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Prey of Nesting Ospreys on the Willamette and Columbia Rivers, Oregon and Washington
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Abstract Tomore effectively use ospreys as a biomonitoring tool and to better assess contaminant pathways, the diet of nesting ospreys (Pandion haliaetus) was studied along the lower Columbia and upper mainstem,Willamette rivers by evaluating prey remains col- lected from wire baskets constructed under artificial feeding perches installed near nest sites and from the ground beneath natural feeding perches and nests. Prey remains from 1997-2004 on the Columbia River and 1993 (previously published) and 2001 on the Willamette River were evaluated and compared. Largescale suckers (Catostomus macrocheilus) were the predominate fish species identified in collections from the Columbia River (61.5% [84.3% biomass]) and Willamette River (76.0% [92.7% biomass]). Prey fish diversity, when based only on ground collections, was higher in the Columbia (2.45) than the Willamette river (1.92) ( P= 0.038). Prey fish diversity in collections from the Willamette River did not differ between this study (2001) and previous study (1993) (P= 0.62). Fish bones recovered in wire baskets are likely more representative of osprey diet compared,to bones recovered from the ground, because prey diversity was higher among basket samples compared to ground collections (wire basket diversity =5.25 vs. ground collection diversity = 2.45, P = 0.011). Soft-boned salmonids (Oncorhynchus spp.), American shad (Alosa sapidissima), and mountain whitefish ( Prosopium williamsoni) were probably underrepresentedin collections obtained from the ground. Study results suggest that baskets provide a better method for assessing osprey diet than other indirect methods. These findings augment available osprey food-habits information and provide additional biological and ecological information to better assess potential impacts of various environmental contaminants on nesting ospreys. Author to whom correspondence,should be addressed.
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... Osprey diet and biomagnification factor (BMF) calculations Osprey diet was assessed by collecting prey fish remains from below nest sites and nearby perches and identifying the species and estimated size of fish in the collection (for details, see Henny et al. 2003; Johnson et al. 2008). Basically , frequency distributions were determined for each fish species with mean mass estimated for each individual based on length–mass relationships determined from similar size reference fish for each species (i.e., length–mass relationships derived from selected prey remains bones). ...
... Ospreys are opportunistic foragers; therefore, overall fish occurrence in the diet was evaluated on a nest by nest basis with each nest weighted equally. Food habits data collected in the study area in 1993 was compared to information collected 8 years later in 2001 (Johnson et al. 2008). No significant difference in diet was detected, therefore, the 1993 dietary biomass calculations were also used in 2001. ...
... In 2001, prey remains and osprey eggs were again collected in the same area. The fish species represented in the larger sampling of prey fish remains (reciprocal of Simpson's Diversity Index) collected in 1993 (24 nests) did not differ significantly from the smaller sampling effort (12 nests) in 2001 (Johnson et al. 2008 ...
