Gizzard Stones from Adult White-Tailed Ptarmigan (Lagopus Leucurus) in Colorado^*

* A partial contribution of Colorado Federal Aid in Wildlife Restoration Project W-37-R.

Abstract

Gizzard stones from 288 adult white-tailed ptarmigan (Lagopus leucurus) collected from 16 locations in Colorado were examined to describe monthly changes in size, weight, type, and wear. Mean grit size was largest during winter months and smallest in July. Seasonal changes in grit size were correlated with dietary changes. Females had a significantly greater mean annual amount of grit than males. Amount of gizzard grit in both sexes fluctuated about a winter level until April when amount decreased in males to a low in June and increased in females to a high in June. Amount of grit then stabilized to the winter levels. Quartz was preferred as grit along with lesser amounts of feldspars, amphiboles, pyroxenes, micas, rhyolite, and andesite. Grit from birds collected in areas of volcanic origin was mixed quartz and tertiary material. Larger stones were more rounded than smaller pieces with more wear evident on grit from birds collected in the San Juan Mountains where tertiary material is softer and quartz is retained in the gizzard. There were no seasonal fluctuations in roundness. There appears to be no shortage of grit in Colorado; however, in some areas ptarmigan have adapted to differences in availability of stones.