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Using Candlers to Determine the Incubation
Stage of Passerine Eggs


Study profile.-- In 1991, we estimated the age of embryos in passerine eggs in the field using information provided for waterfowl eggs by Weller (1956) and for Mourning Dove (Zenaida macroura) eggs by Hanson and Kossack (1963). The success of these efforts prompted us to rate the ability of our field assistants to age embryos in 1992. We compared the estimates of embryo age by 6 field assistants with the actual date of hatching for 40 passerine nests. The sample included Lark Bunting (Calamospiza melanocorys), Horned Lark (Eremophila alpestris), and Vesper Sparrow (Pooecetes gramineus) clutches that were found during the egg-laying stage and that ultimately hatched. Although we had no pictures of passerine egg development in 1992, we described the technique to the field assistants and provided pictures of waterfowl egg development (Weller 1956) scaled for passerine eggs.

The 1992 tests were encouraging so we developed a technique to photograph incubation stages in eggs in 1993 and photographed the daily stages of egg development in 1994. Photographs were made from known-age eggs collected in the wild and incubated in incubators until hatching. Egg photographs were mainly of species of small passerines with incubation periods of about 11-13 d.

Field candling technique.-- For candling eggs in the field we used only ambient light and portable candling tubes. The tubes were made of 15-cm lengths of foam pipe insulation. Foam insulation with inside diameters of 1.3 cm and 1.6 cm is suitable for candling most small bird eggs. The foam is easily shaped around the egg to screen out light from the side when viewing.

Eggs are held at the distal end of the candling tube but are not placed entirely inside the tube. The egg is held vertically, with the blunt end or air cell of the egg upward and the pointed end of the egg downward. Generally, eggs incubated fewer than 3 d are best viewed while looking toward the ground, and eggs incubated 3 d or more are best viewed against a bright sky. Embryonic development is not symmetrical so the egg must be turned on its long axis to view the air cell, embryo, and other details from various positions. In the field, we candled at least 3 eggs in each clutch, because some eggs may be dead or sterile. The eggs of all species, especially passerines, require careful handling because they are quite fragile.

Photographing embryonic development.-- Most of our photographic work on embryonic development was done with Lark Bunting eggs (n = 6), which photographed well because the egg shells are translucent and unmarked. For comparative purposes, we also examined and photographed the incubation stages of eggs from Mourning Dove (n = 3), House Wren (Troglodytes aedon, n = 2), Eastern Bluebird (Sialia sialis, n = 1), American Robin (Turdus migratorius, n = 2), Red-winged Blackbird (Agelaius phoeniceus, n = 5), Brown-headed Cowbird (Molothrus ater, n = 2), and Common Grackle (Quiscalus quiscula, n = 1).

Eggs were placed in an incubator that automatically controlled temperature (380 C), relative humidity (50-55%), and turned the eggs each hour. Incubation period of eggs in the incubator were compared with those reported by Harrison (1979) to ensure that we were quantifying natural embryonic development.

Eggs were removed from the incubator each day to photograph and document embryo development. Each egg was placed horizontally into a cradle that held the egg firmly on its side, but allowed artificial light to be projected from below. The cradle was lined with plasticine to make a light-proof seal. A C-700 Super Chromega enlarger colorhead was used as the source of light. We found that green or cyan light projected from the enlarger gave the most natural portrayal of blood vessels and embryo within the yellowish-hued egg interior. A single-lens reflex camera, equipped with a 50-mm macro lens, was held above the egg on a copy stand, with the lens pointed vertically at the side of the egg. We used shutter speeds of 0.5 to 12 sec and F-stops of 4.0 to 16.0 and three types of black-and-white film: Kodak Plus-X Pan (125 ASA), T-Max (100 ASA), and Tri-X Pan (400 ASA). Use of brand names does not constitute endorsement by the U. S. Government.

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