MONITORING LONG TERM TRENDS OF BIRD POPULATIONS IN SWEDEN

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1 S. SVENSSON, Monitoring long term trends of bird populations in Sweden.In : Anselin, A. (ed.) Bird Numbers 1995, Proceedings of the International Conference and 13 th Meeting of the European Bird Census Council, Pärnu, Estonia. Bird Census News 13 (2000): MONITORING LONG TERM TRENDS OF BIRD POPULATIONS IN SWEDEN S. Svensson ABSTRACT Swedish common birds have been monitored for more than twenty years using territory mapping, point counts and a combination of point counts and line transects. Most work has been based on the participation of volunteers and arbitrary choice of plots and routes. Hence there are geographical and habitat biases. Starting in 1996, a new sampling system was introduced, representatively covering the whole country with a grid of permanent routes with fixed positions. We report population trends over the previous two decades. The Tree Pipit has declined in recent years. The long term decline of the Starling has continued. The Chaffinch has been very stable in all parts of the country. The two subspecies of the Willow warbler show different trends: trochilus has declined recently whereas acredula has not. Among the woodpeckers, Green Woodpecker and Wryneck have declined but Great Spotted and Black Woodpeckers have been fairly stable. The Stock Dove has declined drastically and the Wood Pigeon remained stable. Both Lapwing and Snipe are strongly declining. Department of Ecology, University of Lund, Ecology Building, S Lund, Sweden INTRODUCTION When monitoring bird populations, different census methods are used in different countries and in different monitoring programmes. When the methods are standardized, so that the same method is always used at the same site every year, it matters little which method is used. Results that come from a mixture of methods are equally as useful as results from a single method as long as a standard is maintained through time for each individual count. The main weakness of most monitoring programmes is not the actual counting method but the sampling strategy. In many programmes, e.g. the Swedish one, the sampling strategy has not been considered seriously. The census sites have been chosen arbitrarily, usually by the census takers without any rules. There is of course an awareness of the difficulties that may arise as a consequence of the fact that the samples are not representative in terms of habitat and geographical location. The North American Breeding Bird Survey randomly selects the starting point and direction of a route within a one by one degree block. This stratified random sampling ensures representative counts on a continental scale but does not eliminate habitat bias. In this report I will first briefly describe the Swedish bird monitoring system, which in 1995 had been in operation for 25 years, then provide some examples from the summer point counts, and finally discuss improvements of the sampling strategy now being introduced. The Swedish bird monitoring system Apart from several species specific monitoring programmes, mainly of rare or endangered birds, the multi-species programmes may be grouped into four categories: (1) winter counts of waterfowl (the IWRB counts in autumn and mid-winter), (2) migration counts (standardized trapping at Falsterbo and Ottenby bird stations, and counts of visible migration at Falsterbo), (3) winter bird counts (all species, point routes, five times every winter), and (4) breeding bird counts

2 The breeding bird counts belong to three categories: (1) point counts by volunteers all over the country since 1975, (2) territory mapping by volunteers all over the country since 1969, and (3) territory mapping and 10 km combined point and line transects in eleven specially selected "baseline" or "integrated" environmental monitoring areas (since 1980). Distribution of point count routes in Sweden The distribution of routes over Sweden is given in Fig. 1, both for summer and winter, and the proportion of routes within different regions, compared to proportions of the total land area for each region, are shown in Figure 1. The number of census routes in different parts of Sweden (left is summer, right is winter). The three regions used for some of the index calculations are indicated in the summer map. The southern border of the northern region coincides approximately with the southern limit of the boreal zone. Table 1. It is evident that the number of routes is heavily biased in favour of the southern provinces. The three southern regions contain 73 % of all summer routes and 83 % of all winter routes but they cover only 34 % of Sweden's land area. And even within the regions, there is uneven coverage. The three major population centers, i.e. the Stockholm-Uppsala, Gothenburg, and Malmö/Helsingborg areas, host a much higher proportion of routes than the rest of the regions. Table 2 provides information about the habitats of the routes in comparison with the true proportion of different habitats in southern Sweden, where most of the routes are located. The table clearly shows that there is a strong habitat bias. Only farmland is sampled in the proportion that it covers. Deciduous forest is greatly overrepresented in relation to coniferous forest, and mires and clear-cut forest areas are underrepresented. The habitat bias is even more prominent in the northern provinces, which can easily be seen from the route distribution maps (most farmland is located along the coast; coniferous forest and mire predominate in the interior). The Table 1. Proportion of routes in different regions of Sweden in relation to the proportion of the total land area of Sweden in each region Region Percentage of routes Percentage of land area Summer Winter Northern Norrland Southern Norrland and Dalarna Svealand except Dalarna Northern Götaland Southern Götaland

