Osprey Habitat Model
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March 23, 2000
Osprey, Pandion haliaetus
Use of Study Area Resources:
Reproduction and foraging throughout the study area. Ospreys have a world-wide distribution (DeGraaf and Rudis 1986). Those in the Northeast winter from South Carolina, southwards through Central and South America.
Nesting Cover. Ospreys build large nests of sticks on snags or on living trees (Poole 1989). Stocek and Pearce (1983) quantified 105 natural nests in New Brunswick and found that 50% were in live coniferous trees, 5% in live deciduous, 6% in live trees with dead tops, and 34% in dead trees. Ospreys also readily nest on man-made structures if the configuration provides support; they have been known to use powerline towers, light poles, channel markers, pilings, fence posts, stone pillars, chimneys, duck blinds or to simply nest on the ground, if ground predators and human disturbance do not deter them (Poole 1989). At Lake Umbagog, on the Maine-New Hampshire border, Smith and Ricardi (1983) found that nest trees were considerably taller than the surrounding vegetation, whether in grassland, or shrubland, and Brewster (1924 in Smith and Ricardi 1983) noted that most of the Umbagog nests were placed conspicuously in white pines, near the water's edge.
Nest Location. On the coast, osprey nests are usually adjacent to, if not over, water, whereas on inland lakes and waterways nests are usually more distant from foraging areas (C. Todd, MDIFW, personal communication, 10/99). This distance may be up to 14 km (Prevost 1979, Hagan and Walters 1990), but more typically is up to 3-5 km (Poole 1989). The majority of nests in Oregon and California studies were within 1 km of large lakes and rivers (Zarn 1974, Vana-Miller 1987). The latter distance was confirmed by our observations using data for the Gulf of Maine (see below). Ospreys nest singly in most cases, but readily form loose colonies (Poole 1989) where food supply and nest sites are plentiful (Newton 1976, 1979 in Spitzer, Pool and Scheibel 1983; Hagan and Walters 1990; Kushlan and Bass 1983 in Johnsgard 1990). Limited food supplies may preclude nesting at small lakes; eagles in Maine nest on lakes 30 ha or larger (Livingston et al. 1990), probably because of the need for adequate fishery resources.
Foraging. Osprey diet is almost exclusively live fish (Poole 1989. They will utilize whatever fish species are available, and readily take "any medium-sized (15-35 cm) fish ... near the surface" (Hughes 1983). Ospreys are usually more successful in catching benthic or bottom-feeding fishes that are slower and less wary than piscivorous species (Swenson 1979 in Van Daele and Van Daele 1982, Vana-Miller 1987). Along the coast they usually feed over shallow or intertidal areas (Szaro 1978, Prevost 1979). Eagles, with similar foraging behavior, utilize shallow marine waters, typically < 1.8 m deep (Livingston et al. 1990). Because osprey locate their prey visually, decreased water turbidity and still, shallow waters are to their advantage (Swenson 1981b in Vana-Miller 1987), whereas rippled surface waters interfere with their ability to see prey (Grubb 1977). For this reason "reservoirs often provide improved forging conditions over rivers and oligotrophic lakes, because of a larger percentage of still, shallow, open water with an abundant fish population and reduced turbidity" (Vana-Miller 1987).
Ospreys show a wide range in their tolerance of human disturbance. Predictable disturbance, or disturbance that was ongoing when nesting was initiated, is better tolerated than sporadic disturbance or new disturbance which occurs after nesting has begun (Levenson and Koplin 1984 in Vana-Miller 1987; Poole 1989). Van Daele and Van Daele (1982) found greater productivity for nests more than 1,500 m from human disturbance.
Charles Todd (MDIF&W) supplied locations of over 800 osprey nest sites throughout Maine, which we digitized. In addition, the New Hampshire and Massachusetts Natural Heritage programs supplied data on recent nest site locations in those states. Because of the nest site fidelity of ospreys, the known occurrence locations are likely to continue to be regularly used.
We examined environmental conditions associated with a subset of 320 Maine nests in developing our habitat model, reserving the balance for testing the model. Most nests occurred near the coast, larger rivers, and large lakes. Using a subset of interior nests we found 180 of 199 nests within 1 km of lakes > 100 acres or major rivers; this, and Livingston et al. (1990) observations, led us to selection criteria of a minimum lake size of 30 ha (74 acres) and use of all river polygons from USGS Topographic maps (that is, not single-line streams). Within the distance of 1 km from these lakes, rivers and coast we found 278 of the entire set of 320 nests.
