Variation in yolk ring appearance determined the duration of the rapid yolk formation period in several shore birds and sea birds. After freezing, yolks were fixed in formalin, and unstained and dichromate-stained slices were examined. The range in days required to deposit the yolk was estimated by counting pairs of pale and dark rings, which represent 1 day/pair. With some exceptions, yolk formation in small alcids required from 8-10 days, in large gulls from 10-13 days, in small gulls from 5-8 days and in small shore birds from 4-7 days. This type of analysis can also yield information on the sequence of egg laying in a clutch and the timing of natural pigment deposition in the yolk.