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For the Weller candling method, rubber radiator hose (15-20 cm long; 3.8 cm ID) is used as a candler. We recommend wrapping fluorescent adhesive tape around the tube to help find it if misplaced in dense vegetation. On bright days candling efficiency can be increased by holding the tube slightly above the horizontal axis (Fig. 4). The egg should be held vertically, with the larger end toward the top, and rotated slowly to obtain a view from all sides. This method can be used during days with heavy overcasts. At least two eggs from each clutch should be candled.

Weller (1956) described five criteria for age classification: (1) size of the embryo when visible, (2) shape and appearance of the yolk, (3) development of the extra-embryonic circulatory system, (4) density of the opaque areas, and (5) size of the air cell. Weller described the development of redhead (Aythya americana) embryos as follows (see also Fig. 5):

1. Unincubated. The yolk is barely visible due to its pale yellow color. It is free floating and the air cell is quite small. In unincubated eggs that have been deserted by the female, the yolk soon adheres to the shell membrane and turns brown.
2. 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.
3. Eight days . The outline of the embryo is less distinct, appearing as two isolated dark areas. These are the head and the trunk; the 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.
4. 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 the 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.
5. Sixteen days. When seen from the rear view, this stage differs from that of the 12-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.
6. 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.
7. 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.
8. In embryos that die before hatching, the blood vessels and yolk appear brown. The observer will learn to distinguish these with practice.

Weller's criteria for redhead eggs have been modified slightly for use with other species of ducks. The stages of incubation do not vary appreciably among species through the first 12 days, but some species differ from the redhead during later stages (Fig. 5).