Some Foods of Young Great Blue Herons

Abstract

In the course of investigations by the Wisconsin Geological and Natural History Survey concerning the productivity of some of the lakes in the Trout Lake region of northeastern Wisconsin, it was observed that these waters furnished a supply of food for considerable numbers of fish-eating birds. These birds include the great blue heron (Ar dea her odias her odias), the American merganser (Mergus merganser americanus), the common loon (Gavia immer immer), and the belted kingfisher (Megaceryle alcyon alcyon). All of these are common breeders in the region but the great blue heron is the most conspicuous from the standpoint of numbers as well as size. From the first of June until the latter part of August one or several of these herons can almost always be seen on any lake known to contain fish. Within a radius of a few miles of Trout Lake two great blue heron rookeries have been located and it seems likely that there are more in the vicinity. One of these rookeries was made up of 138 nests in the 1940 season. The other consisted of 75 nests. Only active nests were counted. Active nests were designated as such only when young birds could be seen in the nests, or when whitewash evidence was plain enough to indicate that the nest was or had been recently occupied. When the nest census was made from one to four young could be seen in most of the nests. Later experience revealed that when not being fed some or all of the young birds lie quietly on the nest, completely hidden from view beneath. Thus some of the nests originally thought to contain only one bird, later were found to have two, three, or even four. On this basis it is probably safe to assume that the average number of young to a nest was at least two, and perhaps three. Since it was obvious that the lakes supported this heron population to a great extent, it became desirable to the group engaged in fish growth and population studies to determine what part fishes played in the diet of the herons. This was to be accomplished by first identifying the kinds of food taken by the herons. Then an estimate was to be made of the total pounds of fish consumed by either one or both rookeries during a given period of time. This estimate, calculated for the nesting season, would indicate, perhaps, whether the pr?dation of the herons upon the fishes constituted any appreciable drain upon this important natural resource of the lake region. Obviously any such estimate would have to be based on definite observations as to the number of times a young heron was fed in a certain period and the quantity of food it received at each feeding.