Brood parasitism in the Green-backed Heron (Butorides striatus)

Abstract

The Green-backed Heron (Butorides striatus) was previously considered to be a rare non-breeding visitor to the northern Gulf of Eilat, occurring throughout the year (Shirihai 1996, Snow and Perrins 1998). (Note that this group includes sexually immature birds, floaters without mates, and individuals whose previous breeding attempts have terminated). Breeding in the Middle East was considered limited to the coastal zone of the southern Persian Gulf, south Arabia and the Red Sea to 28°N (Porter et al. 1996). However, Yosef et al. (2002) reported the first confirmed nesting at Eilat Israel (29°33’N, 34°57’E). During the 2003 breeding season (March–May) we successfully documented the nesting attempts of seven Green-backed Heron pairs. One of the most centrally located nests included an atypical number of eggs. We assumed that this was a case of brood parasitism wherein two or more individuals had laid their complete clutches in the parasitised nest. No previous accounts of egg-dumping have been reported for this species (Del Hoyo et al. 1992, Snow and Perrins 1998). During spring in 2003 we monitored the breeding attempts of seven pairs of Green-backed Herons in the north-western part of the Gulf of Eilat/Aqaba. We measured all eggs and checked for variability in egg size of the different clutches in order to elucidate whether different females had laid their eggs in the parasitised nest. Located approximately 500m offshore are aquaculture facilities that attract numerous pelagic bird species. In September 2002 we requested permission from the managers of the aquaculture facility to attach milk crates (300mm L x 300mm B x 140mm H) to the cables of the fish tanks. Green-backed Herons occupied seven of these boxes by mid March but eggs were laid in late June only. All clutches were completed and incubation was in progress when we measured all nests and eggs on 4 July 2003. The length (L) and breadth (B) of eggs in the completed clutches were measured by RY with a Measy 2000 caliper to the nearest 0.1mm. Egg volume (V) was estimated following Hoyt’s (1979) formula: V = 0.51 x L x B2/1000. The withinclutch co-efficient of variation (CV) of egg volume was calculated according to a formula adjusted for small sample sizes (Sokal and Rohlf 1995). Mean values are given with standard deviations. The atypical clutch (n = 9 eggs) was subsequently collected and is now housed at the National Natural History Museum at Tel Aviv University. A total of 30 Green-backed Heron eggs from seven clutches were measured: three eggs (n = 3), four eggs (n = 3)