The osprey (Pandion haliaetus) population nesting along the main stem Willamette River and lower Santiam River was first studied to evaluate contaminants and reproductive rates in 1993 when 78 occupied nests were present. By 2001, the population increased to 234 occupied nests, a 13.7% annual rate of population increase. A sample egg was collected from each of a series of nests along the Upper River (river mile 55-187) in 1993, 2001 and 2006 to evaluate trends of persistent contaminants (organochlorine [OC] pesticides, polychlorinated biphenyls [PCBs], polychlorinated dibenzo-p-dioxins [PCDDs], and polychlorinated dibenzofurans [PCDFs]). Nearly all OC pesticide residues decreased significantly, e.g., p, p'-DDE (DDE) from 2,350 to 1,353 to 210 microg/kg wet weight (ww). PCBs followed a similar pattern over time, e.g., SigmaPCBs 688 to 245 to 182 microg/kg ww, while PCDDs and PCDFs showed a more precipitous decline (often 85-95%) between 1993 and 2001, with no egg analyses warranted in 2006. During 2001-2002, sample osprey eggs were also collected from nests at three Headwater Reservoirs and two lower reaches (Newberg Pool and Tidal Portland) of the Willamette River, as well as the lower portion of the Santiam River to evaluate spatial residue patterns. Significant differences were seldom detected among the different sampling areas for OC pesticides (probably due to small sample sizes), although higher concentrations were often seen in the lower reaches, e.g., DDE 901 microg/kg ww (Headwater Reservoirs), 1,353 (Upper River), 1,384 (Newberg Pool) and 2,676 (Tidal Portland). PCB congener concentrations in eggs were usually higher in the Tidal Portland reach than at other locations and often significantly higher than at the Headwater Reservoirs or Upper River. Mercury (first analyzed in eggs in 2001), PCDDs and PCDFs were extremely low in 2001/2002 with no significant spatial patterns. Whole fish composite samples of largescale sucker (Catastomus macrocheilus) and northern pikeminnow (Ptychocheilus oregonensis), which account for about 90% of the biomass in the diet of this osprey population, were also collected from the Willamette River in 1993 and 2001 and analyzed for the same contaminants as osprey eggs. Contaminant residues in fish from the Upper River decreased between 1993 and 2001, paralleling findings for osprey eggs. Likewise, spatial patterns for fish residues paralleled findings for osprey eggs from the different reaches in 2001. A second empirical estimate of biomagnification factors (BMFs) from fish to osprey eggs for OC pesticides, PCBs, PCDDs and PCDFs (ww and lipid weight [lw] basis) was calculated based on residue data collected in 2001. The two independent BMF estimates (1993 and 2001) for each contaminant from the Upper River provide a measure of consistency, e.g., DDE (ww) 87 and 79, (lw) 103 and 112; SigmaPCBs (ww) 11 and 8.4, (lw) 13 and 12. Mercury did not biomagnify from fish to osprey eggs (BMF = 0.60). Legacy contaminants investigated had limited (perhaps only DDE), if any, effects on reproductive success of the increasing osprey population nesting along the Willamette River by 2001.
... Eggs from these two rivers contained the highest RPBDE concentrations in 2008-2009, relative to other sampling sites (Fig. 3). Prey remains were collected near osprey nest sites along the Spokane River in 2009 using standard protocol (see Johnson et al. 2008). Remains of 109 fish were collected with findings from each nest site weighted equally (N = 7). ...
... Suckers (primarily largescale suckers) were the dominant prey species (72.0% of the biomass), followed by bullheads/catfishes 14.1%, northern pikeminnows (Ptychocheilus oregonensis) (10.7%), bass (2.1%), trout (0.4%) and others (1.9%) ( Table 3). Earlier studies of the osprey diet along the Lower Columbia River and Upper Willamette River also indicated that native largescale suckers were the dominant fish species preyed upon (Johnson et al. 2008). The Washington Department of Ecology emphasized the collection of largescale suckers including 18 composites (3-5 fish) at 6 sites along the Spokane River in both 2005 and 2009 (Furl and Meredith 2010). ...
... Eggs from these two rivers contained the highest RPBDE concentrations in 2008-2009, relative to other sampling sites (Fig. 3). Prey remains were collected near osprey nest sites along the Spokane River in 2009 using standard protocol (see Johnson et al. 2008). Remains of 109 fish were collected with findings from each nest site weighted equally (N = 7). ...
... Suckers (primarily largescale suckers) were the dominant prey species (72.0% of the biomass), followed by bullheads/catfishes 14.1%, northern pikeminnows (Ptychocheilus oregonensis) (10.7%), bass (2.1%), trout (0.4%) and others (1.9%) ( Table 3). Earlier studies of the osprey diet along the Lower Columbia River and Upper Willamette River also indicated that native largescale suckers were the dominant fish species preyed upon (Johnson et al. 2008). The Washington Department of Ecology emphasized the collection of largescale suckers including 18 composites (3-5 fish) at 6 sites along the Spokane River in both 2005 and 2009 (Furl and Meredith 2010). ...
