Post-Nesting Ecology of the Long-Eared Owl (Asio Otus) in Southwestern Idaho

Publication Date


Type of Culminating Activity


Degree Title

Master of Science in Raptor Biology



Major Advisor

Tom J. Cade


To study the post-nesting activities of the Long-eared Owl (Asio otus) in the Snake River Birds of Prey Area, Idaho, I radio-tagged 8 adult males, 6 adult females, and 12 juveniles at 8 nests in 1988 and 1989. I used 10-g transmitter packages (Biotrack, Wareham, Dorset, England) attached with a backpack harness. I studied behavior during the fledgling period, movements after the owls left their nest groves, and their food habits.

Young Long-eared Owls left the nest at about 3 weeks of age, began to fly at 4.5 5 weeks, and became independent of their parents at about 10-11 week of age. Before they could fly, owlets roosted singly and high in trees; after they could fly, owlets roosted in groups more often and lower in trees. These patterns may be predator-avoidance mechanisms. The oldest owlets in a brood began to make long adult-style hunting flights for 1-2 weeks before independence, whereas their younger siblings made only short flightsduration.

Adult females were more protective of the young, whereas adult males delivered 2.5 times more prey than females. Females deserted when the young were about 6.5-8 weeks of age and could fly well enough to escape most predators on their own. The males stayed behind and continued to provide food. The cost of desertion (losing her brood) for the female at this stage should be slight, because her role of protection is over.

In 1988, the owls left the BOPA abruptly, and I did not find any of them during 13,000 km of searches by truck and 18 hrs of aerial searches, covering a 90-150 km radius from the next sites. In 1989, I found 7 of 13 radio-tagged owls after they left the BOPA, 3 adult females, 2 adult males, and 2 juveniles. All went north into the mountains, 72-125 km from the nest groves. Four of the owls eventually moved into forested habitat after first using more open areas. Three moved to within 2 km of active logging, and one moved near year-old logging, perhaps to take advantage of displaced rodents.

I identified 4638 prey from pellets of Long-eared Owls collected between March 1988 and December 1989. Over 99% were 11 genera of small mammals, and 0.8% were birds. The most numerous prey were Peromyscus maniculatus (36%), Perognathus parvus (23%), Dipodomys ordii (23%), Microtus montanus (7%), neonate Thomomys townsendii (5%), and Reithrodontomys megalotis (3%). The most important contributors to the biomass were Dipodomys (12%), Microtus (7%), and neonate leporids (2.5%).

The proportions of the prey taxa in the diet varied over the seasons. Peromyscus and Dipodomys were highest in the winter and spring, Microtus in the spring, and Perognathus in the summer. The increase in Perognathus from spring to summer was gradual and began before owlets fledged in mid-May. The diets of hunting juvenile owls did not differ from those of adults at the same place. One adult owl which remained in the BOPA until September consumed over 80% Perognathus. These data indicate that the increase in proportions of Perognathus in the summer diet of long-eared owls is not owing to prey choice by juvenile owls but rather is a result of changes in the population numbers or availability of this rodent. Seasonal changes in the availability of prey species may be part of the reason Long-eared Owls leave the desert in the summer.

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