Mary F. Willson

Consumption choice by bears feeding on salmon

Consumption choice by brown (Ursus arctos) and black bears (U. americanus) feeding on salmon was recorded for over 20,000 bear-killed fish from 1994 to 1999 in Bristol Bay (sockeye salmon, Oncorhynchus nerka) and southeastern Alaska (pink, O. gorbuscha and chum salmon O. keta). These data revealed striking patterns of partial and selective consumption that varied with relative availability and attributes of the fish. As the availability of salmon decreased, bears consumed a larger proportion of each fish among both years and habitats. When availability was high (absolute number and density of salmon), bears consumed less biomass per captured fish, targeting energy-rich fish (those that had not spawned) or energy-rich body parts (eggs in females; brain in males). In contrast, individual fish were consumed to a much greater extent, regardless of sex or spawning status, in habitats or years of low salmon availability. The proportion of biomass consumed per fish was similar for males and females, when spawning status was statistically controlled, but bears targeted different body parts: the body flesh, brain and dorsal hump in males and the roe in females. Bears thus appeared to maximize energy intake by modifying the amount and body parts consumed, based on availability and attributes of spawning salmon.

Seed Selection in Some North American Finches

Food selection and feeding strategies have recently received considerable theoretical attention. The models of MacArthur and Pianka (1966), Emlen (1966, 1968) and Schoener (1969a, b) focus on the evolution of dietary specialists and generalists, and highlight the importance of the relationship between food preferences and harvest yield. One important measure (but not the only one, Rozin and Mayer 1961) of harvest yield is caloric intake: do predators (seed-eating birds in this case) prefer prey (seeds) of high caloric content, those providing the most calories per unit time, or does preferred prey have other characteristics? How does food selectivity change in response to food density, hunger levels, and metabolic stresses? Food preferences and degree of selectivity might also be expected to vary with the ability of an animal to handle different sizes of prey. In birds, it has frequently been shown, and is generally assumed, that larger-billed birds tend to take larger food items than their smaller-billed relatives. (Lack 1947; Morris 1955; Cade 1960; Hespenheide 1966; Myton and Ficken 1967; Newton 1967; and Holyoak 1.970) and may also take a wider range of food sizes (Snodgrass 1902; but see Bowman 1961; Newton, op. cit. ). The present study deals with seed selection of several species of finches in relation to bill size, seed-husking speed, seed size, caloric content, and rate of caloric intake under two temperature regimes in the laboratory. The data provide evidence relevant to the theoretical models and additional evidence regarding the closeness of the correlation between bill size and food-size preference.

Foraging ecology of avian frugivores and some consequences for seed dispersal in an Illinois woodlot

A 3-year study of interactions between frugivorous birds (11 species) and fleshyfruited plants with bird-dispersed seeds (eight species) documented the diffuseness of the mutualism between the taxa. We found considerable annual variation in degree of frugivory, principal fruits in the diet, and dietary diversity for most of the frugivores. There were no consistent correlations between fruit or seed size and gape width, body size, or diversity of consumers. Mutual dependency of bird species and plant species was very limited. The birds could usually obtain 1 to 2% of their metabolically effective body mass in fruit pulp/minute, but they did not concentrate their foraging on the fruits yielding the greatest intake rate. Subcanopy and understory foragers seldom changed foraging stratum and may constitute two guilds of seed dispersal agents, from the perspective of the plants. These guilds differed consistently in average body size, tendency to void seeds by regurgitation, occurrence in years of low fruit abundance, frequency of foraging on clumped fruiting displays, and in speed of movement away from a fruit source. However, three of the five subcanopy species have increased dramatically in abundance in the past 100 to 150 years, and so the dispersal regime for plant species whose fruits are eaten by these species may be different now from what it was. Most of the frugivores foraged preferentially in treefall gaps, and several species also shifted their foraging behavior toward increased fruit foraging in gaps. These observations reinforce other studies that have shown greater abundances of birds and greater rates of fruit removal in gaps. As a result of the preference for fruit foraging in gaps, bird-dispersed plants growing in gaps may achieve better seed dispersal than those in forest interior. Frugivores usually left a fruit source soon after feeding, so most seeds were carried some minimum distance away from the parent plant. However, the probability of departure varied with bird species and plant species, and bird species also differed in probable sites of seed deposition. The most efficacious dispersal agents were not necessarily the most common dispersers of any of the plants.

RELATIVE IMPORTANCE OF SALMON BODY SIZE AND ARRIVAL TIME AT BREEDING GROUNDS TO REPRODUCTIVE SUCCESS

The importance of body size to reproductive success has often been emphasized, but timing of arrival to the breeding grounds may also be important. To assess the relative roles of size and timing, we investigated the influence of life history and of behavioral and morphological traits on realized reproductive success in wild pink salmon, Oncorhynchus gorbuscha, during the 1997 and 1998 spawning seasons. Reproductive success was quantified by the number of adult offspring returning to spawn (1999 and 2000; parentage determined via DNA microsatellite genotyping). Males that arrived earlier, lived longer in the stream, and were more often observed in dominant courtship positions had significantly more offspring, although the importance of dominance was overshadowed by timing. Female reproductive success was not closely linked to any measured trait, but stabilizing selection on arrival timing and length is possible, as 1998 fish showing intermediate values of these traits were the most productive. Bear predation accounted for up to 50% of the mortality on the spawning grounds, yet did not have detectable effects on reproductive success. The absence of strong linear relationships between reproductive success and such traits as body size and fecundity may indicate that selection has driven the population to equilibrium values.