Several polybrominated biphenyl ether (PBDE) congeners were found in all 175 osprey (Pandion haliaetus) eggs collected from the Columbia River Basin between 2002 and 2009. ΣPBDE concentrations in 2008-2009 were highest in osprey eggs from the two lowest flow rivers studied; however, each river flowed through relatively large and populous metropolitan areas (Boise, Idaho and Spokane, Washington). We used the volume of Wastewater Treatment Plant (WWTP) discharge, a known source of PBDEs, as a measure of human activity at a location, and combined with river flow (both converted to millions of gallons/day) created a novel approach (an approximate Dilution Index) to relate waterborne contaminants to levels of these contaminants that reach avian eggs. This approach provided a useful understanding of the spatial osprey egg concentration patterns observed. Individual osprey egg concentrations along the Upper Willamette River co-varied with the Dilution Index, while combined egg data (geometric means) from rivers or segments of rivers showed a strong, significant relationship to the Dilution Index with one exception, the Boise River. There, we believe osprey egg concentrations were lower than expected because Boise River ospreys foraged perhaps 50-75% of the time off the river at ponds and lakes stocked with fish that contained relatively low ΣPBDE concentrations. Our limited temporal data at specific localities (2004-2009) suggests that ΣPBDE concentrations in osprey eggs peaked between 2005 and 2007, and then decreased, perhaps in response to penta- and octa-PBDE technical mixtures no longer being used in the USA after 2004. Empirical estimates of biomagnification factors (BMFs) from fish to osprey eggs were 3.76-7.52 on a wet weight (ww) basis or 4.37-11.0 lipid weight. Our earlier osprey study suggested that ΣPBDE egg concentrations >1,000 ng/g ww may reduce osprey reproductive success. Only two of the study areas sampled in 2008-2009 contained individual eggs with ΣPBDE concentrations >1,000 ng/g, and non-significant (P > 0.30) negative relationships were found between ΣPBDEs and reproductive success. Additional monitoring is required to confirm not only the apparent decline in PBDE concentrations in osprey eggs that occurred during this study, but also to better understand the relationship between PBDEs in eggs and reproductive success.
... Moreover, shallow divers and surface feeders are more vulnerable, are considered more sensitive indicators than pursuit divers, and show greater variation in breeding performance (Montevecchi, 1993, Monaghan et al., 1994Scott et al., 2006). As one of the more recognized raptors, ospreys have been used as an ecotoxicological sentinel species of environmental health due to their reproductive responses to natural and anthropogenic pressures and life history traits Johnson et al., 2008;Grove et al., 2009). Ospreys exhibit strong nest fidelity and their reproductive status is observable by ground, boat, or aerial surveys which makes them a valuable and efficient sentinel of the ecosystem (Ogden et al., 2014) and an appropriate ERP for menhaden (Buccheister et al., 2017). ...
The Atlantic States Marine Fisheries Commission (ASMFC), the governing body responsible for managing fisheries on the U.S. East Coast, formally adopted the use of Ecological Reference Points (ERPs) for Atlantic menhaden, Brevoortia tyrannus. Scientists and stakeholders have long recognized the importance of menhaden and predators such as ospreys, Pandion haliaetus, that support the valuable ecotourism industry and hold cultural significance. Landings in the reduction fishery are at their lowest levels and menhaden is facing potential localized depletion. Mobjack Bay, located within the lower Chesapeake Bay, has been a focus of osprey research since 1970 and represents a barometer for the relationship between osprey breeding performance and the availability of their main prey, menhaden. Since local levels of menhaden abundance were not available, we conducted a supplemental menhaden feeding experiment on osprey pairs during the 2021 breeding season. Our main objective was to determine if the delivery rate of menhaden had an influence on nest success and productivity. Nest success (χ2 = 5.5, df = 1, P = 0.02) and productivity (β = 0.88, SE = 0.45, CI = 0.049, 1.825, P = 0.048) were significantly higher within the treatment group. Reproductive rates within the control group were low and unsustainable suggesting that current menhaden availability is too low to support a demographically stable osprey population. Menhaden populations should be maintained at levels that will sustain a stable osprey population in which they are able to produce 1.15 young/active nest to offset mortality.
