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Journal ArticleDOI

Breeding season, clutch and brood sizes in verreaux's eagle

01 Jun 1968-Ostrich (Taylor & Francis Group)-Vol. 39, Iss: 2, pp 139-145
About: This article is published in Ostrich.The article was published on 1968-06-01. It has received 15 citations till now. The article focuses on the topics: Brood.
Citations
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Journal ArticleDOI
01 Jun 2015-Ardea

746 citations

Book
01 Jan 1997
TL;DR: In this paper, a major work covering the breeding and non-breeding birds of the Southern African sub-region is presented, which sets new standards in its scope and in its methods, for setting a measured baseline against which to judge environmental trends across the great range of southern Africa.
Abstract: This is a major work covering the breeding and non-breeding birds of the Southern African sub-region. Published in two volumes, Volume One includes introductory chapters describing methodology and the 'avi'-geography of the region, with habitat photos, and coverage of the non-passerines, whilst Volume Two covers the passerines. Some 900 species are covered in total, including 200 vagrants, with detailed species accounts, maps and statistics for at least 500 species. Conservation issues are discussed for most species. '...sets new standards in its scope and in its methods...it will come to be valued ever more as years go by, for setting a measured baseline against which to judge environmental trends across the great range of southern Africa.' - Colin Bibby, "BirdLife International".

347 citations

Journal ArticleDOI
01 Jun 1978-Ostrich
TL;DR: Comparisons are made between Matopos and Tanganyika observations on Black Eagles and between observations on sibling aggression in the Lesser Spotted Eagle Aquila pomarina where the second hatched chick lives longer and does not die directly from attack.
Abstract: Gargett, V. 1978. Sibling aggression in the Black Eagle in the Matopos, Rhodesia. Ostrich 49:57-63. A two-egg clutch of the Black Eagle Aquila verreauxii was observed from the hatching of the first...

56 citations

Journal ArticleDOI
01 Dec 1977-Ostrich
TL;DR: There was no significant difference between the mean reproductive success of five African eagle species that lay two eggs and that of fiveAfrican eagle species laying one egg, even excluding inequalities due to sample size, and other factors.
Abstract: Summary Brown, L. H., Gargett, V. & Steyn, P. 1977. Breeding success in some African Eagles related to theories about sibling aggression and its effects. Ostrich 48:65-71. Previous explanations for fatal inter-sibling strife in eagles (lack of food, extra aggressiveness which enhances survival, and an expression of the innate aggressiveness or territoriality of raptors) can not be supported by recent evidence. The latest theory, that the second egg acts as a “reserve”, is examined. If so, eagles normally laying two eggs should have better reproductive success than eagles laying a single egg. However there was no significant difference between the mean reproductive success of five African eagle species that lay two eggs and that of five African eagle species laying one egg. Even excluding inequalities due to sample size, and other factors, the overall finding is the same. In Verreaux's Eagle Aquila verreauxi for which the largest number of records is available there was a significantly higher total loss in...

36 citations

References
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Book
01 Jan 1962
TL;DR: Wynne-Edwards has written this interesting and important book as a sequel to his earlier (1962) Animal Dispersion in Relation to Social Behaviour, and reviewing it has proven to be a valuable task for one who normally is only at the periphery of the group selection controversy.
Abstract: Wynne-Edwards has written this interesting and important book as a sequel to his earlier (1962) Animal Dispersion in Relation to Social Behaviour. Reviewing it has proven to be a valuable task for one who normally is only at the periphery of the group selection controversy. My comments will be organized into three sections: one regarding the factual content of the book, a second attempting to relate my own expectations and predictions based on soft selection with facts described by Wynne-Edwards, and a third criticizing the argument that has been advanced for group selection. A number of important studies have been summarized in this book. Foremost is the extensive work on red grouse with which Wynne-Edwards has been associated for more than 30years. A great deal of ecological, physiological, nutritional, and behavioural information regarding this bird has been reviewed in seven chapters (pp. 84-170) with additional comments liberally sprinkled elsewhere. Anyone interested in avian biology who has missed this important study must read this book. Also extensively reviewed is the work of Michael Wade on group selection in Tribolium, the flour beetle. Wade's data are presented in enough detail that his work can be understood by those who have not seen the original publications. [Twice (pp. 210 and 233) reference is made to a 40-fold difference between two of Wade's selected lines, whereas the figure on p. 209 suggests that the difference is nearer 7-fold. Figure 11-16 also contains an error.] A number of other studies have been reviewed in some, but not exhaustive, detail. Among these are Smithers and Terry's analysis of immunology in schistosomiasis, Birdsell's studies of the social structure of Australian aborigines and Sewell Wright's shifting balance theory of evolution. Numerous other observations are cited in the text, none in so great detail as those mentioned here, and many in such rapid succession as nearly to overwhelm the reader. Many of Wynne-Edwards' conclusions are based on densityand frequencydependent selection, territoriality and the migration of individuals between and within populations at various heirarchal levels (in-groups, demes, populations and higher categories). Still, there is no mention of authors such as Howard Levene or Wyatt Anderson. Consequently, I feel justified in presenting

