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by
1
Richard L. Glinski2
Teryl G. Grubb3
Larry A. Forbis4
Abstract.--As exemplified by the osprey, bald eagle and Mississippi kite, snag use by raptors is variable and includes nesting, resting and hunting. The concept of a snag should be expanded to encompass opportunistic snag use by raptors.
Research and management is needed to preserve snags in southwestern riparian deciduous forests.
INTRODUCTION
Few studies nave documented the importance of snags to specific raptors, Snag use by birds of prey is diverse and involves perching to rest or hunt, and nesting on exposed limbs or in cavities.
The purpose of this paper is to provide insight on raptor use of snags by presenting information on three selected species: the osprey (Pandion haliaetus), bald eagle ( liaeetus leucocephalus) and Mississippi kite (let nia mississippiensis).
The data presented does rict discuss every population of these species, but it exemplifies the variety of snag structures used by raptors and the management implications of snag use.
Birds of prey are usually opportunistic in their use of available resources, and their use of snags adheres to this tendency. Also, snag use varies amoung species. Ospreys usually nest in large dead conifers, but under local conditions they will nest on rock pinna~les, large columnar cacti or man-made structures that meet certain criteria. Bald eagles use snags extensively for perching to rest and hunt, and at times for nesting. Mississippi kites commonly forage from snags over areas of favorable prey densities, and often their snag perches consist of minute dead sprigs atop a live tree.
1
Paper presented at the Snag Habitat Management Symposium, Flagstaff, AZ, June 7-9, 1983.
2Richard L. Glinski is a Regional Wildlife
Specialist, Arizona Game and Fish Dept., Mesa,
AZ, 85205.
3Teryl G. Grubb is a Research Wildlife Biologist, U.S.D.A. Forest Service, Rocky Mnt.:Forest and Range Exper. Sta., Tempe, AZ 85287.
4Larry A. Forbis is a Forest Wildlife Biologist, Tonto National Forest, Phoenix, AZ 85038.
It is incorrect to assume that raptors that nest in snags are more dependent on snag habitat than those that perch or hunt from snags, especially since opportunism will often mask snag use.
Nests are constantly used during a breeding season and can be readily discovered, but perches are temporarily used and their importance can be quantified only with more intensive research.
By demonstrating the diversity of snag use by selected raptors, we hope to broaden the concept of a snag. A snag is frequently defined as any dead standing tree (USDA 1978). For cavity nesting species, Evans and Connors (1978) defined a snag as a standing dead or partially dead tree, and Co~nor (1978) termed a snag as any dead, dying or living tree suitable as a nest site for a cavity nesting bird. Indeed, the process of a live tree dying is often a continuum rather than an abrupt event, with dead sprigs, limbs, and cavities existing in canopies of trees long before they are dead. Many deciduous trees, like cottonwoods (Populus fremontii), provide both dead and live nesting, foraging and resting substrate in a single tree. In addition, during winter deciduous trees assume leafless, snag-like characteristics that can be important for species like wintering bald eagles.
SNAG USE BY SELECTED RAPTORS
Snag Use By Ospreys
Throughout their range ospreys prefer nesting in snags (Table 1). Nest snags in California pine forests are generally prominent structures that exist in open tree stands with sparse overstory
(Gale and Forbis 1974). Nearly all nests are placed at the top of a snag or snag-like structure rather than on lateral branches. In Florida,
Szaro (1972) concluded that nest heights in the intertidal areas are determined by the heights of available dead black mangrove (Azicenna nitida).
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Table !.--Habitat characteristics of four nesting osprey populations in the·United States.
Locations
(number of nests)
Percent of
Nests in
Snags
Preferred Nest Tree Species
(percent preference)
Distance of
Nest from
Water
References
Oregon
(63)
81 Lodgepole Pine
(Pinus contorta)
--(67)
<400m ( 1300 ft)
(76% of nests)
Roberts (1970)
Florida
(47)
Idaho
(127)
Northern
California
(61)
78 sol
Black Mangrove
(92)
Douglas-Fir
(Pseudotsuga menziessi)
(87)
(intertidal)
(500m (1640ft)
(53% of nests)
<92m (300ft)
(85% of nests)
Szaro (1972)
Van Deale et al. (1980)
Gale and Forbis
(1974)
!Includes power poles and artificial platforms that present a situation similar to a snag.
2Includes spike-tops (live trees with dead tops).
