Northern Prairie Wildlife Research Center
Techniques for Studying Nest Success of Ducks in Upland Habitats in the
Prairie Pothole Region
Determining the Stage of Incubation
Incubation stage of duck eggs is used to determine when laying and incubation
began and to estimate hatching dates. These parameters are used to calculate nest
success. The field candling method described by Weller (1956) is convenient for
upland nesting ducks and is the one we used in our studies. Training, however,
is necessary to obtain reliable data. Westerskov (1950) described a flotation
method for determining the stage of incubation of ring-necked pheasant (Phasianus
colchicus) and gray partridge (Perdix perdix) eggs. The decreasing
specific gravity of eggs during incubation changes their orientation and buoyancy
when floated in water. Inexperienced investigators can use the flotation method
as a guide until they gain competence with the Weller method. They can also compare
their estimates of embryo development with the eggs of a known incubation stage.
This training method is very good when nests are found during the egg-laying stage
and later checked during incubation.
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):
- 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.
- 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; 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.
- 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.
- 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.
- 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
- 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).
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