Trends in the body condition of Rocky Mountain Population sandhill cranes during spring and winter

Abstract Body condition during the spring and winter can have cross‐seasonal effects on the reproduction and survival of migratory birds. The quality of resources on wintering and migratory stopover areas can impact nutrient reserves. During spring migration, greater sandhill cranes ( Antigone canadensis tabida ) in the Rocky Mountain Population rely on resources (e.g., grain, water) in the Middle Rio Grande Valley in New Mexico and the San Luis Valley in Colorado, USA. Our objective was to understand how sandhill crane body condition varies over time and in response to weather and resource availability. We collected sandhill cranes to determine an appropriate body condition index (BCI) for the species in this region and examine temporal trends in lipid and protein content during winter and spring. We then modeled within‐year and across‐year trends in the BCI of sandhill cranes in New Mexico harvested over a 20‐year period (1999–2022) to determine the impact of weather (minimum temperature, drought severity) and habitat (surface water, vegetation, corn area) on BCI. Sandhill cranes increased lipid content throughout spring, with an average increase of 140 g for females and 146 g for males. The BCI that explained the most variation ( R 2 = 0.56) in lipid content was body mass divided by the sum of tarsus length and wing chord. Of hunter‐harvested sandhill cranes, BCI peaked in mid‐November and again in early January; it increased by 22% between the lowest and highest observed values of normalized difference water index. In contrast, we observed a difference in BCI of only 2% between the lowest and highest observed minimum temperature values. We did not find a significant change in BCI over the 20‐year study period, and no other covariates influenced BCI. While surface water and temperature influence body condition of sandhill cranes in this region, resource availability in the San Luis Valley and Middle Rio Grande Valley may be appropriate for meeting their energetic requirements.