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Osprey (Pandion haliaetus) eggs were collected from 1991 to 1997 at nests (n = 121) upstream and downstream of bleached kraft pulp mills and at reference sites in the Fraser and Columbia River drainage systems of British Columbia, Washington, and Oregon. Blood samples were collected from nestling ospreys during the 1992 breeding season on the Thompson River. Samples were analyzed for polychlorinated dibenzo-p-dioxins (PCDDs) and -dibenzofurans (PCDFs). Mean concentrations of 2,3,7,8-TCDD were significantly higher in eggs collected in 1991 at downstream compared to upstream nests near pulp mills at Kamloops and Castlegar, British Columbia. There were no significant temporal trends in 2,3,7,8-TCDD, -TCDF or other measured compounds at a sample of nests monitored between 1991 and 1994 downstream of the Castlegar pulp mill, despite changes in bleaching technology (CIO2 substitution). However, by 1997 concentrations of 2, 3,7,8-TCDD and -TCDF were significantly lower than previous years in nests sampled downstream at both Castlegar and Kamloops. An unusual pattern of higher chlorinated PCDDs and PCDFs was found in many of the osprey eggs collected in this study, and considerable individual variation in the pattern existed among eggs from the same site. For example, eggs from four different nests at one study area (Quesnel) on the Fraser River had concentrations of 1,2,3,4,6,7,8-HpCDD ranging from <1 to 1,100 ng/kg and OCDD from <1 to 7,000 ng/kg wet weight. Higher mean concentrations of HpCDD and OCDD were found in eggs from the Thompson River, a tributary of the Fraser, compared to the Columbia River, and concentrations were generally higher at nests upstream of pulp mills. In plasma samples, 1,2,3,4,6,7,8-HpCDD and OCDD were the main compounds detected, with no significant differences measured between samples upstream versus downstream or earlier versus later in the breeding season. Use of chlorophenolic wood preservatives by lumber processors was considered the main source of higher chlorinated PCDD/Fs throughout the systems, based on patterns of trace PCDFs in eggs and significant correlations between egg concentrations of pentachlorophenol and both HpCDD (r = 0.891, p < 0.01) and OCDD (r = 0.870, p < 0.01).

Eggs of ospreys (Pandion haliaetus) were collected over the period 1991 to 1997 at 111 nests in the Fraser and Columbia River drainage systems of British Columbia, Washington, and Oregon. Contents were analyzed for organochlorine (OC) pesticides, polychlorinated biphenyls (PCBs), and total mercury. Blood samples were taken from nestling ospreys at two time points during the 1992 breeding season on the Thompson River and analyzed for non-ortho PCBs. Concentrations of DDE and related compounds showed high variability among individual eggs within study areas and no significant differences in mean concentrations among study areas. Some eggs contained high concentrations of DDE, up to 20 mg/kg (wet weight), for which there were no evident local sources, suggesting that exposure occurred outside of the breeding grounds. Most other OC pesticides measured in osprey eggs showed a similar distribution, although mean concentrations of chlordane-related compounds were generally, and in some cases significantly, higher in samples collected from the Columbia River sites compared to elsewhere. Significantly greater concentrations of PCBs were found in eggs from the Columbia River basin compared to the Fraser. This was attributed to extensive development of hydroelectric generation and related industries on the Columbia system. Significant differences among sites were also found in the pattern of PCB congeners, e.g., eggs from the lower Columbia River site had proportionally greater concentrations of less chlorinated, Aroclor 1242-type PCBs compared to other sites. In contrast, eggs from upper reaches of the Columbia River had relatively greater amounts of Aroclor 1254 and 1260 congeners. Mercury concentrations in osprey eggs tended to be uniform among sites and comparable to those reported in the literature for ospreys nesting on naturally formed lakes and rivers. There were no significant temporal changes in mean concentrations of any of the measured compounds at sites monitored from 1991 to 1997.

The influence of weather on the foraging behavior of breeding male Ospreys (Pandion haliaetus) was investigated from an energetic perspective. Neither cloud cover, sun brightness, nor precipitation had significant effects on foraging performance. Wind speed and water surface conditions both had an effect, and as expected, they were very highly correlated. As wind speed increased (and surface conditions deteriorated), Ospreys glided more and flapped less while hunting, but each hunt became longer and was less likely to terminate with capture of a fish. Wind speed was by far the strongest effect. Overall, more work was required to capture each prey as wind speed increased, and the data indicate that the detrimental effects increase sharply at wind speeds greater than about 7 m/s. The results are compared with previous investigations of Osprey foraging.

The food-amount hypothesis, proposed to account for sibling aggression in brood-reducing bird species, predicts that broods will be aggressive when hungry, and that as a result, senior siblings will be able to take a greater share of the total available food. We performed a field experiment to test these predictions in ospreys, Pandion haliaetus. Broods were removed from their natal nests and subjected to a series of experimental trials. Each trial consisted of a 3.5-h starvation period followed by a feeding or sham-feeding treatment designed to manipulate the hunger of the nestlings. Following treatment, broods were placed in a viewing nest along with a fish. While the viewing-nest mother fed the fish to the brood, we recorded food allocation and aggresssion. Each of 10 broods experienced a number (one to six) of feeding and sham-feeding treatments over a period of 2 days and was then returned to its natal nest. Broods were slightly but significantly more aggressive after sham-feeding treatments, and senior siblings took a greater share of the meal. The tremendous variation in the level of sibling aggression observed between broods was inversely related to brood mass asymmetry but not to condition, suggesting that siblings adjusted the rate of aggression according to the level of competition within their broods.Résumé : L'hypothèse de la quantité de nourriture comme cause de l'agressivité des oisillons à l'égard de leurs frères et soeurs chez les espèces qui contrôlent le nombre d'oisillons dans les couvées suppose que les oisillons sont agressifs quand ils ont faim et que, en conséquence, les oi3sillons plus vieux devraient pouvoir s'approprier une plus grande partie de la nourriture disponible. Nous avons éprouvé cette hypothèse chez des Balbuzards pêcheurs, Pandion haliaetus, en nature. Les oisillons ont été enlevés de leurs nids d'origine et soumis à une série d'expériences au cours desquelles leur faim était manipulée : ils étaient privés de nourriture pendant 3,5 h, puis nourris ou soumis à une période d'alimentation simulée. Après l'expérience, les couvées étaient placées dans un nid bien en vue avec un poisson. Pendant que la mère distribuait le poisson à sa progéniture, nous avons noté la ration reçue par chaque oisillon et l'intensité des agressions. Chacune des 10 couvées a reçu un certain nombre de repas (un à six) ou de repas simulés au cours d'une période de 2 jours, puis a été retournée à son nid d'origine. Les oisillons étaient légèrement, mais significativement, plus agressifs après les repas simulés et les oisillons les plus âgés s'appropriaient une plus grande part du repas. L'énorme variation de l'intensité des agressions chez les oisillons des différentes couvées était inversement proportionnelle à l'asymétrie de la masse de la couvée, mais pas au coefficient d'embonpoint, ce qui semble indiquer que les oisillons ajustent l'intensité de leurs agressions selon le degré de compétition au sein de leur couvée. [Traduit par la Rédaction]