pre-hatch brood amalgamation, whereby a female lays her eggs in the nest of another female and the recipient thereafter provides all further care of the eggs and resulting offspring, and (ii) post-hatch brood amalgamation, whereby a female abandons or loses her young to another female after hatch, and the recipient subsequently tends the foster young. Some authors have viewed these behaviours as accidental or aberrant and of little evolutionary significance. More recently, a number of alternative hypotheses have been suggested. However, few of these hypotheses have been contrasted as viable alternatives and tested in the field, largely because an appropriate theoretical framework is lacking. We analyze the frequency of occurrence of brood amalgamation in North American anatids. We also review the hypotheses that have been proposed to explain these behaviours and erect a theoretical framework which applies to the evolution of both pre-hatch and post-hatch brood amalgamation, and which may apply to species other than those of the Anatidae. Finally, we show that the occurrence of brood amalgamation in North American waterfowl may be associated with low relative resource availability and K-type life-history traits. EADIE, J. McA., KEHOE, F. P., et NUDDS, T. D. 1988. Pre-hatch and post-hatch brood amalgamation in North American Anatidae: a review of hypotheses. Can. J. Zool. 66 : 1709 -172 1. Deux formes de fusion de couvCes se produisent frCquemment chez plusieurs espkces d'oiseaux de rivage nord-amCricains : ( i ) fusion des couvCes avant 1'Cclosion : certaines femelles pondent leurs oeufs dans le nid d'une autre femelle et la femelle rCcipiendaire donne alors tous les soins aux oeufs et aux oisillons qui en rCsultent et (ii) fusion des couvCes aprks 1'Cclosion :certaines femelles abandonnent leurs oisillons a une autre femelle ou les perdent aprks 1'Cclosion et la femelle rkcipiendaire assure alors les soins aux petits qu'elle adopte. Certains auteurs considkrent ces comportements comme accidentels ou aberrants et de peu d'importance Cvolutive. Plus rCcemment, d'autres hypothkses ont Ct C proposCes, mais peu ont Ct C retenues ou CprouvCes en nature, en grande partie parce qu'elles ne sont reliCes a aucun cadre thCorique appropriC. On trouvera ici une analyse de la frCquence de fusion des couvCes chez les Anatidae nord-amkricains, de meme qu'une rCvision des hypothkses proposCes pour expliquer ces comportements; cette rCvision nous permet d'Ctablir un cadre thCorique qui peut s'appliquer a 1'Cvolution de la fusion prC-Cclosion et de la fusion post-Cclosion des couvCes et qui peut convenir aussi a des espkces autres que les Anatidae. I1 est possible que la fusion des couvCes chez la sauvagine nord-amCricaine soit associCe a une disponibilitk relative faible des ressources et a des caractkristiques d'une stratCgie dkmographique de type K.[Traduit par la revue]

Summary 1.Geographic gradients in population dynamics may occur because of spatial variation in resources that affect the deterministic components of the dynamics (i.e. carrying capacity, the specific growth rate at small densities or the strength of density regulation) or because of spatial variation in the effects of environmental stochasticity. To evaluate these, we used a hierarchical Bayesian approach to estimate parameters characterizing deterministic components and stochastic influences on population dynamics of eight species of ducks (mallard, northern pintail, blue-winged teal, gadwall, northern shoveler, American wigeon, canvasback and redhead ( Anas platyrhynchos , A. acuta , A. discors , A. strepera , A. clypeata , A. americana , Aythya valisineria and Ay. americana , respectively) breeding in the North American prairies, and then tested whether these parameters varied latitudinally. 2. We also examined the influence of temporal variation in the availability of wetlands, spring temperature and winter precipitation on population dynamics to determine whether geographical gradients in population dynamics were related to large-scale variation in environmental effects. Population variability, as measured by the variance of the population fluctuations around the carrying capacity K , decreased with latitude for all species except canvasback. This decrease in population variability was caused by a combination of latitudinal gradients in the strength of density dependence, carrying capacity and process variance, for which details varied by species. 3. The effects of environmental covariates on population dynamics also varied latitudinally, particularly for mallard, northern pintail and northern shoveler. However, the proportion of the process variance explained by environmental covariates, with the exception of mallard, tended to be small. 4. Thus, geographical gradients in population dynamics of prairie ducks resulted from latitudinal gradients in both deterministic and stochastic components, and were likely influenced by spatial differences in the distribution of wetland types and shapes, agricultural practices and dispersal processes. 5. These results suggest that future management of these species could be improved by implementing harvest models that account explicitly for spatial variation in density effects and environmental stochasticity on population abundance.

