Using candlers to determine the incubation stage of passerine eggs

Determining the incubation stage of bird eggs can provide important information to investigators conducting nesting studies. We describe the use of candlers in the field to determine the incubation stage in eggs of Lark Buntings (Calamospiza melanocorys) and other small birds with an incubation period of 11-13 d. Candling was accomplished easily using simple tools and did not involve the destruction of eggs or lengthy disturbance of nests. Candling is often preferable to other methods that rely on egg mass, mass-growth curves, or immersion of eggs in water.