Numbers Of Waders Research Articles

Regime shift and changes in sediment morphology driven by sea level rise affect abundance of migratory waders

Regime shifts are likely to have strong impacts on all trophic levels and their interactions. In addition, weather conditions and sea level rise together with tidal currents may change sediment morphology in coastal areas and estuaries. Here, we studied the effects of these drivers of ecosystems on abundance of seven wader species feeding on macrozoobenthos in the German sector of the Wadden Sea. We hypothesized that regime shift caused decreases in wader abundance and that changes in sediment morphology driven by sea level rise and tidal amplitude affected these changes. We analyzed numbers of waders on a short term, 1998–2016, that matched a large-scale geographical study of sediment morphology in the German sector, and at a long term, 1987–2019. Changes in numbers of waders were estimated as slope of coefficients of the trends of wader species numbers in the two periods. The average (se) year of decreases in numbers of waders was initiated for most species and tidal basins in the year 1992 (± 0.4) that followed a regime shift in the Southern North Sea and in the Wadden Sea in 1988–1989. There were no statistical differences between the years when numbers of waders started to decrease between the northern and the southwestern tidal basins of the German sector, indicating that the drives affected both coastal sections simultaneously. Significant relationships were found between slopes of coefficients of wader species and sediment morphology and physical features as sea level rise and tidal amplitude. Relationships were most pronounced for the long term, 1987–2019 compared to the short term, 1998–2016. We conclude that regime shift and changes in sediment morphology driven by sea level rise and sediment change can affect abundance of migratory wader species in large estuaries and shallow coastal waters.

Synchronous Declines of Wintering Waders and High-Tide Roost Area in a Temperate Estuary: Results of a 10-Year Monitoring Programme

Migratory wader populations are declining across all major flyways, mostly due to habitat loss and human disturbance. Portuguese wetlands act as key wintering and stop-over sites for many waders due to their strategic location in the East Atlantic Flyway. Despite their importance, there are major knowledge gaps in some areas, such as the Sado Estuary. The main goal of this study was to identify the phenological patterns and wintering trends of the most abundant wader species in this wetland, and to compare the results with the population trends found at regional and flyway scales. A comprehensive dataset of count data collected monthly in high-tide roosts under a set monitoring programme (2010–2019) was used. The results revealed strong declines in the overall number of waders and of three of the most abundant species—Avocet (Recurvisostra avosetta), Dunlin (Calidris alpina), and Common Ringed Plover (Charadrius hiaticula)—associated with a 21% reduction of the suitable high-tide roosts across the study area. These trends are similar to those compiled from winter counts for the entire Sado Estuary, and at regional (Tagus Estuary) and flyway levels. Our results highlight the need to maintain suitable high-tide roosts to contribute towards reverting current declines in wader populations.

The influence of different aspects of grouse moorland management on nontarget bird assemblages.

Conflict between stakeholders with opposing interests can hamper biodiversity conservation. When conflicts become entrenched, evidence from applied ecology can reveal new ways forward for their management. In particular, where disagreement exists over the efficacy or ethics of management actions, research clarifying the uncertain impacts of management on wildlife can move debates forwards to conciliation.Here, we explore a case‐study of entrenched conflict where uncertainty exists over the impacts of multiple management actions: namely, moorlands managed for the shooting of red grouse (willow ptarmigan) Lagopus lagopus in the United Kingdom (UK). Debate over how UK moorlands should be managed is increasingly polarized. We evaluate, for the first time at a regional scale, the relative impacts of two major moorland management practices—predator control and heather burning—on nontarget bird species of conservation concern.Birds were surveyed on 18 estates across Northern England and Southeast Scotland. Sites ranged from intensively managed grouse moors to moorland sites with no management for grouse shooting. We hypothesised that both targeted predator control and burning regimes would enhance ground‐nesting wader numbers and, as a consequence of this, and of increased grouse numbers, nontarget avian predators should also be more abundant on heavily managed sites.There were positive associations between predator control and the abundance of the three most widespread species of ground‐nesting wader: strong effects for European golden plover Pluvialis apricaria and Eurasian curlew Numenius arquata and, less strongly, for common snipe Gallinago gallinago. These effects saturated at low levels of predator control. Evidence for effects of burning was much weaker. We found no evidence of enhanced numbers of nontarget predators on heavily managed sites.

