摘要:

The roles of maternal age and experience, on the one hand, and individual, year and random effects on the other, in influencing avian egg‐size and hatching success have been much debated but seldom studied comprehensively. We investigated these topics with Wandering AlbatrossesDiomedea exulansof known age (7–30 years) and experience (1–8 breeding attempts) over a 10‐year period. Older and more experienced birds laid larger eggs. After allowing for year and controlling for experience, significant age effects remained; after controlling for age, no detectable experience effects remained. However, age accounted for only 6% of the overall egg‐size variation. Egg‐size varied significantly between years and has increased over the last decade. Individuals laid eggs of consistent sizes; 55% of the random variation in egg‐weight was due to such effects. Egg‐ and hatchling‐weight were very closely linked; larger eggs also had higher hatching success. The latter was influenced significantly by age and experience but neither remained significant after controlling for the other. Year effects were also detectable.That there are significant effects of age, experience, year and individual on egg‐weight (and hatching success) is probably typical of seabirds generally, though with different balances between factors depending on species and situation; however, insufficient data exist to examine this critically. Our finding that age was a more important influence than breeding experience does not support recent suggestions that hatching success is mainly influenced by experience and that experience will have a greater effect on reproductive success in long‐lived species with high mate‐fidelity. However, Wandering Albatrosses may have acquired much relevant experience before

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03803.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The effects of changing buoyancy on the diving and feeding behaviour of Reed CormorantsPhalacrocorax africanusand DartersAnhinga melanogasterwas investigated at Lake Kariba, Zimbabwe. A qualitative model of the energetic constraints caused by buoyancy changes on the diving behaviour of these two birds is presented and the predictions from the model are tested. The buoyancy of both species declined exponentially at different rates with water depth. Reed Cormorants were neutrally buoyant at 5–6 m while Darters were neutrally buoyant at 2–4 m depth. Buoyancy changes affect underwater swimming speed, which for Reed Cormorants is twice as fast on the bottom than when commuting, and for the Darter is significantly slower when diving than at any other time. Cormorants feeding in water deeper then 6 m spent less time on the bottom and fed less successfully than those birds feeding in shallower water. This is because their bottom times were significantly reduced as a result of the energetic constraints caused by changes in their buoya

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03804.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Theory predicts that skewed progeny sex ratios should be relatively common in vertebrate populations. In most birds this has proved hard to substantiate due to the difficulties associated with identifying the sex of large samples of chicks. This study reports the success of a new molecular DNA technique in determining the sex of 601 newly‐hatched Lesser Black‐backed GullsLarus fuscusThere was no evidence of any adaptive sex ratio within broods. Male chicks were found to be disproportionately large and to grow at a faster rate than females. The overall sex ratio changed significantly from 0.484 (male/male + female) at hatching to 0.399 by fledging, probably due to male susceptibility to starvation. Mortality also increased significantly with hatching order, an effect often observed in species like the Lesser Black‐backed Gull where hatching is asynchronous. I discuss the possibility that hatching asynchrony may in fact be a strategy employed to prevent excessive skews developing in progeny sex ratio whenever variable differential mortality is likely.The results appear to vindicate Fisher's (1930) hypothesis which predicts the overproduction of the ‘cheaper’ sex. However, as the skewed sex ratio may be determined more by unpredictable environmental factors, such as food supply and weather conditions rather than parental strategy, this interpretation should be treated wit

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03805.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Although many age determination techniques have been developed for four of the five vertebrate classes, none has been developed to determine ages of adult birds. Here we report a technique, involving counting endosteal layers in the tibia, that has allowed us accurately to determine the age (in years) of 12 dead birds of species in four different families, FulmarFulmarus glacialis,ShagPhalacrocorax aristotelis,RedshankTringa totanusand Great SkuaCatharacta skua,all of which had been ringed as chicks and so were of known ages up to 16 years old, but provided inaccurate results for one species, theCommonGuillemotUria aalge.Since other bird species have been found to possess endosteal layers and no species has yet been found to lack such layers, this would appear to be a technique of general applicability which could permit invaluable new work in both pure and applied fields of ornithology.

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03806.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Twenty clutches were collected from nests of Fish Eagles Haliaeetusvociferat Lake Kariba, Zimbabwe, and a small dam nearby in 1989‐90. Unaltered DDT, and metabolites DDD and DDE, were found in every egg. Mean levels of σDDT(= DDT + DDD + DDE) generally varied from 14 to 49 mg/kg dry weight per clutch, but 113–223 mg/kg dry weight were found in clutches from the eastern end of the lake and the mouth of the Sengwa River. SDDT and DDE levels were significantly correlated with the Ratcliffe Index of eggshell thickness. Comparison with museum specimens showed that the Ratcliffe Index has declined by 11% since 1936‐41 due to a significant fall in shell weight. Eggshell thinning exceeded 20% at the eastern end of the lake.Aerial surveys in 1987 and 1990 found that hatching success along the southern lakeshore exceeded 72%, but chicks were seen in fewer than half the nests at the eastern end. However, the density of breeding pairs was greatest here.Residue levels have increased by about 8% since 1980, rising more steeply in areas recently sprayed for tsetse fly control and falling in others. The threat from DDT may now be receding as regional use has declined and will end, for tsetse fly control, by 1995. None was used for this purpose in 1991. Mercury levels in adult birds were very high and may pose a significant risk.The breeding population may be limited by availability of safe nest sites. Chicks are sometimes eaten by people. Settlement along the lakeshore is increasing and safe sites are becoming scarcer as dead trees in the lake collapse and large trees onshore are destroyed by ele