Magnitude and Fate of Salmon-Derived Nutrients and Energy in a Coastal Stream Ecosystem

ABSTRACT We quantified the energy and mineral (nitrogen, phosphorous) composition of live pink salmon (Oncorhynchus gorbuscha) and chum salmon (O. keta), their eggs, and carcasses, and tracked the fate of chum salmon spawning in a small Alaskan coastal stream. On average, salmon entered streams with 5.3 kJ·g-−1, 3.3% N, 0.48% P. Much of the energy in female salmon was stored in the gametes because the gonads were both large (20% of their wet body mass) and high in energy density (11 kJ/g). Carcasses following senescent death had lower mass-specific energy and N (but not P) compared to fish at stream entrance. Bears removed nearly 50% of the salmon-derived nutrients and energy from the stream by capturing salmon and dragging the carcasses from the stream. Much of the salmon biomass was made available to riparian scavengers because bears partially consumed the fish. Nutrients bound in salmon tissue at senescent death were quickly exported to the estuary after only a few days because of periodic high flows and low rates of scavenging by bears.

Seed Preferences and Digestive Efficiency of Cardinals and Song Sparrows

Laboratory experiments on seed preferences of several species of North American finches have indicated that a major determinant of seed preferences is ease of handling, and that the number of calories per seed, and the potential caloric intake per unit time from each seed type, are less important in determining seed preferences (Willson 1971). In the present study, we used seed gathered from wild plants rather than commercial seed. Metabolic tests permitted the estimate of actual rather than potential caloric intake from each seed type.

Foraging Behavior of Some Winter Birds of Deciduous Woods

popular exercise, yet all too often work ceases with a simple list of differences between more or less similar, coexisting species. More general treatments are relatively rare (but see, e.g., Cody 1968) and yet are essential if we are to understand the basis of community composition and organization and if we are to relate field and laboratory information to the more theoretical discussions presented by Hutchinson and MacArthur (1959), Schoener (1965), Selander (1966), and others. Of special import are comparative studies of different communities (a variety of habitats, taxonomic groupings, trophic levels, etc.) with respect to the magnitudes and kinds of differences among distinct phenotypic categories, be they species, sexes, or whatever, and to the variation within each phenotypic category. With quantities of these kinds of data in hand, it may then be possible to approach with empirical models the question of the regulation of species (and phenotypic) diversity. The present paper is the first of a projected series dealing with problems of comparative niche segregation and variations in avian species diversity. Since the paper deals only with one woodland, comparative aspects are limited to a consideration of intersexual differences in foraging behavior.

Body size, arrival date, and reproductive success of pink salmon, Oncorhynchus gorbuscha

The influences of adult body size on breeding opportunity and progeny survival are widely recognized in many animals but the timing of reproduction may also be very important. Specifically, the benefits of large size may be offset by selective mortality, and dominance may be related to competition and opportunity; all of these factors may be related to date as well as size. The relative influences of size and breeding date on reproductive success were investigated in a naturally reproducing population of pink salmon (Oncorhynchus gorbuscha), an anadromous, semelparous fish species, in southeast Alaska during the 1997– 2000 breeding seasons. Females showed strong positive relationships between body size and both fecundity and egg size, with a trade off between these traits. Duration of nest defense was associated with entry date (early arrivals lived longer) and bear predation, which also affected the proportion of females completing egg deposition. In males, access to reproductively active females was positively influenced by body size but arrival date favored large males early in the season and small males late in the season. The reproductive life span of males was reduced by bear predation but not influenced by body size or arrival timing. Taken together, the results indicated that both body size and arrival date affect potential reproductive success but complex interactions between these factors and predation are also very important.

NESTING SUCCESS OF FOREST BIRDS IN SOUTHEAST ALASKA AND ADJACENT CANADA

Abstract Predation caused 78% of nest failures in coastal forests of southeast Alaska and interior forests of adjacent Canada. Nest success tended to be better in coastal than interior forests. Mayfield daily nest survival from predation on open-cup nests was higher in egg than nestling phase for most species. Species building large (thrush-sized) nests had lower Mayfield daily survival from predation than species building smaller (warbler-sized) nests, but there was no difference in daily survival (total and from predation only) among species nesting in different vegetation strata. Nesting success differed little with nest cover or nest site diversity for most species. Total nest success within species was only sometimes higher in commonly used nest sites than in less frequently used sites. Nest survival from predation did not generally decrease with increasing nest density within guilds of species with similar nests or with nest-site similarity. We emphasize the likelihood of varied outcomes of natural selection on nest-site selection in differing circumstances.

Heritability of Life History and Morphological Traits in a Wild Pink Salmon Population Assessed by DNA Parentage Analysis

Abstract Heritability estimates were calculated for body size and shape, day of entry onto the spawning grounds, and egg size for two brood years (1997 and 1998) of a wild population of pink salmon Oncorhynchus gorbuscha based on parentage analysis of DNA samples from breeding adults and their mature offspring in a small creek in Southeast Alaska. The majority of heritability estimates for size and shape were not significantly different from zero, probably as a result of the close association of these variables with fitness. The even- and odd-year brood lines are genetically isolated but heritability estimates from the two lines were significantly correlated, suggesting similar selection histories. Consistent with heritability estimates from controlled breeding and captive rearing of pink and other species of salmon, day of entry onto the spawning grounds had the highest heritabilities of the traits examined in this study, especially for males (h 2 = 0.6– 1.38). One plausible method for the maintenance of...