Interacting effects of agriculture and landscape on breeding wader populations

The capacity of different landscapes to sustain viable populations depends on the spatial and temporal availability of key population-specific resources. Heterogeneous landscapes provide a wider range of resources and often sustain higher levels of biodiversity than homogeneous ones. Across the globe, agricultural expansion has resulted in large-scale homogenisation of landscapes with associated declines in many taxa. However, during the early stages of agricultural development, in terms of area and intensity, increased landscape heterogeneity and changes in local productivity through fertilizer inputs can potentially increase resource availability for some species. Agriculture in Iceland is currently neither highly intensive nor extensive, and primarily occurs as hayfields (>90% of agricultural land) embedded within a mosaic of semi-natural wetlands and heaths. These landscapes support internationally important breeding populations of several wader species but the role of agricultural land in promoting or constraining breeding wader densities is currently unknown. Understanding the relationship between cultivation and wader populations is important as the area of cultivated land is predicted to expand in Iceland in near future, largely through conversion of the remaining semi-natural wetlands. Here we (a) quantify relationships between breeding wader densities in lowland Iceland and the amount of cultivated land and wetland in the surrounding landscape using density estimates from 200 transects in common semi-natural habitats, (b) assess the extent to which cultivated land affects wader densities in these landscapes, and the potential effects of future agricultural expansion at the expense of wetlands on wader populations. Wader densities in semi-natural habitats were consistently greater when surrounding landscapes had more wetland at scales ranging from 500 m to 2500 m, indicating the importance of wetland availability. However, the effects of cultivated land in the surrounding landscape varied with altitude (ranging from 0 to 200 m); in low-lying coastal areas, wader numbers decline with increasing amounts of cultivated land (and the lowest densities (<1 km2) occur in areas dominated with cultivated land), the inverse occurs at higher altitudes (>100 m a.s.l., where lowest densities occur in areas without cultivated land). This suggests that additional resources provided by cultivated land may be more important in the less fertile uplands. Further agricultural conversion of wetlands in low-lying areas of Iceland is likely to be detrimental for breeding waders, but such effects may be less apparent at higher altitudes.

George Richard (Dick) Potts (1939–2017)

George Richard (Dick) Potts (1939–2017)

Bird communities at two marshes in Øvre Forra, today and 40 years ago

In the protected area Ovre Forra in Central Norway, birds were surveyed at two marshes in two succeeding years at the beginning of the 1970s. By following the same survey procedure 40 years later in these unchanged marsh habitats, I sought changes in the breeding bird community. While the total number of waders seems to have been quite stable, the abundance of some of the involved species might have changed during this period of time. Due to low numbers, and annual variations, the implications for some of the involved species have to be taken cautiously. The most abundant species in Ovre Forra is now the Common Redshank Tringa totanus . This species has outnumbered the earlier most abundant species in this bird community, the Meadow Pipit Anthus pratensis . The numbers of the Meadow Pipit, Northern Lapwing Vanellus vanellus , European Golden Plover Pluvialis apricaria and Common Sandpiper Actitis hypoleucos all tend to be lower than for 40 years ago. The Common Gull Larus canus and two Tringa species, the Common Redshank T. totanus and Common Greenshank T. nebularia , seem to have increased in numbers, while the situation for the Common Snipe Gallinago gallinago and the Whimbrel Numenius phaeopus appears to be stable. The Ruff Calidris pugnax used to be a characteristic species on the Ovre Forra’s bogs in the 1970s, but has since vanished. Altogether, the study shows an apparently quite stable avifauna over the time span of 40 years in this environmentally stable reserve.

Loss of periodicity in breeding success of waders links to changes in lemming cycles in Arctic ecosystems

Lemming population cycles in the Arctic have an important impact on the Arctic food web, indirectly also affecting breeding success in Arctic‐nesting birds through shared predators. Over the last two decades lemming cycles have changed in amplitude and even disappeared in parts of the Arctic. To examine the large scale effect of these recent changes we re‐analysed published data from the East Atlantic Flyway (EAF), where a relationship between lemming cycles and wader breeding success was earlier found, and new data on breeding success of waders in the East Asian–Australasian Flyway (EAAF).We found that 1) any long‐term periodicities in wader breeding success existed only until the year 2000 in the EAAF and until the 1980s in the EAF; 2) studying these patterns at a smaller spatial scale, where the Siberian–Alaskan breeding grounds were divided into five geographical units largely based on landscape features, breeding success of waders from the EAAF was not correlated to an index of predation pressure, but positively correlated to Arctic summer temperatures in some species. We argue that fading out of lemming cycles in some parts of the Arctic is responsible for faltering periodicity in wader breeding success along both flyways. These changed conditions have not yet resulted in any marked changing trends in breeding success across years, and declining numbers of waders along the EAAF are therefore more likely a result of changing conditions at stop‐over and wintering sites.