We examined the frequency of occurrence of the landcover types of the study area (see table, below) at the 320 nest sites in Maine in order to assign relative suitability scores (see table, below). Certain types occurred around nest sites more frequently than would be expected by chance (based on their relative abundance in the overall study area). Some of these nests were probably in large solitary trees or snags in a generally lower ground cover. These also may have been easier to spot during aerial surveys, and so may have been reported at a higher frequency than more cryptic nests in forest covers. The cover types around nest sites nominally included some marine intertidal and subtidal covers which had structure in the form of pilings and channel markers. Those structures were indistinguishable in the landcover, while the aquatic background itself is not regarded as suitable habitat for nesting and so not scored. Bare ground, palustrine scrub/shrub, palustrine emergent, palustrine coniferous and coniferous forested had nests at a far greater frequency (2.5 to 10 times) than would be expected by chance, and these were scored 1.0.Palustrine deciduous and upland mixed forest occurred somewhat more frequently than by chance, and these were scored 0.5. We were surprised to find that upland deciduous occurred far less frequently around nests than in the overall land cover. Perhaps deciduous forest extensive enough to 'show up' in 30 m satellite imagery is not as suitable as large single trees which may not register in that land cover.
Mapping and Modeling:
We buffered known nest sites at 0.25 mile, and scored appropriate cover types within this distance according to nominal values in the table, below.
Foraging habitat for these sites was mapped within 1.5 km (a radius of about 2 km centered on the nest or winter roost points). Based on Livingston et al. (1990), we selected riverine, lake, and shallow coastal waters (detailed below) within this distance and scored these 0.8.
We also developed a model with which to map supplementary foraging and nesting habitats. Vana-Miller (1987) provided a model for osprey habitat in lakes and rivers. Her key variables were fish standing crop and accessibility of prey, and level of disturbance by humans. We lacked specific information on the forage base in study area lakes and rivers, and so modeled habitat from our above observations (distance to large water bodies, and cover types associated with nest locations) in combination with the relevant factors from bald eagle habitat models of Livingston et al. (1990). Our modeled foraging habitats consisted of lakes > 30 ha, major rivers (designated as polygon features in National Wetland Inventory maps), and coastal waters to - 6' mlw depth. These modeled foraging areas were scored 0.2, except that waters with a relatively high diversity of diadromous fishes were scored 0.4. Nesting habitats were mapped as suitable cover types (see table, below) within 1 km of the identified foraging habitats. The nominal values (table, below) were re-scored to 0.5 or to 0.3 if over 500 m from developed or agricultural landcovers, and to 0.3 and 0.1 if less than that distance.
Nominal Suitability Values for Nesting or Foraging (*, **)
|Cover Types||Cover Suitability
(0 - 1 scale)
|Upland deciduous forest||
|Upland coniferous forest||
|Upland mixed forest||
|PEM, L2EM||Lake/pond, emergent vegetation||
|PFOcon||Palustrine forest, conifer||
|PFOdec||Palustrine forest, deciduous||
|PSSdec||Palustrine scrub shrub, deciduous||
|PSScon||Palustrine scrub shrub, conifer||
|PAB, L2AB||Lake/pond, aquatic vegetation||0.8*|
|L1UB, PUB||Lake/pond, unconsolidated bottom||0.8*|
|L2US||Lake, unconsolidated shore||
|L2RS||Lake, rocky shore||
|R1UB||Riverine subtidal unconsolidated||
|E1AB||Estuarine subtidal vegetated||0.8**|
|E1UB||Estuarine subtidal unconsolidated bottom||0.8**|
|E2AB||Estuarine intertidal algae||0.8**|
|E2EM||Estuarine intertidal emergent|
|E2RS, R1RS||Estuarine, tidal river rocky shore||0.8**|
|E2SS||Estuarine intertidal shrub|
|E2US||Estuarine intertidal unconsolidated shore||
|M1AB||Marine subtidal vegetated||
|M1UB||Marine subtidal unconsolidated bottom||
|M2AB||Marine intertidal algae||
|M2RS||Marine intertidal rocky shore||
|M2US||Marine intertidal unconsolidated shore||
|NOTES||* score as foraging habitat
** foraging habitat if within mid-intertidal to -6' mlw
Model testing: The osprey nest sites occurrences from MDIF&W surveys which were not used in model development, and the Massachusetts and New Hampshire occurrences were used to test the habitat map. We compared the presence of habitat near a random set of 798 upland points to that at sites which ospreys actually were observed. Of the 582 sites with birds, 551 had mapped habitat, while 342 sites out of the 798 randomly distributed sites had habitat. The Chi-square was highly significant, indicating that the overall model does indicate localities useful to ospreys.
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