3 subalpine birch forests and alpine habitat in the western parts of northern Sweden are not sampled at all. The concentration of routes in the most densely populated areas incorporates another bias, that of proximity to urban areas, and also a road bias, since some routes are censused with a car as a means of transport between the stops. Temporal coverage and turnover of routes The number of routes has increased since the start of the project in The number of summer routes has increased from below 100 to more than 250. The number of routes declined during the period 1981-'86 (Fig. 2A), primarily due to low recruitment activity and insufficient feed-back to the volunteers. When the Nature Conservation Agency successively increased its support of bird monitoring during the 1980s it became possible to supply all participants with annual progress reports and newsletters, giving detailed results and lists of participants acknowledging what they had done. This contributed substantially to recruitment. Figure 2. Number and turnover of summer routes. (A) Total number of routes (black bars) and number of routes common in adjacent years (shaded bars). (B) Proportion of routes still active after one year. (C) Proportion of routes still active after five years. (D) Correlation between change in number of routes in relation to the proportion of new routes in the previous year. Table 2. Division of the routes among major habitats in comparison with the percentages of these habitats in Götaland and Svealand (the southern half of Sweden where the majority of the routes are located). Habitat The routes (%) True values (%) Coniferous forest Deciduous forest Farmland Mire, heath, clear-cut forest

4 Since the population trends are estimated by a chain index based on the data from common routes of successive pairs of adjacent years, the number of "effective" routes is lower than the total number of routes (Fig. 2B). In 1975-'81 between 70 and 80 % of the routes were censused at least two years in a row. This figure increased to between 80 and 90 % for the period 1982-'94. Hence, more than 80 % of all route counts have been used for analysis. Fig. 2C shows the proportion of routes, started in a certain year, that were still active five years later, thus quantifying a more long-term turn-over rate. The proportion of routes still active after five years increased from only 30 % in 1975 and 1976 to a stable value about 65% from 1982 onwards. The ten year turn over rate stabilized in a similar way at 55 %. Hence these figures may be used as an estimate of expected route age. If the change in number of routes is plotted against the percentage of new routes recruited each year (Fig. 2D), one can see that the total number of routes has always increased if at least 30 % new routes has been added. If the number of new routes amounted to less than 15 % there was always a decline in the number of routes. In the interval % new routes, the changes were small. Thus, the Swedish system of volunteer route runners must recruit that proportion of new routes every year in order to replace discontinued routes. It should be noted, however, that the Swedish system requires that the same person carries out the census every year. If a new person takes over a route, that route is considered to be new. If old routes could be taken over by new observers and maintained in the system, routes would be active longer. RESULTS & DISCUSSION Two different indices I have only used chain indices based on common routes in adjacent years. But this index may be calculated in two ways. The first method is to total the number of individuals each year and then calculate the change between the two sums. The second method is to calculate the change for each route, then report the average of all these changes. I have found that most often the two methods give very similar results. The example of Tree Pipit is given in Fig. 3 for two different geographical zones of Sweden (cf. Fig. 1). The indices based on totals and on averages are almost identical. In addition, the fluctuations in the two regions are also very similar, indicating that the changes have been parallel over the whole of southern Sweden. The decline since the late 1980s is quite alarming. The population size is now only half of the average level of the period 1975-'90. The next few years will show whether it is an unusually steep but temporary drop or a long term and more serious event. The Starling (Fig. 4) is an example where the two indices do not agree. The Starling is known to be declining in Sweden, particularly in the northern, boreal part of the country. However, when I divided Sweden into two regions, it was only in the southernmost region where a decline was evident when the index was based on annual totals of common routes. But when using an index based on average change, the populations of both regions showed declines. Such a difference could occur if routes with few Starlings show a stronger decline than routes with many Starlings. If that were the case, the proportion of routes with Starling, must have declined more in the northern than in the southern of the two regions. In fact, in the southern zone, there was no decline at all of the proportion of routes with at least one Starling, whereas that proportion declined very much in the northern zone