Shorebirds in the Gediz Delta (İzmir, Turkey): breeding and wintering abundances, distributions, and seasonal occurrences

The present study describes breeding and wintering abundances, distributions, and seasonal occurrences of shorebirds (waders) in the Gediz Delta (İzmir-Turkey). Breeding abundance and distribution, and wintering abundance data were collected by carrying out breeding bird surveys in 2002 and 2006, and waterbird censuses between 1990 and 2007, respectively. Seasonal occurrences were determined using 432 different observations corresponding to 5043 different records between 1990 and 2007. Thirty-eight wader species were observed with a weekly average of 22.5 ± 4.0. Breeding evidence was obtained for 9 species in 2002 and 8 species in 2006. Average number of waders identified during yearly mid-winter waterbird census was 8731 ± 2530, corresponding to 42.2 ± 25.9% of the whole wintering wader population in Turkey. During the present study 11 species were encountered in internationally significant numbers. The Gediz Delta is one of the key sites in the Western Palearctic region for breeding, wintering, and migrating waders.

Birds of the Severn Estuary and Bristol Channel: Their current status and key environmental issues

The Severn Estuary and Bristol Channel encompass a number of designated sites supporting populations of waterbirds and seabirds that are of national or international importance, including the Severn Estuary and Burry Inlet Special Protection Areas (SPAs)/Ramsar Sites and Carmarthen Bay, the UK’s first marine SPA. Here, we provide an overview of the present numbers and trends of the waterbirds and seabirds using these sites, updating previous reviews undertaken prior to these designations. We further provide a summary of the main issues that have affected the status of the area’s bird populations. Declines in the numbers of waders on the Severn Estuary and the southwest over the last two decades have been linked to climate change. The Sea Empress oil-spill impacted both breeding seabirds and the wintering Common Scoters in Carmarthen Bay, though numbers of the latter recovered 3years after the spill. At the Burry Inlet, Oystercatcher numbers have fallen over the last 25years and considerable research has been undertaken into the conflict with cockle and mussel fisheries. A long-term study at Cardiff Bay, at the mouth of the Severn, revealed a significant impact on the survival of Redshanks following its impoundment and has helped to further understanding of responses of waterbirds to estuarine habitat loss. The potential impacts of the construction of a tidal power scheme on the Severn Estuary are also discussed.

The effects of human activities on shorebirds, seabirds and waterbirds of Comerong Island, at the mouth of the Shoalhaven River.

Comerong Island is a 660 ha Nature Reserve 15 km east of Nowra. The island sits at the mouth of the Shoalhaven River estuary (Fig. 1) and is the fifth most important area for shorebirds or waders in New South Wales (Morris 1983, Lane 1987). Ninety species of shorebirds or waders (Charadriformes), waterbirds (including ducks, rails, herons, ibises and cormorants) and seabirds have been recorded on the island (Appendix 1). Up to 7000 ducks and 1200 other waterbirds have been counted on the estuary (Chafer pers. Comm.). I will concentrate mostly on shorebirds as these are more restricted to estuarine habitats. Table 1 lists relative abundance for some of these species. Nearly half the Double-banded Plovers Chadradrius bicintus and almost a quarter of the Red-necked Stints Calidris ruficollis found on estuaries within New South Wales are found on Comerong Island. There is little doubt that habitat changes and loss of wetlands are the most serious problems in shorebird conservation (Senner & Howe 1984). Permanently flooding estuaries (Joyes et al. 1976) and removing intertidal sand flats and mud bars (Lum 1978, Evans et al. 1979) have resulted in significant declines in wader numbers. On the other hand, islands created by dumping dredge materials can create nesting and roosting habitat for some species (Soots & Landin 1978 in Senner & Howe 1984). Unfortunately, we have already lost or altered about 60% of the mainly freshwater coastal wetlands in New South Wales since settlement (Goodrick 1970).

