Shooting and trapping
Historically affected by shooting in North America, although less so than other diurnal raptors. U.S. banding data 1972–1984 (Poole and Agler 1987) show 30% of 451 birds recovered were shot, with 93% from overwintering grounds in Central or South America. From 1914 to 1972, 61% of 270 recoveries in U.S. were reported as shot; hence a significant reduction in shooting pressure in the U.S. since the early 1970s.
Shooting still occurs in the U.S. but at a low level in most areas; e.g., 18 of 82 Ospreys brought to the Carolina Raptor Center 1979–2000 (M. Engelmann, unpublished data), 36% of 33 recent admissions at The Raptor Center at the University of Minnesota (unpublished data). Data for flighted birds (excluding nestlings) for 2000 to 2015 from 4 rehabilitation centers (Carolina Raptor Center, Raptor Center/University of Minnesota, Audubon Center for Birds of Prey in Florida, and Tri-State Bird Rescue in Delaware) show much lower gun-shot rates: 9.2%, 7.2%, 3%, and 4.2% respectively, (M. Englemann, L. Arent, S. Little, and L. Smith, personal communication). In Brazil 1960–1998, 33 of 44 (75%) of recoveries were shot; no significant change in shooting rates during the period (I. Nascimento, unpublished data). Mestre and Bierregaard 2009 report a significant decline in Brazil since 2000 but caution that this may be the result of environmental regulations generating a reluctance to report shooting.
In Colombia, fish farms attract overwintering Ospreys, with increasing numbers shot since the 1970s; in southern portions of the country, as many as 270 shot annually in 2001 (Bechard and Márquez-Reyes 2003). More recent surveys throughout Central and South America found Ospreys were considered a serious problem at aquaculture facilities in many countries, particularly Colombia, El Salvador, Panama, and Guatemala; netting to cover fish impoundments shows promise in lessening the damage Ospreys do at fish farms, but most facilities (except in Ecuador) find shooting easier and less expensive with problem birds (Bechard and Márquez-Reyes 2003). More study needed.
Vulnerable to shooting at fish farms during migration. Four migrating satellite-tagged Ospreys stopped moving, presumably shot, at fish farms in Cuba and Hispaniola (ROB), a bottleneck for migrating eastern Ospreys (see Migration, above); needs study in such areas. See (Poole 1989a) for discussion of shooting and trapping in the Palearctic region.
The U.S Fish and Wildlife Service (USFWS) issued only one depredation permit to kill Ospreys at an aquaculture facility (in the Southeast) in the period 1991–2001 (C. Hunter, personal communication).
Eggs collected most intensively from the late 1800s through 1940, but densities of U.S. breeding populations were apparently sufficient to oversupply the market. More clutches collected from New Jersey than from any other state (248 clutches 1887–1938 in collection of the Western Foundation of Vertebrate Zoology; R. Corado, personal communication). Combination of shooting and egg collecting implicated in a 75% decline in a population of approximately 100 pairs at Seven Mile Beach, New Jersey, 1884–1890 (Stone 1937).
Pesticides and other contaminants/toxins
Persistent organochlorine pesticides, particularly DDT/DDE, had major effects on some regional populations from the 1950s through the early 1970s. These chemicals, stored in fatty tissue and increasingly concentrated moving up food chains, have been correlated with depressed reproductive success (Wiemeyer et al. 1975, Wiemeyer et al. 1978, Wiemeyer et al. 1988, Spitzer et al. 1978). Most noticeable effect was eggshell thinning, which caused problems ranging from changes in gas exchange (including water loss) for developing embryos to eggs breaking under the weight of an incubating adult.
“Of the organochlorines found in [U.S.] Ospreys, DDE has been clearly associated with most, if not all of, the adverse effects on eggshell thickness, reproductive success, and associated population declines” (Wiemeyer et al. 1988: 785). Wet-weight DDE levels of 2.0 parts per million (ppm) in egg contents associated with 10% thinning in eggshells, 4.2 ppm with 15% thinning, 8.7 ppm with 20% thinning (Wiemeyer et al. 1988). Eggshell thinning of 15% is general threshold associated with reproductive rates too low to sustain local populations in birds of prey (Anderson and Hickey 1972). No North American raptor population exhibiting ≥18% eggshell thinning maintained a stable, self-perpetuating population during the pesticide era (Lincer 1975). Complete reproductive failure in Ospreys was associated with eggs showing DDE levels ≥ 12 ppm wet weight (Johnson et al. 1975a, Wiemeyer et al. 1975).
