Advantages and disdvantages of interspecies associations of migratory northern sandpipers (Charadrii, Aves)

Field studies were carried out at two stations of the Ornithological Unit, Institute of Biological Problems of the North, Far East Branch, Russian Academy of Sciences, located in Nizhnekolymsk District, Yakutia, from May 15–20 in 1984, 1985, 1987, 1988, and 1990; on the northern coast of Pukhovoy Bay, the southern island of Novaya Zemlya, from June 1st in 1994; at Cape Beliy Nos, the Yugorsky Peninsula, from June 1st in 1995–1997. Classic associations are detected in interspecies flocks of sandpipers between the following species: the Pacific golden plover and the curlew sandpiper, the pectoral sandpiper and the long-billed dowitcher, the pectoral sandpiper and the dunlin, and the grey plover and the dunlin. However, the total amount of birds forming associations is not large. In species of group “A” (the grey plover, the Pacific golden plover, and the pectoral sandpiper), no difference has been observed in migratory birds’ behavior within interspecies or conspecific flocks. Species of group “B” (the dunlin, the curlew sandpiper, and the long-billed dowitcher), on the contrary, change their behavior sharply depending on whether they belong to an association or not. Species of group “A” do not get any advantages when forming an association. Unlike them, species of group “B” profit from associating: the time spent on foraging substantially increases; more time is spent on rest, and less time is spent on reconnaissance and vigilance (readiness for actions); the safety of the birds is enhanced. On the other hand, in species of group “B” there are also disadvantages related with associating: i.e., interspecies competition for food; foraging in suboptimal habitats which may in turn lead to a notable increase in the time spent by birds on foraging. It is proposed that the advantages and limitations of associating for the species of group “B” cancel each other out to a certain extent, and this explains rather small number of birds forming associations.