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Presence and correlation between the numbers of Black- headed and Yellow-legged Gull was studies between 1998 and 2004 on Kolut fishponds in NW Serbia. Both species occurred there from February to December. Spring migration culminated in late February and early March. The period of the most intensive autumn concentration was from October to December. The difference between the numbers in all months as well as between the numbers in spring and in autumn was statistically significant. The period of the most intensive autumn passage of Yellow-legged Gull was from late October to early Decem- ber. The difference between the numbers between the months was not statistically significant. A high concentration of both species in autumn and early winter was explained by high food availability during fish harvest when ponds were empty. Positive correlation between the num- bers of two species thru the year indicated attractiveness of large flocks of Black-headed Gulls for Yellow-legged Gulls. This is especially true for the autumn period, when feeding niches and food is easily accessi- ble. Correlation between the numbers of two species was negative in spring, when accesibility of aquatic animals is lower and large flocks of Black-headed Gulls are lacking.

rivista italiana di ornitologia

Occurrence and correlation between the numbers of Black-headed Gull Chroicocephalus ridibundus and Yellow-legged Gull Larus michahellis on Kolut fish ponds (Serbia)