ISSN:0012-9658    

DOI:10.2307/1938258

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1938259

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1938260

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

The African estrildid finch Pyrenestes ostrinus exhibits a polymorphism in bill size that is not related to sex. This paper examines the feeding ecology, sessonal changes in abundance, and diet overlap of the two bill morphs on a study are in south—central Cameroon. The morphs fed on two major sedge species, hard and soft. The diets of the small— and large—billed morphs were influenced by their feeding efficiencies on the sedges and seed abundance. When both major sedges were abundant, diet overlap between morphs was high, but overlap declined during the major dry season when overall food abundance was low. During the dry season, large—billed morphs specialized on the hard—seeded sedge species while small—billed morphs broadened their diet to include other foods. This pattern of resource use is consistent with increased intraspecific compeition for food and similar to patterns of resource use between distinct species during food—lean times. A model for the evolution of the polymorphism is presented.

ISSN:0012-9658    

DOI:10.2307/1938261

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1938262

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Field experiments were designed to examine the effects of parental ability on clutch size, for the two sexes separately. To handicap parental ability we removed feathers from the wings and the tail of three tit species (Parus spp.) during the initial incubation period. We then removed their first clutches, and examined the effect of the handicap on the subsequent nesting. In Blue Tits (P. caeruleus), and Coal Tits (P. ater), handicapped females reduced the size of the repeat clutch more than control females. In Blue Tits handicapped females fed their young at a lower rate, lost more body mass during the nestling period, and at fledging time their broods were significantly smaller than those of controls. Their fledglings had a body mass, similar to the control fledglings. Handicapped female Great Tits (P. major) only reduced their clutch size a small amount; consequently, at fledging time, their body mass and those of their young were lower than for controls. The males of the three species of tits did not seem to increase parental investment when their mate was handicapped. When male Blue and Coal Tits were handicapped their mates did not reduce clutch size more than did those of the control group, despite a reduction in parental investment by handicapped males. However, breeding success was not significantly affected by handicapping males. We conclude that the clutch size determination of female tits is primarily based on their own ability to feed their young, and not on the quality of their mate. Male tits seem to have a lesser influence on breeding success than females, and males seem less willing or less able to increase their parental investment.

ISSN:0012-9658    

DOI:10.2307/1938263

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Migratory Dark—eyed Juncos in eastern North America migrate southward in autumn and tend to segregate in the winter range according to sex and age. North to south, the most abundant classes are young males, adults males, young females, and adult females. Because adults tend to dominate young at winter feeding sites, this distribution appears at first to conflict with the view that dominance interactions are responsible for differential avian migrations and that dominant individuals remain nearest the breeding range. However, if young juncos establish winter residency at earlier dates than adults, a prior residence effect might make them dominant. This hypothesis requires that young arrive at wintering sites earlier than adults and that residents of all classes arrive earlier than their transient counterparts. We tested these predictions by comparing first—capture dates of juncos at Bloomington, Indiana, which is north of the latitudianal midline of the junco's winter range. During 13 autumns, individuals were classed as local residents or an transients and were sexed and aged. Contrary to predictions were not caught earlier than transients. Further, adults were caught earlier, not later, than young among transiets and probably also among residents. Thus, settlement of the winter range does not proceed from north to south, and dominance established through prior residence cannot account for the concentration of 1st—yr males in the northern part of the winter range. The fact that some sex—age classes tend to winter south of others predicts that at a northern capture site the classes that migrate farthest should be commoner among transients than among residents. This expectation was fulfilled. In addition, the median capture dates of the sex—age classes were arranged approximately according to the north—to—south order of their distribution, indicating that classes with the farthest to travel passed through (transients) or settled (residents) earliest in autumn. We conclude that comparison of autumn migration schedules of transiet and resident passerine birds at a single location can yield considerable information about the dynamics of settlement of the entire winter range, including possible information about differences in destination of subsets of transients.

ISSN:0012-9658    

DOI:10.2307/1938264

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Seasonal variation in reproductive success and nestling size were studied in a population of Song Sparrows (Melospiza melodia). I tested whether seasonal variation in reproductive success was due to differences among parents or territories, or whether the variation was an effect of time felt by all individuals in the population. Clutch size declined in the population through the breeding season, and the decline was due to some classes of females both nesting later and laying smaller clutches. Late lying and smaller clutches of yearling females were a major contribution to seasonal decline in clutch size, although seasonal decline in clutch size was also observed within age classes of females. Hatching success was roughly 10% lower for second broods than first broods, but did not vary with date that the nest was intiated. Survival of offspring did not vary seasonally during the period that offspring were in the care of their parents. Although nestling size was greater later in the breeding season, the probability of independent offspring recruiting into the breding population was lower for young born later in the breeding season. The seasonal decline in recruitment of offspring was not due to differences in the quality of territories or parents.

ISSN:0012-9658    

DOI:10.2307/1938265

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

The foraging behavior of fruit—eating birds can have important effects on the fitness of plants whose seeds they disperse. An important factor influencing the pattern of fruit consumption by birds may be the amount of friut in the immediate vicinity of a plant. I experimentally examined the effects of neighborhood fruit density and fruit crop size on the rate, proportion, and temporal pattern of removal of northern arrowwood (Viburnum dentatum) fruits by birds in central New York. I manipulated V. dentatum fruit densities in 100 10 X 10 m plots, then monitored fruit removal by birds during 14 wk in the fall of 1985. Each plot had either high or low fruit density in a central 1 m diameter "crop" area, and either high or zero "neighboorhood" fruit density in the rest of the 100—m2plot. I counted marked fruit in the center of each plot weekly. Fruits in high density neighborhood plots were removed significantly faster than fruits in plots with zero density neighborhoods. A significantly lower proportion of marked friut was removed from plots with both low crop size and zero density negihborhoods. Forty—three percent of all removal of marked friut by birds took place during a 2—wk period early in the friuting season. Birds feeding in flocks were observed feeding in high density neighborhood plots almost exclusively, presumably causing the greater removal from these plots. Fruit abscission accelerated after week 9, and by week 14 no fruits were left on plants. White—tailed deer (Odocoileus virginianus) consumed 400 of the 750 marked infructescences during the 14 wk, but destroyed rather than dispersed seeds. These results indicate that the amount of fruit in the area surrounding a fruiting plant, as well as the size of the plant's own fruit crop, can influence fruit removal by seed dispersers. Thus, disperser—mediated plant fitness is affected by neighboring plants. In contrast to previous studies, I found that high fruit density in the area surrounding a plant enhanced friut removal rather than inhibited it.

ISSN:0012-9658    

DOI:10.2307/1938266

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Seeds of 55 species of plants were incorporated into the nests of 31 common bird species in the semiarid shrubland of the southern Karoo, South Africa. However, only three plant species were used abundantly by a variety of birds. Nest lining and structural materials included many viable seeds. Our findings suggest that birds facilitated directed dispersal of the creeper Galiurn tornentosurn to sites near trees, and that they moved shrub seeds away from conspecifics. We postulate that plant species with cottony seed coverings (e.g., Eriocephalus) or indehiscent fruits on woolly or branched peduncles, such as Galiurn tornentosurn, are adapted for directed dispersal by birds as nest material.

ISSN:0012-9658    

DOI:10.2307/1938267

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY