Northern Prairie Wildlife Research Center
The progression through developmental stages appeared to be the same for many species of bird eggs that we examined in the field. By scaling for longer incubation periods, we were able to obtain reasonably accurate ages for eggs of Mourning Doves and Western Meadowlarks (Sturnella neglecta). Some features are recognizable across a broad range of taxonomic groups. For example, the distinct line seen separating blood vessels from clear albumen on days 4-6 for passerines is similar to that seen on days 8-12 in Mallard (Anas platyrhynchos) eggs (24 d incubation, Weller 1956). Some features seem to be species or genus specific. For example, the air cell tips well down the side of the egg in the Mourning Dove and to a lesser extent in waterfowl, but tips little in passerines we examined.
Welty (1982:356) noted that most passerine species lay eggs at 1-day intervals until the clutch is completed. The day of clutch initiation, thus, can be estimated by counting back one day for each day of incubation and one day for each egg present in the clutch, assuming that incubation began on the day that the last egg was laid. If incubation in a given species typically begins before the last egg is laid, appropriate adjustments can be made.
Candling eggs increases the accuracy of the nesting data collected in field studies of passerines because the age of the embryo is known. Candling also increases the efficiency of field work because fewer nest visits are required to determine egg hatching success. Reducing the number of visits at a nest can reduce bias such as increased nest abandonment and increased predation associated with nesting studies (Bart 1977, Westmoreland and Best 1985). Knowing the age of the embryos would be doubly important for researchers wanting to mark or study young at hatching. The candlers we employed were easy to use and easy to transport. Young (1988) described another type of portable egg candler made from a plastic jug, but this style is bulkier and more difficult to carry.