Complementary use of natural and artificial wetlands by waterbirds wintering in Doñana, south‐west Spain

Abstract The Doñana wetland complex (SW Spain) holds more wintering waterfowl than any other wetland in Europe. This study focused on the use made by 12 common waterbirds (eight ducks and four waders) of the natural seasonal marshes in Doñana National Park (DNP) and the adjacent Veta la Palma (VLP) fish ponds created in the early 1990s. Data used were from aerial and terrestrial surveys collected between October and February during six consecutive winters from 1998/99 to 2003/04. Changes in distribution of each bird taxon were related to changes in the extent of flooded marshes within DNP. Up to 295 000 ducks were counted in VLP during dry periods, and up to 770 000 in DNP when it was flooded. The timing and extent of flooding in DNP was highly variable, but there was a consistent pattern in which ducks concentrated in VLP during dry months and winters but redistributed to DNP as more of it was flooded. This refuge effect was also strong for black‐tailed godwits Limosa limosa, but much less so for other waders. Waders feed mainly on invertebrates, and invertebrate biomass in VLP was found to be higher than in DNP. Ducks feed mainly on seeds and plant material, which are more abundant in DNP when flooded. When water levels in DNP were stable over the course of a winter, or controlled for in multivariate models, the numbers of ducks at VLP declined over time, probably due to reduced availability of plant foods. In contrast, numbers of waders at VLP were more stable, and their invertebrate prey became more abundant over time, at least in the winter 2003/4. In this extremely important wetland complex, the value of natural and artificial wetlands for wintering waterbirds are complementary, providing suitable habitat for different species and for different conditions in a highly variable Mediterranean environment. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Climate change causes rapid changes in the distribution and site abundance of birds in winter

AbstractDetecting coherent signals of climate change is best achieved by conducting expansive, long‐term studies. Here, using counts of waders (Charadrii) collected from ca. 3500 sites over 30 years and covering a major portion of western Europe, we present the largest‐scale study to show that faunal abundance is influenced by climate in winter. We demonstrate that the ‘weighted centroids’ of populations of seven species of wader occurring in internationally important numbers have undergone substantial shifts of up to 115 km, generally in a northeasterly direction. To our knowledge, this shift is greater than that recorded in any other study, but closer to what would be expected as a result of the spatial distribution of ecological zones. We establish that year‐to‐year changes in site abundance have been positively correlated with concurrent changes in temperature, but that this relationship is most marked towards the colder extremities of the birds' range, suggesting that shifts have occurred as a result of range expansion and that responses to climate change are temperature dependent. Many attempts to model the future impacts of climate change on the distribution of organisms, assume uniform responses or shifts throughout a species' range or with temperature, but our results suggest that this may not be a valid approach. We propose that, with warming temperatures, hitherto unsuitable sites in northeastern Europe will host increasingly important wader numbers, but that this may not be matched by declines elsewhere within the study area. The need to establish that such changes are occurring is accentuated by the statutory importance of this taxon in the designation of protected areas.

Effects of an agri-environment scheme on wader populations of coastal meadows of southern Sweden

Survey data on breeding wader densities and grazing intensity on coastal meadows on the Baltic island of Öland from 1988, 1998 and 2003 were used to evaluate density and density-changes of four common and widespread wader species, lapwing Vanellus vanellus, redshank Tringa totanus, oystercatcher Haematopus ostralegus and ringed plover Charadrius hiaticula in relation to grazing intensity. Population trends over time and changes in densities in relation to changes in grazing management were evaluated. Both wader densities and the environmental variable grazing intensity demonstrated significant spatial autocorrelation, but all main results were unaffected when accounting for spatial structure in the statistical tests. Breeding densities of investigated species were positively related to grazing intensity and local changes in grazing management affected the local change in wader densities. However, average grazing intensity increased over time whereas wader numbers generally remained constant or declined. Thus, changes in grazing intensity could not explain changes in overall breeding numbers. One explanation for this is probably that grazing mainly affected distribution of birds. Some evidence of a buffer effect was found in the redshank and ringed plover, in which breeding densities declined proportionally more in non-shore than in shore habitat. This was probably due to a redistribution of birds to shore habitat. It is concluded that grazing management is essential for the occurrence of waders, but might not be sufficient to ensure long-term viability of wader populations on coastal meadows.