... Ospreys are highly visible and display their prey, which makes them ideal for observation through nest web cameras. In addition, they acquire fish near nest sites, they are tolerant of short-term nest disturbance, and they habituate to humans , Johnson et al. 2008, Grove et al. 2009). Because of this, private property owners and organizations ranging from nonprofit organizations and foundations to utility companies and universities have made close-up views of nesting Ospreys accessible to the public at large via web cameras and live broadcasts (e.g., on YouTube). ...
Ospreys (Pandion haliaetus) are obligate piscivores and their nesting success depends on sufficient amounts of fish delivered to the nests during the breeding season. Nests are considered successful when pairs raise a minimum of one young to fledging or near-fledging age. Through web cameras and online broadcasts of Osprey nests, citizen scientists quantified daily number of fish deliveries, nest survival, and nest success. We received and analyzed curated data (one to seven seasons, 2014–2020) from citizen scientist groups representing 19 Osprey web cameras from four countries in North America and Europe. We compared the average and the coefficient of variation of the number of fish delivered per day within the early breeding season between the failed and successful nests using a Wilcoxon rank-sum test. We also analyzed the effects of the average and the coefficient of variation of the number of fish delivered per day on the number of days of nest survival and whether a nest was successful or not using generalized linear mixed models. Successful and failed nests had significant differences in the average number of fish delivered per day and the failed nests had a higher variation in the number of fish deliveries. Moreover, the variation and average number of fish delivered per day had strong associations with whether a nest would fail or succeed. The global effort and manner in which these data were collected are novel and can further our understanding of this charismatic species. The combination of citizen science and technology is a powerful modern tool that can provide insights and has the potential to advance raptor research worldwide.
... During the osprey reproductive period, dietary preferences of adults were monitored using a variety of techniques [9,23]. Game camera (Bushnell 8MP Trophy Cam) images of prey items captured, direct identification of fish scraps found in nests, and photographic observations of prey Acc e p ted P r e p r i nt This article is protected by copyright. ...
From 2011-2013, a large-scale ecotoxicological study was conducted in several Chesapeake Bay tributaries (Susquehanna River and flats, the Back, Baltimore Harbor/Patapsco River, Anacostia/ middle Potomac, Elizabeth and James Rivers), and Poplar Island as a mid-Bay reference site. Osprey (Pandion haliaetus) diet and the transfer of contaminants from fish to osprey eggs were evaluated. The most bioaccumulative compounds (biomagnification factor >5) included p,p'-DDE, total PCBs, total PBDEs, and BDE congeners 47, 99, 100 and 154. This analysis suggested that alternative-brominated flame-retardants and other compounds (methoxytriclosan) are not appreciably biomagnifying. A multivariate analysis of similarity indicated major differences in patterns among study sites were driven by PCB congeners 105, 128, 156, 170/190 and 189, and PBDE congeners 99 and 209. An integrative redundancy analysis showed that osprey eggs from Baltimore Harbor/Patapsco River and the Elizabeth Rivers had high residues of PCBs and p,p'-DDE, with PBDEs having a substantial contribution to overall halogenated contamination on the Susquehanna and Anacostia/middle Potomac Rivers. The redundancy analysis also suggested a potential relation between PBDE residues in osprey eggs and oxidative DNA damage in nestling blood samples. Results of this study also indicate that there is no longer a discernible relation between halogenated contaminants in osprey eggs and their reproductive success in Chesapeake Bay. Osprey populations are thriving in much of the Chesapeake, with productivity rates exceeding those required to sustain a stable population. This article is protected by copyright. All rights reserved.