Hardest hit population was along the Atlantic Coast from New Jersey to Boston, Massachusetts. Numbers of breeders fell to 10% of pre-DDT levels (Spitzer 1980), and eggshell thickness in 15 eggs from 2 areas in New Jersey in 1970–1974 was 12 and 19% below pre-1947 means with DDE levels averaging 14 and 16 ppm wet weight, respectively (range 6.5–40; Wiemeyer et al. 1978). Similar findings reported from Connecticut (Wiemeyer et al. 1975) and several parts of Canada (Wiemeyer et al. 1988, Noble and Elliott 1990).
Since 1970, following reduced use (and ultimate 1972 ban) of DDT and other chlorinated hydrocarbon pesticides during the late 1960s in North America, most populations of Ospreys and other fish-eating birds have increased rapidly. Despite localized sources of contamination in Central and South America, overall exposure of raptors to DDE has decreased. Nevertheless, Ospreys remain contaminated with DDE (Elliott et al. 2000). Local and regional “hot spots” for DDE contamination persist in areas of extensive past use—often near areas of industrial or intense agricultural or horticultural activity (e.g., Blus et al. 1987, Elliott et al. 1994). Repeated dredging of shipping channels in Delaware Bay may account for high levels of DDE (and other organochlorines) in Ospreys in western New Jersey because DDE residues in adjacent agricultural lands are re-suspended when sediments are disturbed (Steidl et al. 1991a). Other potential sources of continued exposure include: DDT applied locally on overwintering grounds, exposure to products that contain DDT isomers as contaminants, and atmospheric deposition (Elliott et al. 2000).
In the Pacific Northwest, 1991–1997, DDE levels in Ospreys were high and occasionally variable (20% of 111 Osprey eggs had DDE levels > 5 ppm wet weight, 8% had levels > 10 ppm wet weight, and 2 eggs had levels > 20 ppm wet weight); high levels were found in areas where there had been little historic use of DDT. Lack of relation to DDE levels in prey in the nesting area suggested that adults had been exposed on migration or overwintering grounds. DDT:DDE ratios suggested that the main source of current contamination was past use in overwintering areas (Elliott et al. 2000). In New Jersey, by contrast, most organochlorines appear to have been assimilated from local prey (Steidl et al. 1991a). For the Pacific Northwest study, 54 eggs were removed from nests and placed in incubators. In the 13 that failed to hatch, mean DDE level was 3.3 ppm wet weight, not significantly different from 2.6 ppm in 38 eggs that did hatch (Elliott et al. 2001b).
In the Great Lakes basin of Canada, 1991–1995, organochlorine levels in eggs were generally low, with breeding success well above replacement levels (Martin et al. 2003).
Eggs from Michigan Ospreys of known age in the 1980s showed no relationship between DDE levels and age of female, suggesting that females had reached equilibrium levels by breeding age during this decade (Ewins et al. 1999).
In 2001 in southern Baja California, at a location remote from sources of organochlorines (OC), levels of OC in the blood of Osprey chicks were understandably low and the chicks unaffected (0.002–6.856 ppb; (Rivera-Rodriguez and Rodriguez-Estrella 2011).
Other toxic chemicals reported in Osprey eggs include polychlorinated biphenyls (PCBs), heptachlor, dioxins, dieldrin, chlorodanes, lead, and mercury (Wiemeyer et al. 1988, Noble et al. 1993, Elliott et al. 2000, Elliott et al. 2001b). None of these has been implicated in any recent reduction in Osprey productivity. For example, reproduction of Ospreys in Connecticut and New York improved as DDE levels declined, despite continued high levels of PCBs and mercury in eggs (Spitzer et al. 1978). However, poisoning of adults by dieldrin may have contributed to population declines in some areas (Spitzer et al. 1978).
