A Simple Field Candler for Waterfowl Eggs

Abstract

In nesting studies of waterfowl and other game birds it is often desirable to determine the stages of incubation of clutches of eggs. This information makes it possible to determine when incubation began and to calculate the date of hatching. In some studies, the stage of incubation has been determined by breaking an egg of each clutch. This is an unnecessary waste, and for species which lay parasitically in other birds' nests, such as the redhead and ruddy duck, the breaking of one egg has little value because eggs in a single nest may be in many stages of incubation. Westerskov (1950) discussed the need for age determination of embryos and presented several methods which he used in the laboratory in working with pheasant and European partridge eggs. These methods were weighing, floating, and candling of eggs. The use of egg weights in the field necessitates carrying a delicate balance and making calculations from the weights in order to determine the stage of incubation. In the second method, eggs are floated in water; their buoyancy and position are related to the stage of incubation. While this method is satisfactory for use in the laboratory or for species which nest over water, it is inconvenient for land nesters. This technique permits rapid age determination in the early stages of incubation but becomes less accurate as incubation progresses. By the use of a laboratory candler, Westerskov found that the size of the air cell and the early development of the embryo were good criteria for determining the stage of incubation of pheasant eggs. The increased density of the yolk mass plus the opacity of the shell reduced visibility so that the embryo could not be seen properly after the first week of incubation; but the size of the air cell proved an excellent criterion throughout. The use of the air cell in candling chicken eggs has been reported by many workers, including Banner (1920), and Lamon (1921). Candling has been little used in the field because of the lack of simple and efficient equipment. Evans (1951) used a cardboard mailing tube for candling duck eggs in the field; from the comparative size of the air cell, he determined when the eggs were near hatching. Hanson (1954a) developed a battery-operated field candler which provided excellent visibility of many details of the egg. He presented a list of criteria and photographs of eggs at various stages of incubation (1954b). In order to develop criteria by which the stage of incubation of duck eggs could be determined in the field with the simple mailing tube suggested by Evans, the present study was initiated in the spring of 1952. Twenty-six redhead, ten canvasback, five pintail, and five mallard eggs were candled when brought into the hatchery, found to be fresh, and were placed in an incubator. The development of these eggs was followed with the mailing tube candler.