Photographic Guide for Aging Nestling Swainson's Hawks

The purpose of this guide is to illustrate developmental stages of Swainson’s Hawks (Buteo swainsoni) to assist researchers in timing nest visits for studies in growth, reproduction and nest productivity. The ability to accurately age nestlings is important for determining nest success (Mayfield 1975) and minimizing disturbance at critical nesting periods (Fyfe and Olendorff 1976).

This guide is based on observations of a single brood. A second nest was monitored with known aged young as well, but it was found that developmental rates varied, likely due to factors including nutrition and differences in habitat type. Caution is advised in aging chicks using this guide without consideration of these other factors including gender, nutrition, position in brood and brood size.