Pope, Aimee L. – Juvenile Ferruginous Hawk movements and flight skill development during the post-fledging dependency period in south-central Washington
I observed 25 juvenile ferruginous hawks on the U.S. Department of Energy’s Hanford Site and adjacent areas in south-central Washington in 1995 and 1996 to assess spatial and temporal requirements of juvenile ferruginous hawks and to describe flight and hunting skill development during the post-fledging dependency period. On average chicks fledged at 46.1 days old and dispersed at 72.3 days of age, resulting in a three to five week post-fledging period. Fledging age and duration of the post-fledging period did not differ between males and females. Juveniles dispersed at an average date of 28 July, and all juveniles dispersed by 9 August.
Weekly mean distance of juvenile movement from nests increased from 70 m during the first week post-fledging to 690 m during the fifth week post-fledging. Weekly average maximum distance of juvenile movement from the nest increased from 319 to 999 m during the same period. Juveniles from broods with one young moved farther from nests than juveniles from multiple young broods after the second week postfledging, possibly due to the lack of sibling competition for prey deliveries. Weekly mean dependency areas of juveniles (95% minimum convex polygon) increased from 3 to 13 ha from week one to week five post-fledging, with a total average dependency area of 34.2 ha.
Juveniles flight skills began developing during the first week of the post-fledging period, and mainlv used flapping flights of short duration (21 sec per flight). Juveniles flight skills changed as demonstrated by a decrease in flapping flights and an increase in more elaborate flights such as gliding and circling flights. Juveniles began using gliding flights at approximately 7 days post-fledging and circling flights at 14 days post-fledging. Juveniles from nests located on ridges took more gliding and circling flights than juveniles from nests located in flat terrain, possibly due to the wind lift created by the ridges. Total weekly number of flights observed increased in the second week postfledging to a mean of 11.6 flights, then decreased during the following weeks up to dispersal. Mean flight duration increased from 21 to 197 sec per flight between the first and fifth week post-fledging respectively.
Of 78 prey transfers, 51% occurred after juveniles flew toward adults. Five transfers (7%) were aerial transfers and occurred approximately 6 days prior to independence. I observed hunting behaviors including striking and pecking at sticks, and chasing siblings and potential prey. These behaviors may aid in development of hunting skills necessary to capture prey once juveniles are independent.
Buffer zones of 250 to 800 m in radius around nests have been suggested to protect ferruginous hawks from disturbance. A 250 m radius buffer zone around nests would exclude human disturbance from 73.8% of juvenile locations, and an 800 m radius buffer zone would exclude human disturbance from 94.7% of juvenile locations. Buffer zones sustained until after dispersal of juveniles, which occurs by early to mid-August, would protect juvenile ferruginous hawks from disturbance while they develop their flight, hunting, and social skills.