While less common in avian species in the temperate regions, altitudinal migration still plays a part in migration patterns in montane zones and is seen in most ungulates in the Rocky Mountains. Avian temperate species that migrate altitudinally include mountain chickadee, and the American dipper. Ungulates that have been observed to migrate altitudinally include roe deer, bighorn sheep, and mountain goats. Temperate bat species are also altitudinal migrants. Their migratory patterns are sex-biased altitudinal migrations, with females inhabiting lower elevations during reproductive periods. Altitudinal migration, as a short-distance migration pattern, has been easier to trace than long-distance patterns. Still, while the proximate causes and physiological adaptations for migrations are well understood, determining the ultimate causes have been difficult. This difficulty has been linked to limited success of mark and recapture techniques used to track migratory species. There are many hypotheses for why altitudinal migration may occur, including correlations between food abundance and nutrition-the need to migrate in order to meet specific needs associated with varying abundance and nutrition; reproduction-breeding sites being at elevations different than those of non-breeding sites; anthropogenic-species being increasingly driven to higher altitudes due to human actions. Migration in response to food abundance has been the most accepted hypothesis for why species migrate altitudinally. This hypothesis states that peaks in food abundance along an elevational gradient, such as the slope of a mountain, drive migration patterns as species exploit available food resources. Peaks in food abundance along this gradient often coincide with the breeding season. Some frugivorous birds, such as white-ruffed manakins ("Corapipo altera") migrate to higher elevations to exploit peaks in fruit abundance. Evidence supports the possibility that migrants have a competitive advantage compared to non-migrant (sedentary) species, due to increased foraging ability over a larger area, resulting in greater food and nutrient uptake. It's been shown that diet differs between non-migratory and migratory species in large-scale analyses and species-pair comparisons of frugivorous tropical birds. While this hypothesis is supported, and has been the most accepted, it fails to explain why altitudinal migrants return to lower elevations, or if it is done in response to shifting food resources. It has been proposed that weather-related resource availability may trigger the elevational migration of some species, such as the white-ruffed manakin during storms.