Indicators Of Change 21 Range

2.1.1. Size and Position of Breeding Ranges

Changes in the distribution of birds, especially in their breeding range, were one of the earliest topics discussed among ornithologists in the context of climate change. Already in 1995, Burton [2] listed in his book 123 European bird species which extended their ranges in northern, western and northwestern directions and he attributed these changes to global warming. These observations meanwhile were supported by many other studies showing that range boundaries are moving poleward or upward in altitude as the climate gets warmer.

Sound data comes from comparing standardised breeding bird surveys. Such comparisons have been done on data from the United Kingdom and Ireland [5]. The comparison of the breeding bird atlas of 1968 1972 with the atlas of 1988 1991 showed that in 59 species with southerly distribution within the study area, there was a mean northward shift of their northern border of distribution of 18.9 km (see Table 1). This is equivalent to roughly a 1 kilometre northward shift of their northern range border per year. At the same time 42 northerly distributed species did not show any systematic movements of the southern border of their distribution area. In a comparable study with data from the time periods 1974 1979 and 1986 1989, it was found that the northern border of 119 southerly distributed species in Finland showed a mean northward movement of 18.8 km while the southern border of 34 northerly distributed species did not change [4]. Finally, data from the North American Breeding Bird Survey showed that 26 southerly distributed species moved their northern border of range on average 72.9 km northward between the periods 1968 1972 and 1988 1991 [3] while the southern border of northern species did not move.

Changes in breeding distribution registered, so far, are likely to be the first indications of rather severe ecological shifts and species rearrangements in some areas. Based on museum material of 1179 bird species and some mammal and butterfly species occurring in Mexico, ecological niche models have been developed with a genetic algorithm and were projected onto two predicted climate surfaces (conservative and liberal) for 2055. While extinctions

TABLE 1 Changes in borders of the range of bird species as revealed from breeding bird surveys

Region and

Distribution Number Distance within of and type source Period 1 Period 2 region species of shift Shift/year

Great

Britain and

Ireland [5]

1968 1972

1988 1991

Southerly 59 distribution

Northern border moved 18.9 km northward ca. 1 kma

Northerly 42 distribution

Southern border did not show systematic movements

Finland [4] 1974 1986 1979 1989

Southerly 119 distribution

Northern border moved 18.8 km northward ca. 1.7 kma

Northerly 34 distribution

Southern border did not show systematic movements

North 1967 1998 America 1971 2002 [3]

Southerly 26 distribution

border moved

72.9 km northward

Northerly 29 distribution

Southern border did not show systematic movements and dramatic range losses were expected to be few, the turnover in some regions was predicted to reach more than 40% of the species [6].

The studies mentioned above lead to the expectation that in the first instance species richness should increase in areas with incoming southern species and northern species which have not left. Indeed, the Lake Constance area in Central Europe could be an example for this. Based on 2 x 2 km grid cells breeding birds in an area of 1212 km2 were counted in a semiquantitative way in the periods 1980 1981,1990 1992 and 2000 2002. During this time species numbers increased from 141 to 154, in the last decade with a significant increase of species with a southern centre of distribution [7].

A flagship species for a northward range extension of a southerly distributed bird species in Europe is the Bee-eater Merops apiaster. Distributed mainly in

FIGURE 1 Number of breeding pairs of the Bee eater Merops apiaster in the country of Baden W├╝rttemberg (southwestern Germany) 1982 2007. Data from Boschert and Todte.

warmer areas such as the Mediterranean, the species starts breeding in higher European latitudes as soon as there are periods with warmer temperatures. This has been shown by a comparison between Bee-eater records in Central Europe and the size of growth rings in oak wood. The rings give evidence of warmer periods with higher annual growth rates in the oaks from the sixteenth Century onwards [8]. Currently, Bee-eaters are showing significant population increases (Fig. 1) and now breed as far north as Poland and Scandinavia.

On a European scale climatic variables in the actual breeding ranges of bird species have recently been used to forecast the future distributions based on the assumptions of first generation climate change models in the Climate Atlas of Breeding Birds in Europe [9]. This modelling leads to the prediction that between the two periods 1960 1990 and 2070 2100 breeding distribution areas of European birds shall move on average 550 km northwards and that many species shall suffer from area losses. This predicted value of a northward movement rate of 5.5 kma-1 is higher than has actually been found in the studies cited above.

One major problem with this approach is the lack of any account for habitat availability in Europe where many natural habitat types, due to human activity, will not be available, even when climate conditions would allow them to exist. This problem is not negligible as has been shown when bird distribution data from the breeding bird atlas of the United Kingdom and Ireland from 1968 to 1972 has been used to forecast the distribution in 1988 1991, using the same method as was used in the Climate Atlas of Breeding Birds [10]. The results were compared with real distributions in the second time period. For a series of species the forecasts did not match well and the real distributions were much smaller than the forecasted results. A remarkable example is the Red-backed Shrike Lanius collurio which in the 1970s was restricted to the South and Southwest of the United Kingdom. For the

1990s, models predicted a coverage of almost all of the United Kingdom, including the very north of Scotland. In reality, at that time, the species was almost extinct over the whole of the United Kingdom. However, two new breeding sites were established in Scotland, which was in accordance with the forecast.

2.1.2. Ranges during Nonbreeding Season

Besides breeding ranges, winter distributions are also changing. This is obviously the case where migrating birds can stay closer to their breeding grounds when closer areas become more suitable wintering areas for them, or when closer wintering areas become less suitable (e.g., dryer) and thus birds are forced to migrate longer distances [1].

Data from the Christmas Bird Count in North America between 1975 and 2004 showed a mean northward movement of the northern border of wintering ranges of migratory bird species of 1.5 kma-1. At the same time, winter distributions of non-migrants also moved northward during that time period [11].

In Europe, the effect of the drying-up of the Sahel belt, (a dry savannah area south of the Sahara desert where many Palaearctic long-distance migrants have their wintering areas), has been considered as one of the main reasons for population declines in the Common Whitethroat Sylvia communis, the Sedge Warbler Acrocephalus schoenobaenus and in many other species [12]. This indicates that the potential for rather simple latitudinal shifts of wintering areas, corresponding to changes in climate, is limited and might not be an option for all species.

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