On the Wisconsin River, 1992–1996, dioxin (TCDD; 2,3,7,8-tetrachloro-p-dioxin) levels in eggs collected downstream of pulp mills were 29–162 pg/g wet weight—much higher than in control areas upstream of the mills. Dioxin levels in fish were 30–100 times higher downstream from the mills than upstream. No measurable effects on reproduction were found except reduced weight gain in chicks, suggesting possible effects on postfledging survival (Woodford et al. 1998).
Since 2002, Montana studies found heavy metals (copper [Cu], lead [Pb] and arsenic [As], among others) in blood and feathers of ospreys nesting along steams with historic mining operations (Langner et al. 2012). Elsewhere, in Ontario, PCB levels in chick plasma (but not eggs) declined with distance from a point source (de Solla and Martin 2009). And in Washington and Oregon, OC and Hg levels decreased from 2003 to 2007, but none were at levels suggesting risk to eggs or chicks (Johnson et al. 2009a). Sampling fish in known (via satellite telemetry) Osprey overwintering sites in coastal Mexico showed their contaminant levels had no significant effect on contaminant concentrations in sample eggs from Ospreys that overwintered there; rather concentrations of p,p'-DDE were predicted by contaminant levels in fish at breeding sites (Elliott et al. 2007b). Biomagnification levels (fish to Osprey egg) varied significantly by contaminant: from 0.42 to 174 (Henny et al. 2003).
Polybrominated biphenyl ether (PBDE) flame retardants are now widespread in Ospreys, with reports of contamination from Oregon (Henny et al. 2009b, Henny et al. 2011), Chesapeake Bay (Chen et al. 2010), and British Columbia (Elliott 2005a). In general, differences in PBDE concentrations appear to reflect differences in flow (dilution effect) and the extent of human population and industry (source inputs) contributing effluent to the waters where Osprey fish. Data showing impacts of PBDE on Osprey reproduction remain elusive, but clearly the widespread nature of this contaminant, and the Osprey’s tendency to concentrate it, urge continued monitoring.
Grove et al. (2009) propose the Osprey as a “sentinel species” for assessing contaminant levels in waters worldwide. Few species appear better suited for such assessment, given the bird’s broad distribution and dependence on varied aquatic habitats, as well as providing the ability to sample feathers, eggs and blood – each giving a different window on contaminant histories.
Hg is biomagnified in food chains and tends to be higher in biota from recently created reservoirs than in naturally occurring ponds and lakes (Hughes et al. 1997, DesGranges et al. 1998). Ospreys accumulate Hg in body tissues between molts, but chicks eliminate 85% and adults 95% of body burden into feathers at each molt (Hughes et al. 1997, DesGranges et al. 1998). Some Hg is transferred to eggs, but there is no consensus on how this affects egg viability. Studies in terrestrial birds indicate that 0.5–0.8 ppm wet weight is the threshold above which mercury levels in eggs have detrimental effects on reproduction (Fimreite 1971 in Hughes et al. 1997, Heinz 1979, Newton and Haas 1988).
Studies in Ontario and New Jersey, 1991–1994, showed local similarities among levels in young, suggesting Hg in young reflects exposure levels at breeding grounds, while that in eggs reflects exposure of adults throughout the year (Hughes et al. 1997). Hg levels in feathers and tissues of Osprey chicks were 5 times higher at Quebec reservoirs than at nearby lakes (DesGranges et al. 1998). Other data for eggs from the U.S. and Canada was reported by Wiemeyer et al. 1988, Noble and Elliott 1990, and Elliott et al. 2000.
Since 2002: In the Clark’s Fork River Basin, Montana, Hg levels in chick feathers and blood varied with location and river sediment load, with nests near old mining locations producing chicks with the highest levels (Langner et al. 2012). In a study of Ospreys in 15 watersheds in western Canada, modeling suggested that atmospheric deposition was the primary contributor to Hg loading in chicks, with post-snow melt (glacial meltwater) a lesser factor (Guigueno et al. 2012). On the Willamette River, Oregon, 2001–2006, Hg was found at low levels in Osprey eggs, with no significant differences between prey and egg (i.e., Hg did not biomagnify; Henny et al. 2009a).
