25-Year Study of the Population Development of Central European Songbirds: A General Decline, Most Evident in Long-Distance Migrants

Autor: Rolf Schlenker, Wolfgang Fiedler, Ulrich Querner, Peter Berthold
Rok vydání: 1998
Předmět:
Zdroj: Naturwissenschaften. 85:350-353
ISSN: 1432-1904
0028-1042
DOI: 10.1007/s001140050514
Popis: During the past century a decline hasbeen noted in European bird popula-tions, both in large species [1] andpossibly also in songbirds [2]. Todaythe “Red List” (of endangered spe-cies) includes about 50% of the avi-fauna in most Central Europeancountries; about 30% of species aredeclining to an extent that threatenstheir continued existence in CentralEurope. These include songbirds,mainly open-country species that areeasy to monitor. Trends in the num-bers of many unobtrusive songbirdspecies, however, have remainedlargely unclear [3]. To assess theirpopulation changes our institute be-gan a census program in 1972 with thehelp of hundreds of amateur ornitho-logists [4]. A trapping station in thisproject in southern Germany has pro-vided data on 35 species over 25 years– the first dataset based on strictlystandardized methods over such along period. During this entire periodthe number of trapped birds has fall-en slowly but continuously, by an av-erage of about 1% per year. The de-crease is greatest for long-distancemigrants, and it heralds profoundchanges in the Central European avi-fauna.Populations of large birds such as ea-gles and storks can be determined ac-curately, and most of the Europeanbreeding pairs are monitored. Forspecies that are more common or areharder to observe, such as crows andowls, only rough estimates are possi-ble [3]. Serious problems are encoun-tered in songbirds. Abundant, widelydistributed, and unobtrusive speciescannot be counted completely or di-rectly; regional estimates must bebased on indirect procedures such asrecording territorial song. Such esti-mates are often biased by varying,nonquantifiable “observer quality”[5]. Therefore only a few countrieshave established sufficiently reliableprograms for monitoring songbirdsduring the breeding period [6, 7].In 1968 we began testing standardizedtrapping procedures for monitoringsongbird populations in order to re-duce differences among observers bya more objective method. The resultswere positive [8], and in 1972 wetherefore initiated the “Mettnau-Reit-Illmitz” program (based onthree stations, Mettnau in southernGermany, Reit in northern Germany,and Illmitz in Austria. This programwas later extended, and temporarilygrew into an ESF network encom-passing Europe and Africa [9]). TheMettnau station has operated sincethe outset, and we report here on theresults of a full quarter of a century ofsongbird population monitoring.The samples are obtained by trappingpassage migrants during the autumnmigration while staging at rest sites.A suitable study area must (a) becharacterized by largely unchangingvegetation, (b) include different habi-tats so that birds with different eco-logical requirements can rest there,and (c) be a protected area of guaran-teed long-term existence. The Mett-nau peninsula, with a 50-year-old birdsanctuary, is ideal for the purpose[10]. The 35 songbird species selectedfor monitoring include regularly stag-ing long-, intermediate-, and short-distance migrants (Table 1).The birds are trapped in nylon netsnormally used to catch birds for ring-ing; 2 m high and 7 m long, they areset up between wooden bars on poles.A wall is formed by 52 nets extendingthrough 8 different habitats (from abrush zone through forested and wetregions). The annual trapping periodis June 30–November 6 and includesthe entire autumn migratory periodof songbirds in Central Europe. Thenets are checked hourly from dawnuntil soon after dusk; trapped birdsare ringed (to prevent double count-ing), subjected to biometric tests andthen released. The monitoring condi-tions (annual setting up of the nets,collecting the birds, data collection,etc.) are standardized as far as possi-ble [10]. Most of the investigatedbirds come from Central Europe, al-though some have traveled fromnorthern, eastern, and western Eu-rope, and a small proportion are localbreeding birds. These origins areknown from recoveries of ringed indi-viduals [11] and from biometric dataon population differentiation [10].Test studies have confirmed thatstandardized trapping procedures ef-fectively give reliable estimates ofsongbird populations. Safriel andLavee (1991) showed that the numberof palaearctic songbird migrants trap-ped in Sinai is correlated positivelywith estimates of their European pre-migratory population sizes [12]. Dunnet al. (1997) demonstrated that trendsin the annual capture indices of song-birds trapped during autumn migra-tion in Michigan are correlated posi-tively with trends in breeding birdsurvey data from their breedinggrounds and conclude that “intensivestandardized netting can be a usefulpopulation monitoring tool” [13]. Wehave consistently found that trends intrapping data reliably reflect thosefrom other sources such as single-spe-cies population studies and country-wide population estimates for RedLists, etc. [4]. Thus data from 147,661individuals that were caught at Mett-nau between 1972 and 1996 reflectthe population development of 35Central European songbird species,as follows.
Databáze: OpenAIRE
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