Recent declines in the number of breeding ducks in the Canadian prairie‐parklands have been hypothesized to be due to loss of habitat to agriculture. However, prairie‐parkland also has experienced wetland loss to drought as well as to agriculture. If habitat restoration is to be implemented and monitored successfully, it is important to separate the effects of anthropogenic changes to the landscape on duck populations from those caused by changes in climate. We used data from annual air–ground surveys and from precipitation records to develop relationships between indices of abundance of each of 10 species of ducks and indices of wetland conditions during 1955‐1974. We used these relationships to predict annual abundance of each species during 1975‐1989. We compared predicted and observed abundances over the period 1975‐1989 to distinguish declines in duck abundance greater than those accounted for by drought alone and to determine the magnitude and location of real "deficits" in duck abundance. Average annual deficits within Canadian prairie‐parkland over the period 1975‐1989 were estimated at 1.2 x 106 birds for both Mallard (Anas platyrhynchos) and Northern Pintail (A. acuta), 480 000 for Blue‐winged Teal (A. discors), 190 000 for American Wigeon (A. americana), 175 000 for Northern Shoveler (A. clypeata), 50 000 for Gadwall (A. strepera), 10 000 for Green‐winged Teal (A. crecca), 40 000 for Canvasback (Aythya valisineria), 25 000 for Lesser Scaup (A. affinis), and 5000 for Redhead (A. americana). Overall, the effect of agricultural expansion in the east on prime waterfowl habitat since 1951 appears to have been negligible. There, as much as 90% had been already lost prior to 1951. In the west, however, where prime waterfowl habitat was still relatively abundant in 1951, agricultural development has encroached substantially. The relationship between the lost area of the best breeding habitats and the size of population deficits for Mallards and Northern Pintails in the entire Canadian prairie‐parkland region was significant for both species (P < 0.0027 and P < 0.0001, respectively). Consequently, habitat restoration programs located where the highest quality waterfowl habitat and the lowest quality agricultural lands overlap most should have the greatest potential to affect recovery of breeding duck populations in the Canadian prairie‐parklands.

Diets of dabbling ducks (Anas: Anatidae) usually have been recorded only with respect to taxonomic composition and not prey size. Interspecific diet overlap thus has been termed large. However, using published taxonomic diet lists for seven species of ducks, measurements of bill morphology from museum specimens, and "handbook" data on the sizes and caloric density of invertebrates, prey size distributions were found to differ among species (P < 0.001). Also, the sizes of prey in each species' diet differed from that found in the environment. The number of lamellae per centimetre of bill negatively correlated with mean prey size (P < 0.001); body size and bill length did not. Based on the similarity of environmental and dietary prey size distributions, species were classified as specialist or generalist foragers. This classification corresponded well with one generated from analyses of foraging behaviour alone. An explanation for the diminuitive sizes of ducks on islands was advanced: if interspecific competition for food selects for niche divergence among coexisting dabbling ducks along a prey size niche dimension, then in the absence of competition from small species on islands, large solitary species have evolved to a smaller size which may be optimal for the distribution of energy among prey size classes in the environment.

Annual censuses of duck populations from two localities differing in environmental variability were used to test the effectiveness of six tests of density dependence. The magnitude of Type I error for each test was estimated from simulated density—independent data having the same mean and variance as the observed censuses. The actual census data were used to evaluate the ability to detect density dependence. Only autoregression had the desired level of type I error, but this test was ineffective when populations grew or declined. The standard major axis was effective in the latter cases but was ineffective when populations did not grow or decline. The only effective test applicable to all cases was a regression test with a rejection region based oin simulated data. Results of these latter analyses agree with earlier descriptions of interspecific variation in life—history tactics among duck species and the importance of competition as a factor affecting guild structure in prairie—nesting ducks. Diving ducks, which occupy the most temporally stable wetlands, show a greater degree of density dependence than do dabbling ducks, and competition influences the guild structure of diving ducks more than that of dabbling ducks.