Food Exploitation Patterns in an Assembly of Estuarine Herons
ABSTRACT I examined the food exploitation patterns of three day herons, Great Egret (Ardea alba), Snowy Egret (Egretta thula), Tricolored Heron (E. tricolor) and a nocturnal species, the Black-crowned Night-Heron (Nycticorax nycticorax) that nested together in an estuarine colony. Types and sizes of prey were determined by analyzing the food of nestlings. These data were evaluated in light of relative abundance of prey, which was based on fish collected in nearby tidal creeks. The majority of food items (43%) taken by the heron assembly consisted of a two species of nekton, Mummichog (Fundulus heteroclitus) and Grass Shrimp (Palaemonetes pugio). Among the day herons, dietary compositions overlapped to the extent expected if each individual was foraging randomly. The diet of the Black-crowned Night-Heron diverged significantly from those of the Snowy Egret and Tricolored Heron, but not from that of the Great Egret. Considering relative abundance of different sized Mummichogs, the lengths of dietary Mummichogs overlapped as expected between each heron species. The three day herons coincided in their use of feeding microhabitats, most of which were in the intertidal zone. I found no evidence of social foraging: group sizes of feeding herons were the same as those of herons engaged in other activities, and most birds foraged alone. Foraging birds were rarely aggressive. Similarities (overlaps) in resource exploitation, as measured by type and size of prey and by microhabitat use, were as expected under a null model. Only two of 15 pairwise overlaps were less than expected; two were greater than expected. The four heron species formed an assembly of opportunists, whose members took prey according to abundance. The results provide little evidence that the herons partitioned food resources in any significant way. The lack of competitive or cooperative interactions between herons may be related to the superabundance of a few prey species, a condition probably typical of most estuaries.
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ABSTRACT: The fall and winter population of larval fish in a small intertidal creek was measured. The creek was blocked at high tide, and the immature fish were captured in a channel net designed for consistent quantitative sampling as they left with the ebbing tide. A total of 573,739 individuals with a biomass (preserved wet weight) of 66.1 kg were captured during the eight month sampling period (October 1974–May 1975). Twelve families, 13 genera, and 16 species were represented, with five species comprising 99.3% of the fish captured. The five species were:Leiostomus xanthurus (53.5%),Lagodon rhomboides (31.7%),Brevoortia tyrannus (11.9%),Micropogon undulatus (1.7%), andMyrophis punctatus (0.5%). The net was efficient, the catch was seasonal, and the greatest larval abundance occurred in February and March.Estuaries and Coasts 3(2):89-97. · 2.56 Impact Factor
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ABSTRACT: Sex-ratios of Gambusia affinis populations in freshwater marshes in the Camargue (Rhne Delta), are highly biased in favour of males, whereas the sex-ratios in ditches are close to unity. Studies of the diet of free living birds and experimental studies on prey size selection in captivity show that the abnormal sex-ratios in marshes can be attributed to differential heron predation. Ditches are relatively free from predation. Mature female Gambusia are larger, and have an energy content 5–25 times greater than that of mature males. Handling times of Grey Heron (Ardea cinerea) and Little Egret (Egretta garzetta) are only slightly longer for female Gambusia than males. Hence, females represent a much more profitable prey.Analysis of nestling regurgitates show that Gambusia makes up a considerable proportion of the diet of four species of Camargue herons, and that the majority of Gambusia taken are females. Under experimental conditions, captive herons consume almost exclusively female fish, even when offered in ratios where they are heavily outnumbered by males.The relevance of these results to optimal diet theory is discussed.Oecologia 12/1981; 53(2):146-151. · 3.01 Impact Factor
- Quarterly Review of Biology - QUART REV BIOL. 01/1981; 56(2).