Osprey management involves intensive snag management. Gale and Forbis (1974) suggested retaining all snags 38cm (15 inches) in diameter and 12m (40ft) tall in osprey nesting areas, and large live trees every 250m (820ft) as future replacement snags. Van Deale et al. (1980) recommended leaving live trees near snags to prevent wind damage of snags. Roberts (1980) documented the halt of an ongoing snag-clearing program in Oregon because of osprey nesting concerns. His management plan called for a 61m
(200ft) "no logging zone" adjacent to water, a
341m (1120ft) "restricted-cutting zone," and retention of all snags and two dominant green trees in the restricted zone. Where necessary, snags can be created by killing live trees (Gale and Forbis 1974, Roberts 1970). Artificial snags for osprey nesting can also be created in selected locations using artificial structures
(Van Deale et al. 1980, Gale and Forbis 1974,
Roberts 1970, Kahl 1971 and 1972, and Johnson and
Melquist 1973).
Snag Use By Bald Eagles
Snag use by bald eagles is as opportunistic as the species' foraging and nesting habits.
Comparative size, abundance and location of trees are more important for nesting than absolute tree size or tree species. Topography and surrounding habitat vary with the activity, but perching, roosting and nesting usually occur in forested habitat along major lakes, rivers, or marine coastlines. Use-trees must have sturdy branching at sufficient height to support the nest, and good visibility and accessibility as provided by exposed branching, an open crown, crown dominance or tree isolation. Snags are frequently used, especially for perching, whenever they are present within the eagles' nesting or wintering habitat.
Stalmaster and Newman (1979), using a perchpreference index based on the ratio of utilization to availability, found that bald eagles strongly preferred dead trees for daytime perches. Relative height and morphological characteristics of the perch tree, range of vision from the perch, unobstructed flight paths, and proximity to water, feeding sites and open areas were contributing factors. Grubb and Kennedy (1982) also have documented a preference for daytime use of snags in the Southwest. In a continuing study near Stoneman Lake, Arizona, snags comprise less than 10 percent of the standing timber, but of 164 documented perch observations of 5-15 bald eagles,
65 percent were in dead or dead-topped trees.
Most perches were in ponderosa pine (Pinus ponderosa) trees on a hillside above the lake and scattered in the pinyon-juniper upland, usually in the vicinity of foraging areas.
Limited winter roost data for the Southwest
(Grubb and Kennedy 1982) suggest that som~ snags are usually present among the high-use trees within small, protected ponderosa pine stringers.
Presumably these trees are used for roosting in milder weather, or by subordinates unable to compete for more protective live trees. They may also serve as sential perches during evening entry into the roost, when a bird on such a perch indicates the area is undisturbed to approaching eagles (Ingram and Koller 1981).
During the breeding season eagles favor snags for hunting and loafing perches, and for guard perches overlooking the nest site. Snags are particularly important near foraging areas, but are rarely selected as nest trees. In western
Washington only 12 (6 percent) of 218 nests surveyed were in snags (Grubb 1976). In addition, the more decadent the tree the more likely a nest was to be inactive. In Arizona between 1977 and
· 1983, 19 of 71 nesting attempts have been in trees, of which 7 (or 10 percent of total nesting
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attempts) were in snags.
One pair of breeding bald eagles in Arizona bas consistently selected willow (Salix sp.) snags for nesting. These trees becom;-suitable for eagle use only after they are dead and aged so that the restrictive fine branches have fallen away. However, such decadent condition has caused consistent loss of the nests or nest trees, so a permanent artificial "snag" in the form of a tripod was built in 1977 (Grubb 1980). Fortification of dead nest trees is another management alternative to reduce potential loss, as is relocating nests in live trees nearby. The removal of unsound, dead nest trees or at least the dead nest supporting portions is a legitimate alternative to avoid the risk of loosing active snag nests, but such action should be evaluated on a case by case basis.
Snag Use By Mississippi Kites
The Mississippi kite locally breeds in the southern United States from Florida through
Arizona, nesting in live trees along drainages or in shelterbelts (Parker and Ogden 1979). The principle foraging strategy of this kite has been described generally as soaring and aerial pursuit of insects (Sutton 1939, Skinner 1962, Fitch 1963)
Glinski and Ohmart (1983a) found that in Arizona,
Mississippi kites captured 41 percent of the nestlings' diet (insects, mainly cicadas) by hawking (locating prey from perches) from snags within 150m (490ft) of the nest. Kites perched
10 to 20m (33-66ft) above the ground on dead branches of cottonwood trees. Kites employed this foraging strategy especially during morning and late afternoon periods when thermals and winds were not available to assist in soaring.
