Abstract
Swainson’s hawks (Buteo swainsoni) are a Neotropical migratory raptor species and a common breeding raptor in the High Plains of Texas. Colleagues and I monitored reproduction across a study area in northern Texas over seven years (2012 – 2018) to determine occupancy of large stick nests by Swainson’s hawks and reproductive output. I found that territories were consistently occupied by Swainson’s hawk pairs, and few other raptors or large stick-nesting species occupied the study area at any given time. Reproductive output (55% nesting success and 1.8 fledglings produced per successful nest) was lower than reports from other regions (70% success and 1.9 fledglings per nest) and was likely impacted by frequent droughts. Colleagues and I equipped adult hawks trapped in nesting territories with satellite transmitters in 2012 and 2013 and tracked hawks for up to five years. I examined data to compare migration characteristics, such as timing, routes, distance travelled, and length of migratory periods, to previous research that tracked Swainson’s hawks from other breeding regions. I found that most of the new data suggested agreement with previous conclusions, and differences were mostly attributable to breeding origin/destination. I described new migratory information, such as speed of travel, new staging and stopover locations, and habitat use and selection across the migratory pathway. One interesting finding was that Swainson’s hawks might be capable of surprisingly fast flight speeds (> 100 kmh-1 and > 800 km per day), though average speeds (25 kmh-1 and 189 km per day) agreed with previous research. Habitat use and selection reflected similar patterns as on breeding and nonbreeding ranges, and the most important conclusion was that avoidance of water and other migratory barriers, such as mountain ranges, heavily influences the migration pathway chosen by this species. In 2016 – 2018 I extended the transmitter study by equipping fledgling hawks with lower-resolution satellite transmitters and tracking them for up to four years. I used these data to describe previously unknown information about survival and behaviors during the juvenile and sub-adult life stages, which lasts 3-5 years in this species. I found a longer post-fledging period than previously described and that siblings gain independence from each other at the same time as they gain independence from adults. Juvenile hawks migrated southward with similar timing as adults, though during the first journey several hawks went off track, which proved fatal for most. Some juvenile hawks stopped short at the end of migration to overwinter in northern Argentina, while others completed the journey to the primary wintering grounds across the Pampas. Spring migration took longer than for adult hawks, with some juveniles arriving to the breeding region significantly later, possibly because they had no intention of attempting to breed. During the breeding season, juvenile hawks were nomadic, with only half making visits to natal territories. Survival was lowest immediately post-fledging and increased with time, though I did not track hawks long enough to observe any recruitment into the breeding population. Last, I used transmitter data from both adult and juvenile hawks to assess risk from the wind energy industry (a known hazard for raptorial and migratory species) throughout their global range. I found that Swainson’s hawks are at highest collision risk on their breeding range, though this species may be at lower overall risk than other raptor species, due to differences in behaviors and their long-distance migratory patterns.