Notes on Split Clutch Nesting and Supernumerary Nests in the Mountain plover

Clutch size is traditionally defined as the number of eggs laid by a mated pair in a single breeding attempt. Some avian species increase reproductive output by splitting the breeding attempt across multiple nests that are simultaneously and independently incubated by either the male or the female. In this paper, we define a “clutch” for species with such split clutch nesting as the sum of eggs laid in rapid succession across two or more nest cups in a single breeding attempt and introduce the term “split-clutch” for the fraction of the clutch laid in a single nest cup. We then report cases of supernumerary split-clutches in the mountain plover, a species known to use this strategy. The case of a 10-egg nest represents the largest split-clutch observed in this species. We discuss hypothetical explanations for supernumerary nests in species with split clutch nesting, considering physiological limitations as well as potential for parasitism. We conclude that our observation of a 10-egg split-clutch either increases the known physiological limit for a single laying female or establishes a record of parasitism in the mountain plover. Further documentation of parentage of supernumerary split-clutches is warranted.