Crayfish (Orconectes spp.), Asian clam (Corbicula fluminea), northern hog sucker (hog sucker; Hypentelium nigricans), and smallmouth bass (smallmouth; Micropterus dolomieu) from streams in southeastern Missouri (USA) were analyzed for total mercury (HgT) and for stable isotopes of carbon (δ¹³C), nitrogen (δ¹⁵N), and sulfur (δ³⁴S) to discern Hg transfer pathways. HgT concentrations were generally lowest in crayfish (0.005-0.112 μg/g dw) and highest in smallmouth (0.093-4.041 μg/g dw), as was δ¹⁵N. HgT was also lower and δ¹⁵N was higher in all biota from a stream draining a more heavily populated historical lead-zinc mining area than from similar sites with mostly undeveloped forested watersheds. δ¹³C in biota was lowest at spring-influenced sites, reflecting CO₂ inputs and temperature influences, and δ³⁴S increased from south to north in all taxa. However, HgT was not strongly correlated with either δ¹³C or δ³⁴S in biota. Trophic position (TP) computed from crayfish δ¹⁵N was lower in hog suckers (mean=2.8) than in smallmouth (mean=3.2), but not at all sites. HgT, δ¹³C, δ³⁴S, and TP in hog suckers increased with total length (length) at some sites, indicating site-specific ontogenetic diet shifts. Changes with length were less evident in smallmouth. Length-adjusted HgT site means in both species were strongly correlated with HgT in crayfish (r²=0.97, P<0.01), but not with HgT in Corbicula (r²=0.02, P>0.05). ANCOVA and regression models incorporating only TP and, for hog suckers, length, accurately and precisely predicted HgT concentrations in both fish species from all locations. Although low compared to many areas of the USA, HgT (and therefore methylmercury) concentrations in smallmouth and hog suckers are sufficiently high to represent a threat to human health and wildlife. Our data indicate that in Ozark streams, Hg concentrations in crayfish are at least partly determined by their diet, with concentrations in hog suckers, smallmouth, and possibly other higher-level consumers largely determined by concentrations in crayfish and other primary and secondary consumers, fish growth rates, and TP.
Osprey (Pandion haliaetus) populations were adversely affected by DDT and perhaps other contaminants in the United States and elsewhere. Reduced productivity, eggshell thinning, and high DDE concentrations in eggs were the signs associated with declining osprey populations in the 1950s, 1960s, and 1970s. The species was one of the first studied on a large scale to bring contaminant issues into focus. Although few quantitative population data were available prior to the 1960s, many osprey populations in North America were studied during the 1960s and 1970s with much learned about basic life history and biology. This article reviews the historical and current effects of contaminants on regional osprey populations. Breeding populations in many regions of North America showed post-DDT-era (1972) population increases of varying magnitudes, with many populations now appearing to stabilize at much higher numbers than initially reported in the 1970s and 1980s. However, the magnitude of regional population increases in the United States between 1981 (first Nationwide Survey, ∼8,000 pairs), when some recovery had already occurred, 1994 (second survey, ∼14,200), and 2001 (third survey, ∼16,000-19,000), or any other years, is likely not a simple response to the release from earlier contaminant effects, but a response to multi-factorial effects. This indirect "contaminant effects" measurement comparing changes (i.e., recovery) in post-DDT-era population numbers over time is probably confounded by changing human attitudes toward birds of prey (shooting, destroying nests, etc.), changing habitats, changing fish populations, and perhaps competition from other species. The species' adaptation to newly created reservoirs and its increasing use of artificial nesting structures (power poles, nesting platforms, cell towers, channel markers, offshore duck blinds, etc.) are two important factors. The timing of the initial use of artificial nesting structures, which replaced declining numbers of suitable trees at many locations, varied regionally (much later in the western United States and Mexico). Because of the increasing use of artificial nesting structures, there may be more ospreys nesting in North America now than ever before. Now, with the impact of most legacy organic contaminants (DDT, other organochlorine [OC] pesticides, polychlorinated biphenyls [PCB], polychlorinated dibenzo-p-dioxins [PCDD], polychlorinated dibenzofurans (PCDF]) greatly reduced or eliminated, and some osprey populations showing evidence of stabilizing, the species was proposed as a Worldwide Sentinel Species for evaluating emerging contaminants. Several emerging contaminants are already being studied, such as polybrominated diphenyl ethers (PBDE) and perfluorinated acids and sulfonate compounds (PFC). The many advantages for continued contaminant investigations using the osprey include a good understanding of its biology and ecology, its known distribution and abundance, and its ability to habituate to humans and their activities, which permits nesting in some of the potentially most contaminated environments. It is a top predator in most ecosystems, and its nests are relatively easy to locate and study with little researcher impact on reproductive success.
