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S U M MARYI . The effects of 4-6 h of water immersion on the renal excretion of water and electrolytes were studied in thirteen normal male subjects in balance on a constant diet containing 150 mEq of Na and 100 mEq of K per day. Each subject was studied during a control period, consisting of quiet sitting, and during water immersion to the neck.2. Immersion resulted in a natriuresis beginning within the first hour, with the rate of sodium excretion eventually exceeding that of the control period by 3 4 f o l d ; potassium excretion also increased. Despite a progressively negative water balance during the immersion studies, urine flow was greater during the first 4 h and free water clearance was greater during the first 2 h of immersion than during the control study.3. The demonstration of a highly significant increase in fractional excretion of sodium during immersion suggests that the natriuresis of water immersion is not attributable to changes in filtered sodium load.4. The prompt onset of the natriuresis, the concomitant kaliuresis and the fact that aldosterone secretion under the conditions of study was probably already suppressed make it unlikely that the natriuresis of water immersion is mediated solely by decreases in aldosterone activity.5. The data suggest that the natriuresis caused by water immersion is the result of decreased fractional reabsorption of sodium proximal to the renal diluting site. The mechanism whereby increased proximal tubular sodium rejection occurs in relation to immersion remains unclear.

ABSTRACT. It has been postulated that the decline of the Northern Pintail (Anas acuta) population is related to the propensity of female pintails to nest in cropland. Using spatial modeling at multiple scales, we estimated that the long-term average breeding population of Northern Pintails in prairie Canada would have initiated a mean of 974,260 nests/year, of which 47% (457,900 +/-43,270) would have been in cropland. Nest success rates are very low (5%) in spring-seeded cropland with predation and agricultural activity responsible for approximately 78% and 22% of the nest loss, respectively. We estimated that a long-term mean of 94,750 (+/-19,680) nests representing 524,725 pintail eggs would have been destroyed by agricultural seeding and tillage operations on cropland annually. The number of nests/eggs lost in any given year would vary by an order of magnitude dependent primarily upon the size of the pintail population nesting on the prairies in that year. Our estimate of incidental take is quite robust because it is based on multiple, long-term studies using data from across the Canadian prairies. Our analysis provides additional support for the theory that the pintail's habit of nesting in cropland is the probable reason for the decline in the pintail population, irrespective of the cause of nest loss. Although predation is the primary cause of the loss of pintail nests in cropland, the proportion of nests lost to predation in cropland is similar to that in other upland habitats on the prairies. Thus the additional loss from agriculture could well be incremental and may be the proximate causative factor in the pintail population's decline and failure to recover in recent decades. Destruction de nids de Canard pilet causée par l'agriculture sur les terres cultivées des Prairies canadiennesRÉSUMÉ. Une des raisons avancées pour expliquer la diminution de la population de Canard pilet (Anas acuta) est en lien avec la propension des femelles à nicher sur les terres cultivées. Au moyen de modélisation spatiale à de multiples échelles, nous avons estimé que la population nicheuse moyenne de longue date de Canard pilet des Prairies canadiennes aurait initié une moyenne de 974 260 nids/année, dont 47 % (457 900 +/-43 270) se seraient trouvés sur les terres cultivées. Le taux de succès des nids est très faible (5 %) sur les terres à culture de printemps, où la prédation et les activités agricoles sont responsables de 78 % et de 22 % des pertes de nids, respectivement. Nous avons estimé qu'une moyenne de 94 750 (+/-19 680) nids équivalant à 524 725 oeufs de pilets auraient été détruits annuellement par les opérations de travail du sol et d'ensemencement sur les terres cultivées. Le nombre de nids/oeufs détruits au cours d'une année donnée variait d'abord en fonction de la taille de la population de pilets nichant dans les Prairies cette année-là. Notre estimation de la prise accessoire est assez fiable parce qu'elle est fondée sur de multiples recherches de longue date utilisant des données provenant des Prairies ca...

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Sources of variation in the egg size of wild and captive northern pintails (Anas acuta) were examined in southern Alberta. Egg size, estimated by volume index (length × breadth2), was strongly correlated with weight of the fresh egg (r2 = 0.89) and weight of the 1-day-old duckling (r2 = 0.89). The body weight of pintail hens was weakly correlated with their egg size in the wild (r2 = 0.11) but was not correlated with egg size in captivity. Captive birds fed a 29% protein diet laid larger eggs than did those fed a 14% protein diet and egg size increased from first to second clutches in birds maintained on a constant diet. Egg size was not related to laying date or clutch size, and did not differ between adults and yearlings. Individual hens tended to lay eggs of a consistent size but there was no significant heritability for egg size.

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