While lab studies suggested that Osprey eggs/embryos have relatively high sensitivity to methylmercury (Heinz et al. 2009a), no study of wild Ospreys has yet clearly demonstrated an Hg impact on breeding success. At lakes in northern California (one subject to mine runoff), for example, Hg levels in Osprey feathers declined significantly during the 1990s (20 mg/kg DM to 2 mg/kg), but increased to 12 mg/kg by 2006; reproduction, nonetheless, remained well above replacement level throughout, and the population continued to grow (Anderson et al. 2008c). In the Delaware River and Delaware Bay (Rattner et al. 2004, Toschik et al. 2005) Osprey brood sizes were smaller in more contaminated regions, but those nests fledged young at or above replacement levels; in addition, other factors affecting reproduction (food supply, nest site vulnerability) were not taken into account as factors in the smaller broods (e.g., see Morrissey et al. 2004c). See also Lounsbury-Billie et al. 2008(Florida Bay), Henny et al. 2003and Henny et al. 2008b (Oregon), Rattner et al. 2008(Chesapeake Bay and Delaware Bay), and Morrissey et al. 2004c(British Columbia) for details on Hg contamination in other regions.
Ingestion of lead, plastics, etc.
Individuals nesting along the Coeur d’Alene River, Idaho (where nearby mining and smelting had released significant quantities of lead into river sediments), had higher levels of lead in blood than those at reference areas (Henny et al. 1991b). No effects on reproductive success detected.
Collisions with stationary/moving structure or objects
Adults, and especially fledglings, at nests near highways are vulnerable to collisions with vehicles. Of flighted Ospreys admitted to rehabilitation centers from 2000 to 2015, incidence of injuries caused by vehicular collisions ranged from 23.6% in Florida to 2.4% in Minnesota. (Audubon Center for Birds of Prey, Raptor Center/University of Minnesota, unpublished data). While collisions with power lines do happen, more threatening are electrocutions, particularly when adults land or attempt to nest on double-crossarm power poles with transformers, which often provide prominent perches or nest sites (see discussion in Poole 1989a, and references therein). Highest rates of electrocution admissions (5.3% and 5.7% of flighted birds) at rehabilitation centers have been near dense human populations (Florida and Delaware; Audubon Center for Birds of Prey, Tri-State Rehabilitation Center, unpublished data)
Collisions with aircraft are an increasing problem with this species. Between 1985–2003, 25 strikes with military aircraft were documented at Langley Air Force Base in Virginia, with damage over $1.4M, including a single strike causing $750,000 damage to an F-15 Eagle fighter jet (Olexa 2006). Washburn 2014 documented 255 collisions with aircraft between 1990 (2 strikes) and 2011 (28 strikes). Management efforts can mitigate such collisions (see below).
Collisions with stationary objects are rare, but the dramatic increase in the use of cell towers as nesting sites is increasingly an area requiring management; the effects of radiation emitted by the towers on birds and eggs is an area needing study (Washburn 2014).
A few individuals were found drowned in pound nets (nets set perpendicular to shoreline that trap fish in a small net-enclosed weir, open at the top) on Long Island, New York, in the 1950s and 1960s (P. Spitzer, personal communication). Since 2000, non-nestling “entanglement” (predominantly fishing line) admittances at 4 rehabilitation centers were 12.7% in Delaware region, 10.5% in the Carolinas, 5.6% in Minnesota, and 5.6% in Florida (Tri-State Bird Rescue, Carolina Raptor Center, Raptor Center/University of Minnesota, Audubon Center for Birds of Prey; unpublished data). Ospreys often bring fishing line or other plastic objects into nests, potentially endangering their young with entanglement. In Westport, Massachusetts, a small percentage (1–2%) of nestlings die each year from such entanglement (AFP). In Saskatchewan (1999–2004), 9 of 77 young banded were found entangled in baling twine in nests, a few severely enough to prevent feeding; one adult died of entanglement (Houston and Scott 2006); nearby farming operations made extensive use of the twine, which was often discarded in fields. Similar data reported from Yellowstone River in Montana (Seacor et al. 2014); the issue presumably affects Ospreys nesting across much of western ranchland.
