Info

Figure 14.6 Annual variations (as reflected in standard deviations, SD) in the first arrival dates of migratory species in relation to their mean first arrival dates (MFAD) over many years. (a) An area in western Poland, 1913-1996 (Tryjanowski et al. 2002); (b) an area in northern Russia, 1931-1999 (Gilyazov & Sparks 2002). In both areas, earlier arriving species had more variable first arrival dates. They were mainly short-distance migrants that wintered within Europe, while the later arriving species were mainly long-distance migrants wintering in Africa south of the Sahara. Regression equations: western Poland, SD = 19.6-0.103x, where x is the mean first arrival date, r = 0.65, P < 0.01; northern Russia, SD = 11.7-0.0724x, r = 0.80, P < 0.001.

1 Most studies of arrival dates have used the first date each spring that an individual of each species was seen. There are obvious problems with this in that the first record may represent only a single individual, and the chance of seeing such a bird depends on the area monitored, and the numbers of observers involved. The first dates from wide areas, such as counties, tend to be earlier (cont./)

above any temperature effect. The Brambling Fringilla montifringilla, for example, arrives on its northern breeding areas earlier in years of good spruce crops than in other years (Mikkonen 1981). As the cones open, the seeds provide an early food supply, enabling the birds to survive in their breeding areas until their main summer food (caterpillars) becomes available. In one area over a number of years, the size of the spruce crop, together with air temperature and snow cover, explained 89% of the annual variation in Brambling arrival dates.

It is seldom certain to what extent early arrival in particular years is due to earlier departure of the species from wintering areas, or to faster progress en route but, because the same weather patterns influence large parts of the migration route, migrants come under the influence of warmer or colder conditions long before they reach their breeding areas. Several studies have found a relationship between arrival dates in breeding areas and temperatures back along the migration route, including the wintering areas (Huppop & Winkel 2006, Sokolov 2006). At least among short-distance migrants wintering within Europe or North America, birds may leave their wintering areas earlier in warm than in cold years, as found for 17 species wintering in Spain (Sokolov 2006). Both long-distance and short-distance migrants arrived at the Courish Spit in the southern Baltic earlier in years when spring temperatures were higher, and apple flowering was earlier (Sokolov 2006).

Those species that advance northward in short flights of perhaps 50-200 km at a time may be able to keep in more precise step with their food supplies than species which arrive in their breeding areas after a long flight from a locality several hundreds of kilometres away. This difference in flight lengths may account for why the arrival dates of short-distance migrants generally show good year-to-year correlations with spring temperatures in the breeding locality, while the arrival dates of long-distance migrants show much poorer correlations or none at all (Tryjanowski et al. 2002). In any case, earlier arrival in breeding areas normally leads to earlier breeding, as documented for 10 out of 15 species studied at the Courish Spit. In addition, however, the later that birds arrived, the shorter was the period between arrival and breeding (Sokolov 2006; see also Dalhaug et al. 2001, Tryjanowski et al. 2004, Hupp et al. 2006).

0 0

Post a comment