NEST BOX USE AND REPRODUCTIVE BIOLOGY OF THE AMERICAN KESTREL IN LASSEN COUNTY , CALIFORNIA by

During 1976 we implemented an American Kestrel (Falco sparverius) nest box program in the Great Basin of Lassen County, California. The primary goal was the creation of nesting habitat where no habitat existed, and the reestablishment of such habitat where it had been eliminated. Of 247 functional nest boxes examined between 1977 and 1980, 31% of these were active and 82% of these were successful. 3.1 young were fledged per active nest box. With careful placement of nest boxes, the percent active may be increased to more than 50%. Introduction Despite its widespread occurrence in California, the American Kestrel has received relatively little study in the state. Except for the major ecological study by Balgooyen (1976) and on seasonal weight variation (Bloom 1973), habitat partitioning (Koplin 1973), winter territoriality (Cade 1955), and predatory efficiency (Collopy 1973), basic natural history information is lacking for most of California. In 1976 we implemented an American Kestrel management-study program in Lassen County, California. The objectives were, 1) to determine if kestrels would nest in artificial nest boxes in areas which lacked suitable natural nest sites, and 2) to investigate the species' reproductive biology. Data were collected over 4 breeding seasons (1977 1980). Study Area The study area covered about 900 km 2 of the Great Basin in Lassen County, California. Elevation ranged from 1,260 to 2,340 m. The dominant plant association was western juniper (Juniperus occidentalis) and big sagebrush (Artemesia tridentata). Other sub-dominant plant associations i cluded ponderosa pine (Pinus ponderosa), white fir (Abies concolor), and the greasewood-shadscale complex (Sarcobatus vermiculatus, Atriplex confertifolia). •Address all correspondence to Peter H. Bloom. Current Address: National Audubon Society, Condor Research Center, 87 N. Chestnut Street, Ventura, CA. 93001. 9 Raptor Research 17(1):9-14 10 RAPTOR RESEARCH Vol. 17, No. 1 Methods Nest box design followed Hamerstrom, et al. (1973), with modifications to meet dimensions of natural nest cavities (Fig. 1). Nest boxes were constructed of 1.9 cm thick pine and measured 18 cm deep, 20 cm wide and 33 cm tall inside. The hole was 7.6 cm in diameter and located 7.6 cm from the top, in the middle of the front of the box. The back of the box extended 5 cm above the top and 5 cm below the bottom and was fastened to the supporting structure by one nail through each extension. Boxes were placed 2 to 6 m above ground and faced all directions. The top of each nest box was completely removeable and fastened on by eye hooks. This was later modified by bending the hook or by wiring the lid to the box since some nest boxes were later found without tops. Presumably they were blown off or sun warped. Nest boxes without tops were rendered useless; no birds of any species ever used them. Thejuniper-big sagebrush abitat was chosen as the primary habitat for nest box placement because kestrel nesting densities were believed low and junipers made logical support structures for nest boxes. Junipers do not readily form natural cavities by limb breakage or rot; thus, kestrels relied primarily on woodpeckers, particularly the Common Flicker (Colaptes auratus), to excavate their nest holes. Much of the Great Basin juniper country is composed of relatively oung trees (Burkhardt et al. 1976). Generally, if large mature junipers are not present, Common Flickers are also limited in their choice of nest trees; thus, large areas may be devoid of suitable nest trees for either species. Although the majority of nest boxes were placed on western junipers some nest boxes were placed on white fir, ponderosa pine, aspen (Populus tremuloides), cottonwood (Populus trichocarpa), and telephone poles to determine site preferences.