Degradation of habitat
Generally tolerant of land development; e.g., Florida Keys population is thriving, despite widespread development there (Bowman et al. 1989); likewise in Baja California, Mexico, where pairs now nest on platforms, highway signals, and channel markers close to boat and vehicular traffic, in towns and industrial areas (Castellanos and Ortega-Rubio 1995, Washburn 2014). In southern New England, Ospreys often nest in locations with high levels of human activity, and can habituate readily to that (see below and Poole 1989a). The species is probably more vulnerable to changes in water quality, but this is not well quantified; a recent decline in the Florida Bay Osprey nesting population has been attributed to a reduction in water quality (Kushlan and Bass 1983a, Bowman et al. 1989).
Some regional population declines may be associated with loss of nest sites, related in turn to increased lumbering and agricultural activities (Ewins 1997). Populations in Oregon declined dramatically prior to 1940; where there were hundreds of pairs in a colony in south-central Oregon in 1899, drainage of Tule Lake and cutting of nest trees eliminated the colony (Henny 1988a). By 1940, numbers had declined elsewhere (Gabrielson and Jewett 1940), probably because of a shortage of nesting sites associated with cutting of trees along rivers. Timber harvesting and conversion of land to agriculture reduced availability of nest sites around the Great Lakes, leading to gradual declines in populations beginning in the 1930s (Ewins 1995, Ewins 1996). Similar loss of nest sites may have affected Chesapeake Bay populations (Reese 1969).
Precipitous decline in beaver populations with intensive trapping of 1600s and 1700s probably affected Osprey distribution and numbers, but this was not documented. Beaver swamps provide key Osprey habitat in northern forests.
Felling of rain forests has likely affected overwintering Ospreys: increasing sedimentation rates, potentially increasing turbidity and making fishing in rivers more difficult; also killing coral reefs that provide prey (Poole 1989a). Gold miners in tropical South America discharge mercury into rivers, where it is already found in alarming levels in fish and humans (Lodenius and Maim 1998). Deforestation also contributes to elevated mercury levels in the Amazon basin through increased erosion (Roulet et al. 2000). Satellite-tracked birds known to overwinter in the Amazon should be tested for mercury levels.
Disturbance at nest and roost sites
Habituates easily to human activity nearby. Pairs that begin nesting near humans usually develop high tolerance; those nesting away from disturbance may be sensitive to human presence (e.g., Swenson 1979a). In coastal New York and New England, “coexistence [of humans and Ospreys] is firmly established in the culture of both species” (Spitzer 1989); e.g., late 19th century farmers used to erect nesting poles for Ospreys near their dwellings, hoping the fish hawks would drive other hawks (e.g., “chicken hawks”) away (Bent 1937b). Guidelines for limiting access around nests have been proposed (Cline 1990b), but tolerance levels are too variable for such guidelines to be broadly applicable. Little information on effects of forest management activities in Canada (Ewins 1997).
Jet-skis are a potential threat to nesting pairs; needs study. Ospreys nesting along Wyoming rivers flushed when boats moved toward them, but ignored boats moving parallel to the shore (Alt 1980). Ospreys nesting on channel markers along the Intercoastal Waterway from Florida to Maryland are habituated to boats near their nests. In Labrador, low-flying jets had little or no effect on Osprey behavior or breeding success (Trimper et al. 1998).
Direct human/research impacts
In southern New England, reproductive success is unaffected by brief (10–20 min) visits to nests during the breeding season (Poole 1981). With care to avoid visits in extreme weather (hot or cold and wet) and just prior to fledging, research activities have little effect on nesting individuals. Helicopter surveys in Idaho did not affect reproductive success (Carrier and Melquist 1976). Helicopter surveys in the Great Lakes provided more accurate data on chick numbers than surveys with fixed-wing aircraft (Ewins and Miller 1995).
The impacts of radio-telemetry devices on Ospreys needs study, although work in the United Kingdom suggests the birds adjust well (Mackrill 2013). Harness-mounted satellite transmitters may be more of a problem, especially for breeding adults, which have shown reduced nesting success and increased mortality, with analysis of the data ongoing (ROB).