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Methodology

Introduction

Systematic counts of migrating birds at Falsterbo were carried out for the first time during 1942-44 by Gustaf Rudebeck (Rudebeck 1950). Then, during 1949-1960 counts were organised by the Ornithological Society of Scania. Most counts were carried out from Nabben, the southwesternmost point of the Falsterbo peninsula (Ulfstrand et al. 1974). A large number of observers took part in the counts over the years. Depending in the availability of observers, the coverage of the migration season varied between years.

Standardised counts 1973-2000 (Gunnar Roos)

In the autumn of 1973 strictly standardised counts were introduced. The annual observation period was set to 11 August-20 November. The observations started at about 30 minutes before sunrise every day, independently of weather conditions, and continued till 2 p.m. (CET). One single observer at Nabben counted the migrating birds. The observer mainly used binoculars to find migrating birds, and telescope was only used to determine species in remote flocks/individuals. In 1978, the project was included in the National Monitoring Scheme run by the Swedish Environmental Protection Agency (Naturvårdsverket), and has remained there since then. Observers were Bengt Bengtsson 1973-1974 and Gunnar Roos (GR) 1975-2000.

All migrants were counted except Great Cormorant, some Gulls species and Sandwich Tern. A number of species, less easy to determine or separate, were put together in pairs: Black-throated/Red-throated Diver, Common/Arctic Tern, Chaffinch/Brambling and Parrot/Common Crossbill. During the first years there were also some groups of species-undetermined birds like goose sp., buzzard sp., swallow sp. etc. No ageing or sexing of the birds was carried out (and not required). The results were presented in annual reports. (Roos 2001 and before).

Raptor counts 1986-2000 (Nils Kjellén)

During the autumns 1986-2000 a special study of the raptor migration was carried out by Nils Kjellén (NK). The observation period was 1 August-20 November and the daily effort from dawn and as long as significant migration was going on. All raptors were counted and, if possible, aged and sexed. Additionally and time allowing, a number of other species were counted. Most species occurring in relatively small numbers were always counted, while for example Common Eider and Wood Pigeon were registered more irregularely and very common passerines were left out. The results were presented in annual reports (Kjellén 2001 and before) and were a large part of NK's thesis (Kjellén 1999).

Standardised counts 2001--> (Nils Kjellén)

In 2001 the standardised counts were slightly modified when Nils Kjellén took over after Gunnar Roos. Since then, the counts start on 1 August and two observers work together 11 August–10 November. The season ends on 20 november as before. All species are counted untill 2 p.m. (CET), while raptors are counted as long as significant migration is going on. Exceptionally, during bad weather conditions when no birds migrate, the counts stop before 2 p.m.

All migrating species are counted, except Great Cormorant, Herring Gull, Great Black-backed Gull and Sandwich Tern.

"Paired species" like Common/Arctic Tern, Guillemot/Razorbill and Parrot/Common Crossbill are separated to species by percentages of the number of species-determined birds in each pair.

A various sample of swans, geese, raptors, cranes and gulls are aged in order to get an indication of annual breeding success.

The results are presented in annual reports (like Kjellén 2006).

Recalculations

In order to increase the comparability between the standarised counts carried out by GR and NK respectively some recalculations were made. GR's counts were completed with numbers from the Falsterbo B.O. log on days when the species in question was not counted by NK (this is during 1986-2000). The amount of additional material varies between years, but this should be of less importance in the long-term perspective view. Averages 1985-2000 from Gunnar Roos’s counts were then compared to the corresponding numbers in the raptor counts. Some species, that were counted simultaneously by GR and NK show significant correlations and thus they were easy to recalculate, mainly by enumerating GR's numbers with the average difference, since the numbers in the raptor counts generally were larger.

In some raptors and sparsely occurring passerines the original figures were tripled with this method of recalculation. It also includes compensation for the first ten days in August and for raptor counts continuing after 2 p.m. (CET).

Other groups of species, like waders and terns, migrating already during the first ten days in August, were enumerated with the average percentages from the same period 1986-2000.

Black-throated/Red-throated Diver in GR's counts were considered as all Red-throated, since it is by far the most frequent at Falsterbo, and thus compared to this species in NK's counts.

Common/Arctic Tern, Guillemot/Razorbill and Parrot/Common Crossbill in GRs counts were separated to species by percentages of the number of species-determined birds in each pair per ten day period during 1986-2000.

Species-specific recalculation details are presented by Kjellén (2002).

Although the totals have been recalculated for many species, the original figures 1973-2000 have been stored for future analyses.

At this website, two sets of data are used:

  • Recalculated seasonal totals - used for long-term trends.
  • Original seasonal totals - used in every other context. In species which were counted simultaneously 1986-2000, like raptors, the highest totals were selected.

References

Kjellén, N. 1999. Differential migration in raptors. Lund.

Kjellén, N. 2001. Rovfågelsträcket över Falsterbohalvön hösten 2000. - Fåglar i Skåne 2000: 51-69 .

Kjellén, N. 2002. Sträckfågelräkningar i Falsterbo förr och nu. - Anser 41:114-123.

Kjellén, N. 2006. Sträckfågelräkningar vid Falsterbo hösten 2005. - Fåglar i Skåne 2005:7-44.

Roos, G. 2001. Sträckfågelräkningar vid Falsterbo hösten 2000. - Fåglar i Skåne 2000: 77-90.

Rudebeck, G. 1950. Studies on Bird Migration. - Vår Fågelvärld, Suppl. 1.

Ulfstrand, S., Roos, G., Alerstam, T. & Österdahl, L. 1974. Visible Bird Migration at Falsterbo, Sweden. -Vår Fågelvärld, Suppl. 8.