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A Simple Field Candler for Waterfowl Eggs¹

Milton W. Weller

Table of Contents


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.

Westerkov (1950) discussed the need for age determination of embryos and presented several methods which he used in the laboratory 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, Westerkov 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 pheasent 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.


The simplicity of the mailing tube is its principal advantage. It can be made sturdy by a coat of shellac and will hold an oval shape, thus reducing light leakage. Any long flexible tube can be used. Evans (personal communication) prefers a large diameter rubber hose (as used in automotive cooling systems) because of its flexibility and durability. A notebook or magazine works well because it can be rolled so that its diameter is slightly less than that of the particular egg being examined. The egg is held tightly against the open end of the candling tube so that its long axis forms a right angle with that of the tube.

Light Source

The sun, even on cloudy days, provides sufficient illumination. Practice with the tube lends skill in using available light; the egg is usually viewed toward the sun, but can be held slightly away from the sun in order to clarify minor details. The tube should be held so that its shadow falls upon the inner corner of the eye, thus reducing the chance of light leakage between the nose and the candler.

Criteria for Age Determination

The appearance of the developing embryo and its associated structures as seen through the candler are shown in Figure 1. Three views are provided of each stage because changes are often more apparent in one view than in another. There are five criteria which aid in age classification: size of the embryo when visible, shape and appearance of the yolk, development of the extra-embryonic circulatory system, density of the opaque areas, and the size of the air cell. Characteristics of redhead eggs at four intervals are as follows:

Unincubated -- The yolk is barely visible ue to its pale yellow color. It is free floating nd the air cell is quite small. In unincubated eggs which have been deserted by the female, the yolk soon adheres to the shell membrane and turns brown.

Four Days -- The embryo and the extra-embryonic vascular system are clearly visible and red in color (the "spider" stage of the poultryman). The yolk is yellow-orange and less solid than in the fresh egg. The margin of area vasculosa, the sinus terminalis, is often visible.

Eight Days -- The outline of the embryo is less distinct, appearing as two isolated dark areas. These are the head and the trunk; thin neck is barely visible. A rocking motion of the embryo may be seen at this time but is retarded by handling. The yolk now appears to be more solid because it has been enveloped by the yolk sac and in the side view, the sinus terminalis is distinct; its margin may parallel the long axis of the egg or may be diagonal.

Twelve Days -- The yolk sac has completely enclosed the yolk except for a small area opposite the embryo where the albumen is attached. The vascularization causes an increased density and less distinct outline of yolk mass. The growth of the embryo forces the remaining yolk into two lobes, one lying on either side of the duckling. The most opaque areas outline these lobes where they are pressed against the shell. The embryo is indistinctly visible as a dark area isolated from the yolk.

Sixteen Days -- When seen from the rear view, this stage differs from that of the twelve day stage in that the light band separating the embryo and the yolk mass is much narrower, due to an obvious increase in the size and opacity of the embryo. The air cell is noticeably larger than in the 12 day stage.

Twenty Days -- The egg is now opaque, except for the air cell and the area immediately adjacent, plus a minute area at the small end. The lobes of the yolk mass are barely discernible.

Twenty-two Days -- The bill of the duckling is now pressed against the inner shell membrane and is visible as a projection in the air cell.

GIF -- Figure of egg incubation stages
Figure 1. Criteria for the determination of the stage of incubation of redhead eggs. Where three views are shown, they are from left to right: (1) Front view, with the embryo adjacent to the viewer, (2) Side view, with the egg rotated 90 degrees to the left, and (3) Rear view, with the egg rotated 180 degrees to the left from the front view.

Hatching occurs at twenty-four to twenty-eight days in the incubator according to Hochbaum (1944), Peter Ward, hatchery superintendent at the Delta Waterfowl Research Station (personal communication), and my own observations.

In embryos which die before hatching, the blood vessels and yolk appear brown. The observer will learn to distinguish these with practice.

Figure 1 and the age criteria described above are satisfactory for other species with a similar incubation period (canvasback and mallard). The pintail, and probably the blue-winged teal, which have an incubation period of 21 to 23 days, can be aged by using these same illustrations if it is realized that the last three stages represent 15, 18, and 20 days of incubation in these species. The technique is not sufficiently refined to make distinctions between species having incubation periods between 21 and 23 days.

Among the species examined, this method worked best with the eggs of the redhead, mallard, blue-winged teal, baldpate, and gadwall; it was more difficult with eggs of canvasback, pintail, and lesser scaup, because of the greater density of their shells. The density of the shell structure or coloration was so great in the eggs of the ruddy duck, Canada goose, coot, sharp-tailed grouse, and Hungarian partridge, that the size of the air cell was the only practical criterion.

¹Contribution from the Delta Waterfowl Research Station, Delta, Manitoba, and the Missouri Cooperative Wildlife Research Unit: U.S. Fish and Wildlife Service, Wildlife Management Institute, Missouri Conservation Commission, Edward K. Love Foundation, and University of Missouri cooperating. The writer wishes to acknowledge the advice and assistance of Wm. H. Elder and T.S. Baskett, Missouri Cooperative Wildlife Research Unit, and H. Albert Hochbaum and Peter Ward, Delta Waterfowl Research Station.

Literature Cited

BANNER, E. L. 1920. The Cornell reading course for the farm, Lesson 156: 329-364. N. Y. State Coll. Agr., Ithaca.

EVANS, C. D. 1951. A method of color marking young waterfowl. Jour. Wildl. Mgt., 15: 101- 103.

HANSON, H. C. 1954a. Apparatus for the study of incubated birds' eggs. Jour. Wildl. Mgt., 18:191-198.

------. 1954b. Criteria of age of incubated mal- lard, wood duck, and bob-white quail eggs. Auk., 71:267-272.

HOCHBAUM, H. A. 1944. The canvasback on a prairie marsh. Amer. Wildl. Inst., Washington, D.C. 201 pp.

LAMON, H. M. 1921. The incubation of hens' eggs. Farmer's Bul. 1106. U.S. Dept. of Agric. 8 pp.

WESTERSKOV, K. 1950. Methods for determining the age of game bird eggs. Jour. Wildl. Mgt., 14:56-67.

This resource is based on the following source:

Weller, Milton W.  1956.  A simple field candler for waterfowl eggs.  Journal of Wildlife Management 20(2):111-113.

This resource should be cited as:

Weller, Milton W. 1956.  A simple field candler for waterfowl eggs. Journal of Wildlife Management 20(2):111-113.  Northern Prairie Wildlife Research Center Online. (Version 18AUG98).

Milton W. Weller Missouri Cooperative Wildlife Research Unit University of Missouri, Columbia, Missouri

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