Egg Formation in Cassin's Auklet

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Abstract
Female Cassin's Auklets (Ptychoramphus aleuticus) were given oral doses of lipophilic and proteophilic dyes during their prelaying period. Yolk ring structure in freshly laid eggs was analyzed to determine the timing of yolk deposition using the dye-stained layer as a date marker within the yolk. Additional information from necropsies of five females killed at specific intervals during the formation of a replacement egg indicated that only a single follicle enlarges in the ovary during a reproductive event and this yolk requires about 8 days to complete rapid yolk deposition (RYD). Once RYD is completed, the egg is not laid for a period of 4.2 ± 1.0 days, an interval termed the lag period (Astheimer et al. 1979). Albumen synthesis appears to begin at, or just before, completion of the yolk and continues until approximately 1 day before oviposition. During this time, however, the yolk remains in the follicle. Replacement eggs are laid 14.5 ± 1.7 days following egg removal or natural loss. Combining the above information with egg composition data, the daily deposition of lipid and protein from the female into the egg was computed. Evidence from the size distribution of small (<5 mm) primordial follicles suggests that the auklet ovary undergoes a cycle of follicular growth and atresia before a single follicle is selected for RYD. Based on these findings, and the correlation between initiation of spring egg laying and decreased sea surface temperatures for Farallon Island auklets, I suggest that the onset of RYD is modulated by environmental conditions.