1,842 citations

Book
01 Jan 1966

1,572 citations

Journal ArticleDOI
28 Jun 2008-Ibis
TL;DR: The breeding cycles and related biology of the White Boorby Sula dactylatra and the Brown Booby S. leucogaster at Ascension were investigated between November 1957 and April 1959 as discussed by the authors.
Abstract: SUMMARY The breeding cycles and related biology of the White Boorby Sula dactylatra and the Brown Booby S. leucogaster at Ascension were investigated between November 1957 and April 1959. In both species there were marked peaks of laying, with intervening periods of very few new clutches. In the White Booby, although only one full season was studied, an annual cycle was deduced; in the Brown Booby (two full seasons) the cycle appeared to be eight-monthly. The breeding season (laying of eggs to fledging of chicks) was however roughly the same length in both species, namely six to seven months. Individuals of both species for the most part conformed to the breeding seasons of the population as a whole, i.e. if out of phase for some reason, had a longer or shorter “rest” period as necessary to bring them into phase again for the next season. These discoveries indicate that external factors are controlling the times of breeding. The proximate factors are not known; at Ascension there is no appreciable seasonal variation in day-length or climate, and possible cycles in the surrounding sea could not be investigated. The two species' periodicity was such that, if maintained, every two years they would lay at almost the same time of year, July–August. The possible relation of these observations to a single underlying factor, perhaps annual variation in oceanic conditions with their origin in the seasonal melting of the Antarctic ice and consequent effect on the food cycles, is discussed. Details of incubation, desertion, feeding, care and growth of the chick are given. The regular death of the second chick as a result of asynchronous hatching, and experiments with twins are described. A low breeding-success, and deaths of chicks from starvation, indicated that both species suffered a shortage of food over an ill-defined period around August-September 1958. Regurgitations of food examined throughout the study, and a few observations on feeding, indicated a big overlap in the two species' diets, mainly flying-fish, but some differences did appear. Details of moult, mainly in the White Booby, are given. Shedding and re-growth of the juvenile primaries began at about seven months of age, proceeding from the innermost outwards in three spaced concurrent cycles, this remarkable pattern continuing throughout adult life. A period of pause in the primary moult was correlated with the time when most birds had chicks, but some birds were known to be moulting their primaries while breeding. A prolonged moult with a pause during breeding was indicated by examination of some other Ascension species; the relevance of this to the availability of food and breeding seasons in tropical sea-birds is discussed.

