# Reproductive Success and Diet of the Swainson’s Hawk (Buteo swainsoni) in the Grasslands of Janos, Chihuahua, Mexico

## TL;DR

This study examines the breeding success and diet of Swainson’s Hawks in Mexican grasslands, finding low nestling survival and a diet rich in small mammals and reptiles.

## Contribution

The study provides new insights into the breeding ecology and diet of Swainson’s Hawks in northern Mexico, emphasizing conservation needs.

## Key findings

- Nest success was 44.4%, with lower survival rates during the nestling stage compared to incubation.
- Swainson’s Hawks primarily consumed vertebrates like small mammals and reptiles.
- Nests were located in mesquite trees in areas with low human disturbance.

## Abstract

The Swainson’s Hawk is a migratory raptor that breeds in the grasslands of northern Mexico, but little is known about its ecology in this region. We studied its reproduction, nest locations, and diet in the Janos grasslands of Chihuahua. We monitored 18 nests and found that less than half produced fledglings, mainly because young hawks (nestlings) had much lower survival than eggs during incubation. Nests were usually placed in mesquite trees located away from towns and in areas with low human disturbance. By analyzing pellets and prey remains, we found that the hawks fed mostly on vertebrates, such as small mammals and lizards. These findings highlight the importance of conserving natural grasslands and monitoring populations over time to support this species’ long-term survival.

Understanding the breeding ecology and trophic dynamics of the Swainson’s Hawk (Buteo swainsoni) is essential for conserving grassland raptor populations in northern Mexico. We evaluated reproductive success, nest-site characteristics, and diet of the species in the grasslands of Janos, Chihuahua, during the 2006 breeding season. Eighteen nests were monitored to estimate daily survival rates (DSRs) using the Mayfield method. Overall nest success was 44.4%. DSR declined significantly from incubation (0.99 ± 0.00079) to the nestling stage (0.98 ± 0.00087; z = 8.5, p < 0.001), resulting in cumulative survival of 79.9% and 56.2%, respectively. Successful nests tended to occur farther from towns, although this trend was not statistically significant. Most nests were built in mesquite trees at intermediate elevations and in areas with low human disturbance. Diet analyses of 56 pellets and 91 prey remains revealed a predominance of vertebrates (63.17%), mainly mammals and reptiles, with vertebrate frequency significantly exceeding that of invertebrates (χ2 = 23.19, p < 0.001). These results highlight the species’ reliance on vertebrate prey and the vulnerability of the nestling stage, underscoring the importance of long-term monitoring in semi-arid grasslands.