Determining clutch size and laying dates using ovarian follicles

Ovarian follicles of Eastern and Western Meadowlarks (Sturnella magna and S . neglecta) and Hermit and Townsend's Warblers (Dendroica occidentalis and D. townsendi) grow and regress sufficiently rapidly for laying intervals to be distinguished. We describe how to deduce clutch size from counts of ova rian follicles and to estimate laying dates from curves describing the grow th of preovulatory and regression of postovulatory follicles. Our estimates of clutch size for warblers and meadowlarks were not different from those published in the literature or obtained from museum records. For birds coll ected during or near laying, dates of egg laying could be estimated over a 5-6-day period for warblers and a 7-9-day period for meadowlarks. Our recor ds increased the number of clutch initiation dates for Hermit Warblers by 5 4%. The window of time during which clutch size can be deduced from ovarian follicles depends on: (1) how quickly follicles grow and regress, (2) how long postovulatory follicles can be identified, and (3) clutch size. This w indow of time is fairly narrow for both species because their follicles gro w and regress quickly and because they lay relatively large clutches. Nonet heless, the clutch sizes we deduced from ovarian follicles for Hermit Warbl er increased the number of records by 48%. We encourage researchers and col lectors to record ovarian follicle measurements for all female specimens, a nd recommend a standard format for recording follicle size measurements on museum labels.