We compared techniques to assess diet at 17 Hen Harrier (Circus cyaneus) nests. Diet was measured directly from observations and compared to estimates from pellets, prey remains and a combination of pellets and remains. For data pooled across nests, pellets over-represented mammalian prey and under-represented avian prey. Prey remains over-represented large prey and under-represented small prey. Combining pellet and remains data did not eliminate these biases. Pellets gave higher diversity values than direct observations and detected more small prey species. For data analyzed on a nest by nest basis, estimates from pellets were significantly related to estimates from direct observations for three prey types by frequency and all types by biomass. These linear relationships were used to predict frequencies and biomass of prey types in the observed diet at five new nests. Our findings suggest that pellets are useful for estimates of prey diversity and as an index of the frequency of certain prey types in the diet, but direct observations are necessary to help quantify the biases inherent in diet estimates.
Great blue heron (Ardea herodias) eggs and prey items were collected from six colonies in Oregon and Washington, USA, during 1994 to 1995. Contaminant concentrations, reproductive success, and biomagnification factors were determined and effects of residue levels were measured by H4IIE rat hepatoma bioassays. Mean residue concentrations in heron eggs and prey items were generally low. However, elevated concentrations of polychlorinated biphenyls (PCBs) were detected in eggs and prey from Ross Island on the Willamette River. Biomagnification factors varied among sites. Sites were not significantly different in H4IIE tetrachlorodibenzo-p-dioxin equivalents (TCDD-EQs), although the TCDD-EQ for Karlson Island was 9 to 20 times greater than that of any other site. Large differences existed between toxic equivalents calculated from egg residue concentrations and TCDD-EQs, which indicated nonadditive interactions among the compounds. Tetrachlorodibenzo-p-dioxin equivalents and nest failure were positively correlated with TCDD concentration. Fledging and reproductive rates were similar to those determined for healthy heron populations, however, indicating that any adverse effects were occurring at the individual level and not at the colony level. Our results support the use of great blue herons as a biomonitor for contamination in aquatic ecosystems. Their relatively low sensitivity to organochlorine contaminants and high trophic position allows contaminant accumulation and biomagnification without immediate adverse effects that are often seen in other, more sensitive species.