212 citations

Journal ArticleDOI
03 Apr 2008-Ibis
TL;DR: The observed timing of breeding seasons can be secured only by external factors regulating an internal rhythm, and in some communities and categories of birds the “reasons” for the observed breeding seasons are intelligible.
Abstract: Summary. 1 For the present purpose Africa is divided into everyreen, semi-arid and intermediate (deciduous) types of count?, all of which occur in all latitudes south of the Sahara. 2 “Breeding season” is limited to the months during which eggs are laid by the species concerned in the given area and all records are interpreted accordingly. As a basis for ascertaining the curve of breeding activity in a given area through the year the number of species laying in each month has been ascertained and calculated as a percentage of the total for all species for the whole year. 3 A definite peak in the curve of breeding activity is evident everywhere except in certain areas within about four degrees of the Equator. 4 In one part of this inner tropical belt there may be no distinct breeding season for must groups of birds (Congo), but in East Africa a double breeding season is the rule, with peaks coinciding with the two rainy seasons. 5 Even so close to the Equator as 5°s. the (single) breeding season in evergreen forest is as restricted as in other types of country and its time-relation to the rains varies locally. 6 In the “ intermediate” type of country characterized by 4–6 months drought each year the timing of the peak breeding season varies from the end of the rains, at Cape Town, to the start of the rains in Natal and several weeks before the rains in areas 23° - 10° S. 7 The key to this local difference is that at Cape Town the rains fall in the cold season, so that vegetation and insects are slow to flush. In the warmer conditions in which the rains begin in Natal the flush comes at once. And further north the dominant vegetation and its associated insects flush towards the end of the drought and well in advance of the rains. 8 From Natal northwards the breeding season for all birds combined shows a progressively less marked peak. The reason is that the seasons of certain ecological categories (1) water birds, (2) raptors and scavengers, (3) ground birds, (4) grass birds, (5) the other birds, tend to diverge. 9 The raptors and scavengers are everywhere the earliest breeders, the biggest species laying by the middle of the dry season. The water birds lay to a large extent towards the end of the rain, and after. The ground birds tend to lay as soon as the grass fires are over and before the heavy rains have induced a lush growth of herbage. The grass birds lay later than most-others, when the grass has grown high. 10 In semi-arid areas the breeding seasons are on the whole similar to the foregoing, with most birds breeding when the vegetation flushes, whether just before or after rain has fallen. But the “semi-and” birds are notably sensitive to rainfall; breeding that has begun is checked if the rains are interrupted. 11 In some communities and categories of birds the “reasons” for the observed breeding seasons are intelligible, the best food-supply or the safest nesting apparently being secured. In others the reasons are not obvious; and the degree to which the breeding seasons are restricted is often incomprehensible. 12 The observed timing of breeding seasons can be secured only by external factors regulating an internal rhythm. Day-length, rainfall and humidity, temperature and visual stimuli are each considered briefly. Each may be effective on some species in some areas, but no one generally.

123 citations

Journal ArticleDOI
01 Apr 1959-The Auk
TL;DR: I suggest that by "staggering" the ages of larger-brooded raptors, the task of sustaining a large and growing family of carnivorous young, instead of becoming progressively more difficult and perhaps ultimately impossible, is simplified, and to a great extent eased, by being spread over a prolonged period.
Abstract: For the majority of birds it is manifestly of considerable importance to synchronize, so far as is possible, the hatching date of their eggs. Indeed, this is essential if the entire brood is to leave the nest at approximately the same time, which, in most species, is necessary if they are all to survive. The approximate synchronization of the hatching date is, of course, normally effected by postponing full-time incubation until the clutch has been nearly, or wholly, completed; but it may be further assisted if the more recently laid eggs require a slightly shorter time to hatch than those deposited earlier (See Jourdain, in Witherby et al., 1: xxi; 1938). As a uniform hatching date is indubitably a desirable feature in the breeding biology of most birds, why is it that with the majority of raptors there is no synchronization in the hatching of the clutch? On the contrary with many of them, intervals of one, two, or more days commonly occur between the birth of each nestling. In large-brooded species, since incubation usually starts with the first egg laid, this may even result in differences of a fortnight or more between the ages of the first and last hatched nestling. What are the advantages gained by graduating the ages of their young in this way? I suggest, at any rate so far as the larger-brooded raptors are concerned, that the habit is beneficial for the following reasons. First, by "staggering" their ages, the task of sustaining a large and growing family of carnivorous young, instead of becoming progressively more difficult and perhaps ultimately impossible, is simplified, and to a great extent eased, by being spread over a prolonged period. And, secondly, as it is eugenically preferable to rear, let us say, one or two healthy well-nourished progeny rather than six or seven weaklings, this "staggering" is of the highest importance since it offers perhaps the only satisfactory way-namely by controlled cannibalism -of numerically reducing the family to a feedable size when this has become vitally necessary owing to a food shortage. As the food supply of some of the owls-and this is especially true of Asio flammeus-is often largely dependent on fluctuations in the population of certain rodents, a ready and rapid means of adjusting the size of the brood to the amount of food available is clearly desirable. Without a marked disparity in the age and size of the fledglings, fratricide would be virtually impossible and consequently no such

110 citations