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In previous studies of waterfowl, we (Lokemoen et al. 1984, Koford et al. 1992) used field candlers (Weller 1956) to age embryos in eggs because candlers are a rapid, simple, and accurate method. Eggs can be candled in the field using a simple candling device, ambient light, and a chart illustrating incubation stages. A candler can be used to view the embryos in most eggs, but not within shells that are dense or too heavily colored to transmit light. In comparison, other embryo-aging methods are more difficult to use in the field or require sacrificing embryos. The flotation method of aging eggs has drawbacks due to errors in aging larger eggs or later-laid eggs (Westerskov 1950). In addition, warm water must be carried in the field. Determining embryo age from egg density requires accurate field measurements of egg mass, length, and width, and development of a linear regression of egg size to egg mass from eggs of known age for each species of interest (Green 1984). Examining the embryo directly to determine age is a problem because one or more eggs in each clutch must be destroyed (Caldwell and Snart 1974).

During nesting studies in 1991-1993, we adapted and developed a candling method to age embryos in eggs of passerines and other small birds.