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03807.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The Seychelles KestrelFalco araeais an endemic species confined to several granitic islands in the Seychelles archipelago. Over the three years 1975‐77 a total of 227 nesting attempts were observed on the island of Mahe. Eggs were laid between August and October and only one brood was reared each year. Laying occurred consistently at a time of increasing food abundance and young were in the nest when food was at a maximum. Clutch‐size was invariably 2 or 3, incubation lasted 30 days, the nestling period was 38 days and the post‐nestling period about 14 weeks. In upland areas (above 200 m a.s.l.) the majority of pairs (69%) nested on cliffs and the remainder in trees. At lower elevations (below 200 m a.s.l.) most used coconut palms (46%), with smaller numbers in cliffs (28%), trees (13%) and buildings (13%). Only 19% of pairs nesting in coconut palms fledged young compared with 35% in buildings, 68% in trees and 76% in cliffs. These differences may be related to differences in predation pressure. Overall breeding success was significantly depressed in 1977 compared with 1975 and 1976. This coincided with food shortage, notably of green geckosPhelsumaspp., in

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03808.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Clutch‐size, nestling growth and predation rates on eggs and nestlings in two sympatric alaudids, the Crested LarkGalerida cristataand the Desert LarkAmmomanes deserti,were studied in the Negev desert of Israel. Logistic growth constants were similar for both species, and experimental brood reduction did not accelerate the growth rate of their nestlings. It is suggested that the predation rate on nests of both species has been sufficiently high to select for maximal growth rate of nestlings. Overall probability of predation was 0.85 on a Desert Lark nest and 0.76 on a Crested Lark nest. The smaller clutch‐size of the Desert Lark (3.7) may be an adaptation to a higher predation risk, as compared to that of the Crested Lark (4.6) which is more vigilant and nests in more concealed si

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03809.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

This paper examines how a returning migrant assesses the quality of an area as a breeding territory before the period of peak food demand and how effective the assessment is in terms of breeding success. Male WheatearsOenanthe oenanthereturn from Africa to choose territories in the Breckland of eastern England about March, females arriving shortly after males. The food supply was predictable: prey densities during the breeding period (egg‐laying to chick independence) were strongly correlated with prey densities at the same sites during the period of arrival and territory establishment. Prey densities were also related to vegetation structure, averaging highest on short turf. Male arrival date and territory size were not significantly related to prey density but were strongly related to vegetation structure, implying that birds used vegetation as an indirect clue to prey availability. Neither territory size nor nest spacing appeared to affect nest losses caused by predators. The major variations in number of young fledged (other than predation) were caused by the number of nestlings hatched and presence of a second brood. Both early arrival and an early first brood improved first‐brood success and were necessary for a second brood. Not all birds which arrived early bred early enough for a second brood. First‐brood hatching date was strongly negatively correlated with pre‐breeding prey availability but not significantly related to vegetation structure. Thus by using vegetation as a clue to habitat quality, some pairs suffered reduced breeding success. This result implies that birds may not be able adequately to assess prey density directly at the time of territory establishment. The critical period for food availability may not be the period of peak demand (nestling period), when food is relatively abundant, but is probably the pre‐breeding period when females must accumulate reserves for eggs and when the food supply is poor. Food supply during this period may determine the timing of breeding and the ability to rear a second brood, and may thus have a greater effect on breedin

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03810.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The Willow WarblerPhylloscopus trochilusis one of the few bird species that undergoes two primary moults a year, a post‐nuptial moult in the breeding area and a moult in the wintering area. Primary‐moult data for Willow Warblers from Finland, Sweden, Britain, the Netherlands, Belgium. Guinea‐Bissau, Uganda, Kenya, Malawi, Zambia, Zimbabwe, Botswana and South Africa are analysed. The parameters of primary moult (mean starting date, standard deviation of starting date, and duration) are estimated using the techniques of Underhill&Zucchini (T.988Ibis 130: 358–372) and Underhill, Zucchini&Summers (1990Ibis132: 118‐12 3). The scheduling of moult in relation to theother main components of the annual cycle, breeding and migration, is considered. The mean durations of post‐nuptial moult for P.t. trochilusand P.t. acredulaare 36.5 and 38.3 days, respectively; the start and termination of moult forP. t. trochilusare about 3.5 days later for each degree of latitude northwards, and the start and termination of moult for P.t. acredula,are about 10 days later than that of the most northerly populations of P. t.trochilusstudied. Females start their postnuptial moult about 10 days later than males. Southward migration commences as soon as post‐nuptial moult is complete. There is an increasing constraint on the timing of breeding and post‐nuptial moult events at higher latitudes, leading to overlap between them. The duration of pre‐nuptial moult is longer than that of post‐nuptial moult, and is completed shortly prior to

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03811.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1992.tb03812.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY