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Using Candlers to Determine the Incubation
Stage of Passerine Eggs


Candling proved to be a useful technique for determining embryo ages in eggs of Lark Bunting and species with similar incubation periods. Most passerines have egg shells that are sufficiently translucent to be viewed easily with field candlers. Some species that have heavy shell markings, such as the Red-winged Blackbird, can be aged by candling, but other species, such as the House Wren, are more difficult to view. Also, in developing our technique, we found that it was not possible to age eggs of most shorebirds, such as Killdeer (Charadrius vociferus), Marbled Godwit (Limosa fedoa,), and Willet (Catoptrophorus semipalmatus), because the dense shell blocks light penetration.

Egg development.-- Development of eggs in the incubator paralleled that we observed in the field. For the focal species in this study, the period of egg incubation that we observed in the incubator was similar to that listed by Harrison (1979). For eggs artificially incubated in 1994, the two Lark Bunting eggs hatched in 12 d, the American Robin egg in 12 d, the Mourning Dove egg in 14 d, and one Red-winged Blackbird egg in 11 d and the other in 12 d. Major egg features are depicted in Figure 1.

jpg -- Major components of Lark Bunting egg

jpg -- Major components of Lark Bunting egg

Figure 1. Major components of a Lark Bunting egg early in development. The heart is distinguishable as the darkened area within the embryo.

The general stage of incubation is immediately apparent as soon as the egg is viewed through the candler. Eggs that are relatively clear are either infertile, unincubated, or in the first 4 d of incubation. Eggs that show dark patterns mainly in the center of the egg are in mid-incubation, about 5 - 8 d, and eggs that are quite dark are in late incubation, about 9 - 11 d for passerines or 10 - 13 d for Mourning Doves. Development of embryos in Lark Bunting eggs is shown in Figure 2 and the comparative development of embryos in Mourning Dove, American Robin, and Red-winged Blackbird eggs is shown in Figure 3. A description of the daily stages of embryonic development is presented in Table 1.

jpg -- incubation stages of Lark Bunting egg jpg -- incubation stages of passerine eggs

Figure 2. The incubation stages of a Lark Bunting egg. The top two rows of pictures show developmental stages from day 0 through day 5 with the yolk or embryo in the center of the egg. The third row of pictures shows the same developmental stage as row two, but the egg is turned 900 and the embryo is at the side of the egg. The fourth row shows egg development stages of day 7, day 9, and day 11 with the embryo having grown so that it fills most of the egg interior.

Figure 3. The incubation stages of the American Robin, Mourning Dove, and Red-winged Blackbird. The top two rows of pictures shows development during the third day of incubation for the three species. The embryo is in the center of the egg in row one, but the egg is turned 900 in row two and the embryo is at the side of the egg. The third and fourth rows of pictures show egg incubation during the sixth and ninth day of development when the embryo begins to fill the center of the egg.

For the 40 nests in our test of field assistants, the mean error rate between the hatch date estimated when the nest was located and the actual date of hatch was 1.27 d (SD = 1.037, range = 0 - 4 d). Given the inherent variability involved in egg incubation, it is probably not possible to estimate hatching with much greater accuracy.
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