The possible impact of climate change on the future distributions and numbers of waders on Britain's non‐estuarine coast

The relationship between weather and wader distributions on non‐estuarine coasts is described and used to predict how future wader distributions may respond to climate change. The distributions of eight out of nine species of wader commonly wintering on the non‐estuarine coasts of Britain altered between two similar surveys, in 1984/85 and 1997/98, that covered 78% and 38% of Britain's 12 594 km of non‐estuarine coastline, respectively. These eight species moved at least in part either eastwards along the winter isotherms or northwards. These changes in distribution broadly coincide with a distributional shift towards the species’ respective breeding grounds and are correlated with the local winter weather over the period: increasingly mild extreme temperatures and changes in mean rainfall, mean wind speed and wind‐chill. Based on the scenarios for Britain's climate in 2020 and 2080, it is predicted that the distributions of the waders will move away from the west. The non‐estuarine coasts of Britain hold particularly high proportions of the international flyway populations of Ringed Plover Charadrius hiaticula, Sanderling Calidris alba, Purple Sandpiper Calidris maritima and Ruddy Turnstone Arenaria interpres that are all expected to show continuing decline to 2080. Overwintering waders appear to be good indicators of the effects of climate change.

The importance of the introduced hedgehog ( Erinaceus europaeus) as a predator of the eggs of waders (Charadrii) on machair in South Uist, Scotland

Populations of waders (Charadrii) on machair habitats on the island of South Uist were counted and wader nests were monitored to determine nest success and causes of failure. There were large declines in wader numbers and nest success following the spread of the introduced hedgehog throughout the island. Predation of wader eggs by hedgehogs was frequent, but susceptibility varied among wader species, being between 0 and 60% of the nests started. Wader species that were susceptible to hedgehog predation tended to suffer larger population declines than those that were less susceptible, with the exception that the ringed plover ( Charadrius hiaticula) population declined markedly even though their nests were depredated by hedgehogs infrequently. A combined analysis of mark-resighting and radio-tracking data produced an average hedgehog population density estimate of 57 adults km −2. It was estimated that wader eggs only provided 0.7–5.5% of the energy requirements of hedgehogs, even during the season when most eggs were taken. From this it is argued that the abundance of hedgehogs and the intensity of their predation on wader eggs are unlikely to diminish in response to wader population declines. Unless some presently unidentified density-dependent mechanism compensates for low nest success, local extinctions of susceptible wader species are likely if no action is taken to reduce hedgehog predation. This appears to be the first documented case of hedgehogs threatening an internationally important bird population with regional extinction.

The dispersion of arctic breeding birds according to snow-free patch dimensions during the spring thaw in the north-eastern Taimyr Peninsula, Russia

The dispersion of passerines, ptarmiganLagopus mutus and waders in relation to available snowfree patches is described after spring arrival but prior to the main thaw at Pronchishcheva Lake, north-eastern Taimyr, in June 1991. The numbers of ptarmigan and passerines were proportional to patch perimeter but disproportionately more birds occurred on smaller patches. In contrast, wader numbers were proportional to patch area but disproportionately more occurred along the perimeters of large patches. These patterns are discussed in relation to risk of predation and food availability. We speculate that the predation risk is higher in small patches for uniformly coloured birds but may be lower for white and pied species. The availability of overwintering plants, fresh plant growth and insects is thought to be highest at the perimeters of snow-free patches.

Consumption of benthic invertebrates by waterbirds in the Oosterschelde estuary, SW Netherlands

The number of waders in the Oosterschelde, S.W. Netherlands, declined after a reduction in intertidal area due to the construction of a storm surge barrier and secondary dams, suggesting that the carrying capacity had been reached (Schekkerman et al., 1993). In this paper we present data on consumption and predation pressure by birds to explore whether the reduction in their numbers is due to prey depletion or to other factors.