5 Figure 3. Population indices calculated by two different methods for Tree Pipit Anthus trivialis in two different zones (see Fig. 1). Figure 4. Population indices for Starling Sturnus vulgaris for two different zones (see Fig. 1), calculated in three different ways. Figure 5. Population indices for Willow Warbler Phylloscopus trochilus in southern and northern Sweden (see Fig. 1)

6 Chaffinch and Willow Warbler The Chaffinch and the Willow Warbler are the two most abundant species in Sweden. The Chaffinch (Fig. 6) is the most stable of all species, with no trend in any region of Sweden. The larger variation in the northern part of the country may reflect the smaller sample there. The Willow Warbler shows a different picture compared to that of the Chaffinch (Fig. 5). The two southern zones 1-7 and 8-12 show the same trend, first an increase over several years, then a decline. There is no significant difference between the two curves, so they have been combined. But there is a clear difference between southern Sweden and the northern, boreal zone. For the first fourteen years the two parts of Sweden had the same trend, a slow increase, but then the northern population continued to increase whereas the southern population declined. This difference is interesting because the two populations are different subspecies, trochilus and acredula, distinguished by morphology, colour, migration routes and wintering areas. Woodpeckers Among the forest birds, the woodpeckers are important components. Certain species have proved to be sensitive indicators of the state of the forests, for example the Middle Spotted, White-backed, Three-toed and Lesser Spotted Woodpeckers. The first species is now extinct in Sweden, the second one has become extremely rare and the other two are declining. There are four common woodpeckers, the Black, Green and Great Spotted, which are true woodpeckers, and the Wryneck. Fig. 7 shows the population trends of these species. For the Black and Great Spotted Woodpecker, there is no clear evidence of long term change, whereas Green Woodpecker and Wryneck have declined. I have included in the figures the proportion of routes where the species has been recorded irrespective of numbers. These measures, whcih exclude some of the excess variation between years caused by factors such as weather, calling activity and phenological differences, show the same trends as the indices of the totals. The two species that have not declined are primarily restricted to coniferous forests where they are able to excavate nest holes even in healthy pines and spruces. Thus, as long as there is a sufficient number of trees with a sufficient diameter, these species are able to cope with forestry. This suggests that food is not a limiting factor. The two declining species occur in deciduous woodland, the Green Woodpecker preferring deciduous trees such as Aspen for nesting and the Wryneck is a secondary hole nester. They also share a common trait in food choice, being ant-eaters to a large extent. My guess is that the main factor for the decline of the Green Woodpecker is the declining amount of old deciduous trees. The decline of the Wryneck is more difficult to explain. One factor could be the expansion of forested areas, but this has not happened at the rate shown by the decline of the Wryneck. The Wryneck differs from all the other woodpeckers in being a tropical migrant. Hence, one cannot exclude the possibility that limiting factors operate in the winter quarters. But in that case the Wryneck has reacted very differently from most other tropical migrants, which have done well during the last twenty years. The pigeons The Wood Pigeon and the Stock Dove both migrate to about the same areas of southwestern Europe. They differ in breeding ecology, the Stock Dove being a hole nester whereas the Wood Pigeon is an open nester. From the mid 1980s, population trends of the two species have been very different (Fig. 8). The Stock Dove has declined to a level below half of what it was in the 70s, but the Wood Pigeon has remained at the same level

