistent with their larger Dipper populations. Duchray Water supported no Dippers although prey biomass was not significantly different from that of the Keltie Water. However, more than half the biomass of the Duchray Water was in the form of stoneflies, an order known to favour interstices and leaf packs rather than the substrate surface (Hildrew et al., 1980; Ernest and Stewart, 1986). These prey items may be less available, or more costly to exploit than many epibenthic mayfly and caddis fly families, thereby placing additional demands on foraging Dippers (Ormerod and Tyler, 1991a). Although it has been suggested that plecopterans may also be avoided by Dippers (Ormerod and Tyler, 1991a), this seems unlikely in habitats where other prey are scarce. Fish, known to make up a considerable, if not the principal, component of the winter diet of Dippers (Ormerod and Tyler, 1986, 1991a) may also be scarce under acid conditions and the Duchray area is known to support only limited trout Salmo trutta numbers (Harriman and Morrison, 1982). A reduction in the reproductive success of Dippers on acid rivers has been noted (Ormerod et al., 1991; Vickery, 1991). However, this would not appear to explain the recorded density differences between rivers in this study as juvenile Dippers are known to undertake considerable autumn dispersal movements within and between catchments (Newton, 1989). Of 17 juvenile Dippers captured in the studied section of the River Devon in the autumn of 1990, only 4 (24%) had been ringed within the Devon catchment, indicating substantial intercatchment juvenile movements. All rivers are known to be in catchments with local areas of high juvenile production (Bryant et al., unpublished data) and although a territory selection component may limit populations along acid streams (Ormerod, personal communication) it appears unlikely that suitable habitats would not be colonized by dispersing juveniles. The differences in Dipper density recorded between the Devon and Knaik appear to reflect the foraging area required per individual to fuel their daily energy expenditure (DEE). Increased territory sizes have been noted in Dippers breeding under acid conditions (Ormerod et al., 1986; Vickery, 1991) and the difficulty of following individual birds for time-activity logging on the Keltie further implies the use of widely spaced foraging areas on this river. Although O’Halloran et al. (1990) found differences in DEE between rivers of different chemistry to be non-significant, the reduced prey capture rates in acid waters demanded a greater time allocation to foraging activities. The energetic demands of foraging have not been considered here (see Bryant and Tatner, 1988); nevertheless, it does appear that a greater time allocation to feeding was required on the Knaik compared with the Devon during the hours before midday (Figure 4).Ormerod and Tyler (1990) found body mass to peak in both sexes 2-3 hours after sunrise and the increased foraging in the early hours of the day may reflect the greater time required to replace reserves used at night and so regain mass. The lack of difference in time allocation in the afternoon hours indicates that, even with the lower available food on the Knaik, birds may not be unduly stressed by this time. However, all results were recorded under low water flows. Fluctuations in stream discharge mean that opportunities for feeding are generally unpredictable for Dippers in winter (Da Prato, 1981) and differences in food availability and hence foraging effort will be more important under less favourable conditions when the efficient 32 J . W . LOGlE / B 15- 4.5 5.5 6.5 PH 1.5 8.5 4.5 5 .5 6.5 1.5 8.5 PH Figure 5. (a) Densities of Dippers in upland Wales in winter and during breeding in relation to pH (after Ormerod et a/., 1985, 1988). (b) Population energetic demands of Dippers in upland Wales at different seasons in relation to pH (after Ormerod et at., 1985, 1988; Bryant and Tatner, 1988). food collection is at a premium. Consistent with this, data from Wales have shown birds on acid streams to be of lower mass than those in circumneutral waters (Ormerod and Tyler, 1990) even though they may spend up to 60% more time foraging (O’Halloran et a(., 1990). A comparison of the breeding and autumn census data collected by Ormerod and co-workers in upland Wales (including data from Round and Moss, 1984) shows evidence of significant density differences between seasons (analysis of covariance, F1,30= 27.591, p < 0.001). However, energetic studies in Wales (O’Halloran et al., 1990) and Scotland (Bryant and Tatner, 1988) have shown that peak (adult) energetic demand per individual occurs during chick-rearing, a period combining high adult activity with a requirement to meet the needs of their chicks. If the energetic demands of the population are considered (Figure 5b, see Appendix A for methods), rather than simply the population density (Figure Sa), breeding can be seen to be not only the period of peak individual energetic demand but also peak population demand. The more acid waters, where autumn energetic requirements appear greater, are clearly suitable for self maintenance but may be unable to suport the peak requirements of parents with a brood to feed and, perhaps for this reason, are unused during breeding. However, the additional energetic demands associated with low ambient temperatures, coupled with the reduced active day lengths, will still place considerable importance on the efficient collection of food during winter (Bryant and Tatner, 1988). Further, the effects of winter spates on food acquisition will be most severe where prey availability is already low. As birds in acid habitats are known to be of lower mass (Ormerod and Tyler, 1990) this may indicate that they are unable to withstand periods of additional stress which may lead to the elimination of even non-breeding