Hatching asynchrony results in competition between the nestling Black-billed Magpies Pica pica hudsonia and is of interest because long-lasting social ranks are probably initiated during the nestling and fledgling stages. The first-hatched birds grow at a significantly faster rate than their day-2 siblings. Flocks of young males and separate flocks of young females were held in an aviary until their dominance relations were known. Strictly linear hierarchies were quickly established among the young males, but females did not exhibit linear hierarchies. The only morphological correlate with dominance was bill length, and then only in the males. During the course of two years brother pairs were entered together into an aviary flock of five or more wild caught, non-sibling hatching year (HY) males. Supplantings were scored until a linear dominance hierarchy was compiled. Most males who were heavier at fledging were also dominant to their smaller brother(s) four to eight months later. It is concluded that the relative nest rank (i.e. hatching order and fledging weight) affects dominance relationships among male siblings. During the winter we observed dominance interactions of a wild flock of Magpies. A linear dominance matrix with less than 2% reversals was constructed from 547 interactions for 19 HY males. Of these males, 13 were from known nests and none were siblings. The weight ranks of these 13 males at fledging were split and compared. The mean nest rank of the top (rank = 0.94) and bottom (rank = 0.61) birds differed significantly. Hatching order of males may affect relationships even between nonsiblings. Young males apparently know each other individually, and this may indicate flock stability in this aggressive age class. Young male Magpies often take risks near predators in an apparent attempt to advertise their status, and perhaps gain social recognition. It appears that sibling competition is a factor that may determine which birds stay in the natal area and fledging dominance rank has fitness correlates.
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