At two nesting sites where foraging behavior was intensively observed the hunting snags were on trees with less than 25 percent canopy die-out
(44 percent), trees with 25-50 percent canopy dieout (37 percent), and trees that were half or totally dead (19 percent). Many of the cottonwoods with less than 25 percent canopy die-out appeared to be healthy trees, and some of these trees have sustained 10-20·percent canopy die-out from 1975 through 1983 (Glinski, pers. obs.).
The spatial arrangement of cottonwoods around a kite nest is critical to kite reproduction, and the variation of vegetation structure (horizontal and vertical) surrounding nest trees accounted for 71 percent of the variation in nesting success
(Glinski and Ohmart 1983a).
The importance of vegetation structure and snag perches to Mississippi kites in other areas of their breeding range is undocumented. These nest site components may be most operative for kites in Arizona since the breezes that may enhance aerial foraging and are typical of kite nesting areas in the Great Plains are usually absent or reduced in central Arizona riparian habitats, particularly at the time when kites are feeding nestlings.
The optimum kite habitat in Arizona is cottonwood-salt cedar (Tamarix chinensis) association.
Although salt cedar generally will displace cottonwood (Robinson 1965), the vegetation associated with kite nesting areas in Arizona appears to be in the transition period when salt cedars are attaining heights of 3-8m (10-26ft) and when cottonwood trees 10-20m (33-66ft) are dying and providing snag perches. Salt cedar provides excellent habitat for the cicadas on which kites prey (Glinski and Ohmart 1983b), and tbe.~presence of cottonwoods with numerous snag forage perches protruding above the salt cedar presently combines to make certain riparian habitats in the Southwest excellent habitat for the Mississippi kite. Levy (1971) first documented the Mississippi kite in Arizona in 1970, and records for New Mexico began in 1950
(Parker and Ogden 1979). The recent occurrance of this kite in the Southwest may well coincide with the invasion of salt cedar and the initial stages of cottonwood degeneration, and thus with the development of favorable prey and foraging habitat.
CONCLUSIONS AND RECOMMENDATIONS
Snag use by raptors involves nesting, foraging and resting activities which, for most species, have not been sufficiently quantified to prescribe snag management guidelines. The definition of a snag needs to be expanded to encompass opportunistic snag use by raptors.
Ospreys commonly nest in prominent snags in open tree stands, and on artificial snags that retain similar features. Snag management for ospreys involves preserving snags, creating snags by killing live trees, and reserving living trees for future creation of snags. These management techniques are particularly applicable in pine habitats where large trees are common.
Bald eagles commonly perch and occasionally nest in snags. Throughout their breeding and wintering habitat, .maintenance of existing snags and long-term provision for continued snag availability is an important management objective for bald eagles, particula'rly in foraging and loafing areas. Creation of snags or the construction-of artificial snags also offers management potential for supplementing marginal habitat or redirecting eagle use away from disturbed areas.
The Mississippi kite in Arizona forages from snag perches. Its dependence on snags is not obvious since kite snag perches are often a ·dead sprig or limb in the canopy of a living tree. In the Southwest, many riparian deciduous trees provide snag habitat (dead limbs and cavities) throughout much of their lives, and thus to manage for broadleaf riparian snags generally is to manage for regeneration and maintenance of riparian trees. This effort will likely require a greater committment than the maintenance of pine snags since more ecological factors are involved with riparian plant succession (Johnson et al.
1977). As the salt cedar-cottonwood association degrades to a salt cedar monoculture and cottonwood nest trees and snag perches are eliminated,
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the kite population in the Southwest will likely decline. The potential for riparian deciduous trees in the Southwest to afford snag habitat for raptors will likely diminish as plant succession occurs in degraded river bottoms. Intensive research and management techniques are needed to preserve riparian deciduous snags.
Acknowledgements
We express our gratitude to Laura Jean Gable for typing the manuscript, and to Jerry A. Davis,
Gregory A. Goodwin, Gary C. Bateman, Richard A.
Ockenfels and Glen c.
Dickens for providing a format to convey the importance of snags.
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