Intra population polymorphism of Caspian gull (Larus cachinnans) from the North-Western Coast of the Azov Sea (oological aspect)

This study presents the results of a long term study of nesting colonies of the Caspian gull (Larus cachinnans Pallas, 1811) on the islands of the Molochniy Liman and in Obitochnaya Bay (Azov Sea), in the South of Ukraine (Zaporizhia region), conducted between 1988 and 2013. A description of the size and coloring of eggs of Caspian gull was conducted by generally accepted methods. We measured 1000 eggs from 5 colonies of Caspian gulls. The background coloration of the eggs’ shells was classified into 7 types, the pattern of markings on the surface of the shells was classified into 4 types. In the nesting colonies, comprising different nesting settlements, the study tested differences in the distribution of typical and atypical coloring types and patterns on the surface of the shells. The background color and character of the shell marking patterns is dominated by eggs of phenotypes 3 and 4: gray-green, with a pattern of spots, of medium size (5–60%) and brown, with a pattern of large spots (2–40%). In different settlements the Caspian gull egg sizes vary in length and diameter of 54.5–86.3 x 39.2–60.4 mm, volume 61.7–113.7 cm3 and index of roundness 63.6–85.3%. The study revealed that the linear dimensions of eggs also depend on the number of birds in the nesting colonies. We found that morphological and dimensional characteristics of Caspian gull eggs can vary at certain intervals and characterize individual colonies, settlements and populations. Based on cluster analysis, conducted in terms of the average of the linear sizes of eggs of Caspian gull from several populations within the range of the species, the study identified three groups of colonies – Danube-Sivash, Azov-Black Sea and Caucasus-Caspian. In region of the Azov-Black Sea, the greatest similarity was shown between the settlements of Sivash and the South of Crimea, which in turn is similar to Lebiyazhyi Islands and Kaniv Nature Reserve (river Dnipro). A related link combined the Azov and Black Sea branches, while the Caucasus-Caspian group is relatively separate. As a result this study the influence of the number of nesting birds on the dimensional characteristics of the eggs was established; in the period of increase in the Caspian gull population from the 1970s to the early 1990s they had the highest scores, and in periods of decreasing population in 2001–2005, their values were lowest. Consequently, the use of oomorphological indicators turned out to be a fairly easy and objective method for studying the monitoring data for separate colonial bird species.