Swainson's Hawks arrive in the survey area and begin breeding much later than our other migrant raptors. They nest exclusively in trees, both deciduous and evergreen, about the open basins and valleys of the Great Basin. Their late arrival may attribute to their relatively small nests when compared with nests of other area raptor species. Often constructed of smaller sticks atop a juniper crown or within a poplar, the nests appear more hastily built than the massive structures of the Ferruginous Hawks and larger Red-tailed Hawk nests. The disc shaped platforms with shallow bowls are lined with grasses and juniper bark. Nests are sometimes rebuilt with material added to an existing base. Often a new nest will be in close proximity to older nests and a tree stand will have many old nest structures. Long-eared Owls will utilize the older nests that remain in good condition.

Nesting is initiated in mid Spring and fledging frequently occurs in the hottest part of mid summer. A standard clutch is one to four eggs. Incubation lasts 34 to 35 days and the Swainson's Hawk fledge 38 to 46 days after hatching.

Utah Swainson's Hawk Occupation and Nest Starts

The number of known and occupied territories, and nest starts (active nests) represent all Swainson's Hawk nests monitored in Utah from 2001 to 2005. More than one nest is often present in a territory, an area used by a single pair during a breeding season. In Utah, the number of nests in a known territory averages just under two, but up to six nests have been found in a territory. Adults on territory are often air borne over the nest area or perched in the general area of the nest. Nest starts can be a challenge to determine. An adult on the nest will use the surrounding foliage to hide their presence.

Utah Swainson's Hawk Success and Productivity and Nest Fledgling Production

Nestling development and behavior cues indicate nestling age and probable fledgling dates. The pre-fledged nestlings often are active at the nest, standing, playing with sticks or exercising their wings (hop-flapping). They are mostly covered with feathers, except for small patches on the back of their head and under the wing pits. Just a few days before fledging nestlings will begin to explore the areas immediately about the nest, walking out on branches. A nest is considered successful once nestlings have reached at least 80% of the species median fledgling age (32 days for Swainson's Hawks). All nestlings reaching at least 80% of the median species fledge age are considered to have fledged, unless observations indicate otherwise. Most mortalities discovered after the 80% fledge age appear to have occurred outside of the nest, post-fledging. Immediately after fledging the young hawks walk and hop about their surroundings, quickly learning to make short flights. Shortly after fledging they can be seen perched atop prominent features or on tree limbs and clumsily flying in the general area of the nest.

Utah Swainson's Hawk Trends

While the number of known and occupied territories, and nest starts increased during the surveys, there was a noticeable jump in 2005. The overall result of the increase in density and the high rate of hatching and fledgling successes was a record number of fledglings, nearly double the number of fledglings in 2003 (the third year of the survey). The unchanging fledglings per successful nest and the increased fledglings per nest start also indicated a high success rate for initiated nests in 2005. Like other buteo species, the increased precipitation and mild winter seem to have been beneficial to Swainson's Hawk nesting success in Utah. For more information of Swainson's Hawk trends in the Great Basin please see our state specific annual reports Northwest Utah Nest Survey and Northeast Nevada Nest Survey.