7 Figure 6. Population indices for Chaffinch Fringilla coelebs for three different zones and all Sweden. Figure 8. Population indices for Wood Pigeon Columba palumbus and Stock Dove Columba oenas. Figure 9. Population indices for Lapwing Vanellus vanellus and Snipe Gallinago gallinago. Figure 7. Population indices and mean number of birds per route for four woodpeckers: Black Woodpecker Dryocopus martius, Green Woodpecker Picus viridis, Great Spotted Woodpecker Dendrocopos major, and Wryneck Jynx torquilla

8 As yet there is no clear explanation for this. It is unlikely that the number of suitable nest holes has declined greatly, at least not at the rate shown by the decline of the Stock Dove. One possibility is predation by the Marten. The evidence is partly indirect - a strong increase and expansion of the Marten. More direct evidence comes from a study plot that has been censused since This plot had a population of about 5-7 pairs of Stock Doves until After the appearance of the Marten in the wood the population declined to 3, 2, 0, 1, 0, and 0 pairs in 1989-'94. This interpretation was supported by a parallel decline of the Jackdaw, from about 20 pairs in to 14, 14, 1, 0, 0, and 0 pairs in 1989-'94. There was considerable Marten predation on nest-boxes which had hinged roofs without locks, implicating Marten predation as the cause of the Stock Dove decline in this plot. The fact that both the Stock Dove and the Wood Pigeon winter in about the same areas seems to preclude differential mortality caused by hunting or other winter factors. I suggest that the decline of the Stock Dove in Sweden has been caused mainly by increasing predation by the Marten. Farmland birds Apart from a few species, most woodland birds, both resident and migratory species, have done well during the last twenty years. The situation is very different for farmland birds. Most of them are declining, for example Skylark, Linnet, Wheatear, Starling, White Wagtail, Red-backed Shrike and Pheasant. The decline has been still more drastic for two wetland birds that often breed associated with farmland, the Lapwing and the Snipe (Fig. 9). The Snipe population has declined at an almost constant rate over the two decades, and the South Swedish population is now only about a quarter of what it was in the 1970s. The Lapwing population also declined rapidly until the mid 1980s but has since stabilised at a lower level. I believe that the cause of these declines is drainage, both of true farmland and of woodlands. The drainage of farmland has been much improved during the last decades when many of the old drainage piplines were replaced by modern ones. Drainage of woodland is today a standard procedure integrated with clear-cutting before new trees are planted. Populations of these species breeding on mires in northern Sweden have not declined since the 1960s, though hard data about this come from a small number of census plots. A new bird monitoring system I have focussed on the two main problems of the present monitoring system: the uneven geographical coverage of the country and the low degree of habitat representativeness of the individual plots. In order to create a satisfactory sampling design, a new way of distributing the count sites will be introduced in There will be a basic system with one sample in each of the km squares of the Swedish national map grid, in total just below 200 samples. Within each square there will also be three supplementary samples that may be used in regions with many ornithologists, in areas where the habitat pattern is so patchy that one sample is not sufficient, or in areas where for some other reason it is judged appropriate to have more than one sample in a square. By this system we will achieve both goals, a more even distribution of routes, and samples that are not biased by observer choice. The counting method will be a combination of a point count and a line transect. A square route, 2 2 km, will have the same standard location in all squares. There will be eight points along the route, one kilometre apart, and consequently eight line transects, each one kilometre, between the points. From each point, all birds seen or heard will be counted irrespective of distance but excluding birds that have been counted from a previous point. From the lines, all birds will also be counted irrespective of distance, excluding all double counts between kilometre sections. This new system will not replace the old one with routes selected by the observers, but it will be given priority and, if possible, financial support, particularly in northern Sweden