Wind effects on prey availability: How northward migrating waders use brackish and hypersaline lagoons in the sivash, Ukraine

Large numbers of waders migrating northward in spring use the Sivash, a large system of shallow, brackish and hypersaline lagoons in the Black Sea and Azov Sea region (Ukraine). The bottoms of these lagoons are often uncovered by the wind. Hence, for waders the time and space available for feeding depend on wind conditions. In hypersaline lagoons the benthic and pelagic fauna was very poor, consisting mainly of chironomid larvae (0.19 g AFDM·m −2) and brine shrimps Artemia salina, respectively. Brine shrimp abundance was correlated with salinity, wind force, wind direction and water depth. Dunlin Calidris alpina and curlew sandpiper Calidris ferruginea were the only species feeding on brine shrimp. As brine shrimp densities are higher in deeper water, smaller waders such as broad-billed sandpipers Limicola falcinellus are too short-legged to reach exploitable densities of brine shrimp. In brackish lagoons the benthic and pelagic fauna was rich, consisting of polychaetes, bivalves, gastropods, chironomid larvae, isopods and amphipods (8.9 to 30.5 g AFDM·m −2), but there were no brine shrimps. Prey biomass increased with the distance from the coast, being highest on the site that was most frequently inundated. Dunlin, broad-billed sandpiper and grey plover Pluvialis squatarola were the most abundant birds in the brackish lagoon. Due to the effects of wind-tides only a small area was usually available as a feeding site. Gammarus insensibilis was the alternative prey resource in the water layer, and their density varied with wind direction in the same way as brine shrimp. Curlew sandpipers and dunlins in the hypersaline lagoons and broad-billed sandpipers in the brackish lagoons often changed feeding sites, probably following the variation in prey availability. Only because of the large size and variety of lagoons are waders in the Sivash always able to find good feeding sites.

Seasonal changes in the importance of coastal wetlands in Ghana for wading birds

Forty-two species of waders, comprising 29 Palearctic migrants, eight residents, two partial migrants and three vagrants, were recorded in three years of montly surveys of shorebirds in coastal Ghana. Eleven species occurred in internationally important populations: curlew sandpiper, little stint, sanderling, spotted redshank, grey plover, ringed plover, black-winged stilt, avocet, black-tailed godwit and turnstone. Arrival of migrant waders in coastal Ghana began in August and reached a peak in November/December. Most waders left the Ghana coast in January, leaving only a small proportion which remained until final departure in April. Another peak in numbers was observed during the spring passage in some years for some wader species. The proportion of migrant wader populations oversummering on the Ghana coast varied with species, and from year to year. While the total populations of some species departed, as much as 30% of the populations of others remained on the Ghana sites during the northern summer. The wader numbers recorded in the different sites and the relative importance of the key sites showed annual fluctuations. Two sites, the Keta and Songaw Lagoons, qualified as internationally important wetlands in terms of their total wader numbers and by virtue of the fact that each supported internationally important populations of seven species of waders. Three other sites also supported internationally important populations: Sakumo Lagoon (4 species), Korle Lagoon (3) and Esiama (1). Two sites, Keta and Korle Laggons, face immediate threat of coastal erosion and/or proposed developments involving drainage and land reclamation. The Sakumo and Songaw Lagoons have great potential for development as conservation areas and proposals have beeb submitted to this effect.

Perspectives in using shorebird counts for assessing long-term changes in wader numbers in the Wadden Sea

Over the past 20 years, a tradition has developed in western Europe of carrying out shorebird surveys in estuaries. These counts are mostly conducted by amateur ornithologists, usually well acquainted with the area in which they are active. The results of these surveys are well reproducible for at least the abundant species, and information is available on the size of the errors which occur. In several parts of Europe, shorebird surveys have been carried out long enough to allow trend analysis of population size. Four examples from Great Britain demonstrate that monitoring schemes may also be used for management purposes, especially if combined with specific research projects. Long-term shorebird surveys can be used not only for simple analysis of trends in bird numbers, but also to provide a deeper understanding of the functioning of the ecosystem. They may also act as a tool to document the effects of management activities in the area concerned. Shorebird surveys in the Wadden Sea have not only revealed the extremely large importance of the area, especially for waders, but also show that different areas are exploited in different ways. They have also provided data on changes in bird numbers throughout the year. Apart from the counts carried out in January, the frequency of counts in most areas in the Wadden Sea has been too low to allow the results to be used for monitoring purposes. In an area as large as the Wadden Sea, some sites, often supporting large numbers of birds, may not always be covered during simultaneous surveys. This may seriously affect the completeness of the count. The best way to overcome this problem is to calculate index values, using only those sites which have been surveyed in successive years. Two monitoring schemes are proposed which may be effective in providing relevant information for management purposes.