Factors Affecting the Use of Hunting Sites by Ferruginous Hawks

Even under the most favorable conditions, the use of space for foraging by free-living animals is difficult to study.This is especially true for the large avian predators, most of which are highly mobile and may range widely in their search for prey.Some investigators have obtained useful information through radiotelemetry ( Nicholls and Warner 1972)) but that technique has the disadvantage that foraging movements often cannot be distinguished from other activities.This paper reports on a study of the foodsearching behavior of free-living Ferruginous Hawks (Buteo regalis). It examines the searching patterns of hawks in relation tospatial variations in vegetation type, cover density, and prey distribution within their foraging ranges.It also considers the influence of previous experience on the birds' searching patterns.STUDY AREA The study area was located in Curlew Valley, Idaho, about 8 km N of Snowville.Utah.It consisted of the home ranges of two adult male Ferruginous Hawks, designated as Male 1 in 1974 and Male 2 in 1975.I judged the two males to be different individuals from plumage differences, although both birds may have had the same mate.Observations were made during the nestling stage of the nesting season, when Male 1 had three young and Male 2 had two.Range boundaries were determined by drawing a line around the outermost sightings of each bird.I believe that nearly all of the birds' diurnal movements occurred within those boundaries.The two hawks occupied ranges of similar size and shape and nested less than 1 km apart (Fig. 1).Each nest was located about 3 m above the ground in the crown of a Utah juniper (Juniperus osteosperma) tree.Males 1 and 2 occupied ranges of 21.7 and 17.2 km' , respectively.Most cultivated fields in the study area were laid out in 16-ha squares which formed a convenient grid system for recording the location of the hawks' ac-t&ties (Fig. 1).Each grid square was designated bv its row and column headings (e.g.. E-5) and had the same designation in both iears.-''The study area was divided into eight vegetation types (Fig. 1).Most plant names were determined from Holmgren and Andersen ( 1971).Bare ground.Plowed fields without vegetation.Included some hard-packed dirt areas.Pasture.Areas of grass or forbs cropped closely by livestock grazing.Common species were ARTOpyron &stat&, Bromus tectorum, B. mollis, T&xscum officinale, Eragrostis hypnoides, Salsola kali, Descurainia sophia.Kochia scoparia.Grindelia sauarrosa, Lepidium perfoliatum, and Hordeurn fub&m.Grass or grain.Cultivated grain (barley, wheat) or ungrazed crested wheatgrass ( Agropyron crtiaturn) fields; these were combined into a single vegetation type due to their similar structure.Alfalfa.First-growth alfalfa (Medicago sativa) fields.None was harvested during the observation periods.Old field.Previously cleared or cultivated land which had reverted to weedy vegetation.Common species in some or all of these fields were Bromus tectorum, B. mollis, Ranunculus testiculatus, Lepidium perfoliutum, Tragopogon dubius, Salsola kali, Lappula redowski, Medicago sat&a, Camelina microcarpa, and Agropyron cristatum.Grass-shrub.Ooen shrubland with grass understory.Shrubs were mainly Artemisia tridentata in the west and A. tridentata and Chrysothamnus nuuseosm in the east.Major grasses were Agropyron cristatum in the west and Bromus tectorum in the east.All areas contained Ranunculus testiculatus and Lepidium perfoliatum.Juniper.Juniperus osteosperm forest with some Artemisia tridentata and a sparse Bromus tectorum understory.Rush-grass.Wet-meadow area surrounding Deep Creek.Dominant plants were Juncus balticus, J. torreyi, Agropyron smithii, and Hordeum jubatum. METHODSTo minimize the risk of nest abandonment, I started observing after the young had hatched.Observations began on 19 May in 1974 and on 26 May in 1975, when the hawks' nestlings were about 1 and 2 weeks old, respectively.In each year, the observation period lasted four weeks, although some additional data were collected during a fifth week in 1975.During each week of observation, all hours of the day were sampled, from 06:OO to 21:00 MDT.Sunrise and sunset occurred at about 06:15 and 20:30, respectively.An average of about ten hours per day was spent watching the hawks, five days per week.Observations were made from a blind in 1974 and from the cab of a pickup truck in 1975.In each year, the observation post was about 300 m from the nest.During each foraging bout, I recorded the hawk' s location relative to the grid at 2-min intervals.Data were taken until the bird either captured a prey item, returned to the nest without prey, or was lost to view.Occasionally the bird captured one prey item, ate it, and continued to hunt without returning to the nest.In that case, the next search effort was recorded as beginning at the place of capture of the first prey, after that item had been consumed.Each time the hawk attempted to capture a prey item, I noted the time, grid location, vegetation type, and success.In 1974 and 1975 respectively, 1,916 and 1,637 location records were classified as prey searching.These records were tabulated to give an estimate of the percentage of a hawk' s foraging time spent in each grid square.Flights for purposes other than for food searching, such as those to repel intruders or to carry prey to the nest, were not included in the distributions.t3I61