Dippers from some acid rivers. Of particular concern is not only the persistence of acidity effects both in time and space, but also the strong coincidence in the range of Dippers (Gibbons et al., 1993) and the distribution of areas with acidified waters (Battarbee, 1993). Further, in many catchments, stream acidification related to acid deposition has been accentuated by recent changes in land use, particularly commercial afforestation with non-native conifers. Highly afforested catchments, are known to hold reduced numbers of breeding Dippers relative to streams draining catchments with few exotic conifers (Ormerod ef al., 1986). Consistent with these concerns has been a 9% reduction in the breeding range of Dippers in Britain between 1968/72 and 1988/91 (Gibbons et al., 1993). Riparian management has been suggested as providing an important first step in protecting many of Britain’s rivers (Boon, 1991) and recent studies have pointed to the importance of bankside rather than EFFECTS OF STREAM ACIDITY ON DIPPERS 33 solely catchment vegetation in determining the breeding densities of several riverine birds (Ormerod et al., 1986; Flousek, 1987; Ormerod and Tyler, 1987, 1991b). Whereas the abundance of invertebrate prey may be reduced in acid relative to circumneutral streams, these differences may not be apparent within the riparian zone, although invertebrate prey may be significantly reduced in areas of exotic conifers relative to both moorland/grassland and deciduous woodlands (Ormerod and Tyler, 1991c). In contrast with the Dipper, relationships between stream pH and the breeding densities of Grey Wagtails Motacilla cinerea and Common Sandpipers Actitis hypoleucos, both species characteristic of upland streams but known to utilize a variety of riparian habitats, have been inconsistent or lacking. The breeding abundance of Grey Wagtails, however, has been positively related to the presence of bankside broadleaved cover (Flousek, 1987; Ormerod and Tyler, 1987). Riparian broadleaved cover may also provide an important input of terrestrial prey into the aquatic ecosystem (Mason and McDonald, 1982). It appears likely that at least a proportion of this would be utilized by Dippers, known to accept soil invertebrates carried into the stream system (Ormerod, personal communication) and the emerging adults of aquatic invertebrates (Shaw, 1978). Recent results have also noted elevated biomass within aquatic invertebrate communities at sites of increased coarse particulate organic matter (CPOM) input and retention, independently of stream acidity (Richardson, 1991; Dobson and Hildrew, 1992). This would have obvious benefits for foraging Dippers whilst the provision of additional shelter or nest sites by deciduous trees may also be important. Accordingly, positive relationships between deciduous cover and Dipper densities have been noted during both the breeding (Ormerod et al., 1986) and non-breeding seasons (Logie and Bryant, 1994). However, recent evidence from Scottish populations suggests that the benefits of bankside broadleaved trees may be subsumed under the more powerful gradient of stream acidity. As the riparian zone usually covers only a limited area of any catchment and hence contributes only a minority of stream run-off, chemical effects from the major part of the catchment may still dominate water quality, prey availability and Dipper densities. Riparian options, principally the maintenance of broadleaved cover along the stream margins, are clearly of importance for riverine species utilizing terrestrial prey (Ormerod et al., 1987, 1991b,c), forest birds within the riparian margins (Bibby et al., 1989) and for Dippers themselves (Ormerod et al., 1986; Logie and Bryant, 1994). However, with the reliance of Dippers on aquatic prey, the benefits of such riparian management options may be insufficient to mitigate the effects on prey availability determined by water chemistry. Management strategies to protect and conserve Dipper populations may, therefore, only be truly effectual if implemented at the catchment or supra-catchment level. ACKNOWLEDGEMENTS I am grateful to Dr S. Newton for help and advice during the initial stages of this work and to Professor D. M. Bryant, Dr S. J . Ormerod and an anonymous referee for valuable comments on earlier drafts of this document. Additional funding from the Charles and Barbara Tyre Trust, Argyll, is also gratefully acknowledged. REFERENCES Balen, J . H. van. 1980. ‘Population fluctuations of the Great Tit and feeding conditions in winter’, Ardea, 68, 143-164. Battarbee, R. W. 1992. Critical Loads and Acid Deposition f o r UK Freshwaters. A n Interim Report to the DOE from the Critical Load Advisory Group (Freshwater sub-group), Research paper 5 , Environmental Change Research Group, University College London. Battarbee, R. W. 1993. Critical Loads and Acid Deposition for UK Freshwaters (Summary). A Report to the DOE from the Critical Load Advisory Groupfor Freshwaters, Research paper 10, Environmental Change Research Centre, University College London. 34 J . W . LOGIE Bibby, C. J . , Aston, N. and Bellamy, P. 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Energetic calculations (Figure 5b) Breeding demands refer to total daily energy expenditure (DEE) of a nestling-feeding parent and two young (assumes broods of four in all habitats). Winter demands assume no mortality between autumn censuses and winter, both autumn and winter calculations assume equal numbers of males and females. All energetic costs are from Bryant and Tatner (1988) and could slightly exaggerate the thermoregulatory costs in Wales. Breedinga Autumnb WinterC Male DEE (kJ d - I ) Female DEE (kJ d - I ) 529 224 265 507 192 214 aNestling period from hatching until 15 days, the period of peak breeding energetic demand bOctober, time of autumn censuses. January, period of peak non-breeding energetic demand.