Various aspects of Osprey biology, Pandion h. haliaetus, were studied from museum specimens and in the field. Size, breast markings and crown markings were found insufficient to discriminate populations, but underwing coverts were sufficient. Four subspecies were recognized, corresponding to the Palearctic, North America, the Bahamas and Australasia. However Ospreys can be divided into two groups, a Holarctic group, consisting of Palearctic, North American and Bahaman Ospreys, and the Australasian Ospreys. Ecology was studied in Senegambia during two visits in 1977-80. Over 800 Ospreys were estimated to winter there, mostly at river mouths and in mangrove. Marked Ospreys returned to the same area in consecutive years, and stayed within that area during winter. Dispersion along the coast was random except at a few localities where the birds concentrated. In mangrove, birds were regularly spaced at high tide but went to the coast or more open mangrove at low tide. Along the coast, Fish Eagles, Haliaeetus yocifer, and Ospreys seemed independently distributed, but in mangrove Ospreys avoided hunting in sight of a Fish Eagle, while along rivers and lakes eagles might have excluded Ospreys. The diet consisted mostly of Mugilidae, various Clupeidae, and, during part of winter, of Exocoetidae. Average fish size was generally 200-300 g, but fish were smaller in areas well protected from the open sea. Dive success and search time per capture varied between sites, partly because they were significantly correlated with fish size. Immatures 6 months old were less successful at catching fish than older birds. Foraging efficiency varied from 2.9 to 10 kcal/minute of foraging. Most Ospreys were found in mangrove even though foraging efficiency was lower there. This was most likely because hunting from a perch in mangrove was less demanding than hunting from flight elsewhere. Moult was studied from captured Ospreys and museum skins. A few Ospreys were caught more than once so that the development of the moult pattern of the primaries could be traced. Limits to the growth rate of feathers are emphasized as a major reason for the evolution of the Staffelmauser pattern of moult in large birds. Ospreys do not breed in the tropics, except in Australasia, even though the habitat seemed ecologically suited. In particular, foraging efficiency was as high in Senegambia as on the north-temperate breeding grounds. It is suggested that migrants Ospreys are physiologically inhibited from breeding in the tropics because daylength is too short, while south of the tropic of Capricorn, where migrants might be stimulated to breed, but six months out of phase, they are too scarce to start a permanent breeding population. It is suggested that non-migratory Ospreys have not spread their range south because of the presence of large numbers of migrants in the tropics.
Ospreys (Pandion haliaetus) once nested throughout most of the U.S. The decline in this population due to biocide use has been well documented, as has its recovery following the U.S. ban on DDT in 1972. A general increase in the nesting distribution and abundance of Ospreys was reported in the U.S. in 1981 but there was limited dispersal into states with low or extirpated populations. We conducted a nationwide nesting survey of nesting Ospreys in 1994, updating the 1981 data. Our data indicate a dramatic increase in die U.S. Osprey population from ∼8000 nesting pairs in 1981 to ∼14 200 in 1994. The most dramatic increases were seen in traditional nesting areas, with some new nesting in the interior U.S. Hacking projects, construction of reservoirs, nest platform management and increased public relations have contributed to the growth of this nesting population.
A quick dip into the literature on diversity reveals a bewildering range of indices. Each of these indices seeks to characterize the diversity of a sample or community by a single number. To add yet more confusion an index may be known by more than one name and written in a variety of notations using a range of log bases. This diversity of diversity indices has arisen because, for a number of years, it was standard practice for an author to review existing indices, denounce them as useless, and promptly invent a new index. Southwood (1978) notes an interesting parallel in the proliferation of new designs of light traps and new permutations of diversity measures.
Although studies of bald eagle (Haliaeetus leucocephalus) food habits are numerous, few authors have quantified biases inherent in the techniques used. In our study of food habits of nonbreeding bald eagles on the northern Chesapeake Bay, we examined biases associated with pellet analysis, food remains analysis, and direct observation. We assessed these biases through controlled feedings of 2 captive bald eagles and through observations of free-ranging eagles. Fish fed to 2 captive bald eagles were underrepresented (P < 0.001) in egested pellets. Most bird and mammal food items were detected in ≥1 pellet; however, species and carcass condition affected frequency of occurrence. Analysis of captive eagles' food remains overrepresented birds, medium-sized mammals, and large bony fish (P < 0.001); small mammals and small fish were underrepresented (P < 0.001). This bias increased over time due to greater persistence of some remains in shoreline plots. Direct observations of free-ranging eagles resulted in biases toward easily identified species such as eels and catfish, but also documented the use of small, soft-bodied fish, which were not well documented by the other techniques. Because of the variety of biases present, accurate assessment of bald eagle foods requires use of multiple techniques.