Communal Breeding in the European Starling: Evidence from DNA Fingerprinting

Abstract

The mating system and reproductive strategies of the European Starling (Sturnus vulgaris) recently have received considerable attention. Both polygyny and intraspecific nest parasitism have been regularly recorded in several populations (Merkel 1980, Lombardo et al. 1989, Pinxten et al. 1989, 1991a, b, Pinxten and Eens 1990, Romagnano et al. 1990). Although cooperative breeding has been reported in several African starlings (Wilkinson 1982, Craig 1983, 1987), it has only rarely been recorded in the European Starling (Stouffer et al. 1988). Here, we report a case in which two female European Starlings laid eggs in the same nest and cooperated in feeding the nestlings, while only one male was observed feeding the nestlings. Using DNA fingerprinting, we could confirm that the three adults attending the nest were the biological parents of the nestlings, and that the male was simultaneously bigamous. The nesting occurred in 1992 in a nest box colony in Zoersel, near Antwerp, Belgium. This colony has been monitored since 1985 as part of an ongoing study of reproductive strategies in European Starlings (see Pinxten et al. 1989, 1991a, b). During 1985-1990, nearly all breeding adults were banded with metal bands of the Belgian Ringing Scheme and individually color marked with wing tags. Age (second-year bird vs. older bird) was determined by measuring (to nearest millimeter) the length of iridescence of the throat feathers (Delvingt 1961). The behavior of the birds was recorded throughout the breeding season. Polygyny and intraspecific nest parasitism occurred regularly with frequencies ranging from 20 to 40% and 0 to 37%, respectively (Pinxten et al. 1989, 1991a, b, Pinxten and Eens 1990). From 1991 onwards fieldwork in this colony was restricted to routine inspections of the nest boxes to collect breeding data. During a routine inspection of the nest boxes on 21 April 1992, one (nest box 3) was found to contain a nest with 10 starling eggs. Clutch size in the other nests in the colony ranged from five to seven eggs, and was on average 5.9 ? SD of 0.6 (n = 15). During 1985-1991, large clutches (8 or more eggs) comprised only 2% (4/187) of all clutches in the colony. Inspection of the eggs suggested that the clutch was laid by two different females in that five eggs of the clutch were darker than the others and appeared to be smaller. In the European Starling, interclutch variation in eggs is considerably greater than intraclutch variation (Greig-Smith et al. 1988), and the eggs of certain females may be distinguishable on the basis of size, shape and color (Feare 1991). All 10 eggs were measured (maximum length and breadth) to the nearest 0.1 mm with a Vernier caliper, and an index of volume (V) calculated using