摘要:

Summary.The anatomy and histology of the proventriculus in several species of petrels were examined and compared with the conditions in other birds.The proventriculus in petrels is comparatively very large and its mucosa is raised into longitudinal ridges; it thus has a much greater surface area than in other birds. The presence of lipoids other than triglycerides concentrated in large quantities, particularly in the outer parts of the epithelial cells of the proventricular glands of petrels, is demonstrated. Lipoids in the corresponding cells in other birds are minute in quantity.The probability that these lipoids represent the stomach oil, or its precursor, immediately before secretion, is discussed and compared with other theories about the origin of the oil. The observations provide strong evidence, but not absolute proof, that the oil originates in the cells of the proventricular glands.Suggestions about the possible function of petrel stomach oil are discussed. The oil may supplement the secretion of the preen gland; it may be an excreted by‐product of the metabolism of excessively fat foods; or it may play an important part in water metabolism, especially in the nestling. Without further work, none of these theories can be upheld or refuted, and it is possible that all are partly correct, for they are not mutually exclusive.Many petrels shoot stomach oil from the beak when they are disturbed. The possible origin of this habit from the escape reaction, found in many birds, of vomiting up the stomach contents when alarmed, is pointed ou

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02288.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Summary.Mourning Chats (Enanthe lugensin the Helwan area in Lower Egypt winter along the lower spurs of the mountains. Each bird occupies a separate territory from which it excludes other Mourning Chats, and wherein it sings.“Winter” song is most energetic in the months of July to October; at this season the main singing hours are in the early morning and early evening. In the period November to February song is less frequent, and is most often heard in the late afternoon.In the hot weather of the autumn the birds select perches in shade during the mid‐day hours.Evidence is advanced that there is a measure of interspecific territorial competition between Mourning Chats, Hooded Chats (Enanthe monachaand White‐tailed Chats (Enanthe leucopyga.The possible relevance of this interspecific territorial practice to the hypothesis of the “food value” of territory i

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02289.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02290.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02291.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02292.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Summary.1The thesis that birds lay clutches of eggs which will produce as many nestlings as they can, on the average, adequately nourish, is considered in relation to the Central American avifauna.2Birds whose usual rate of bringing food to the nest is slow can greatly augment this rate if, after an exceptionally long period of neglect, they find their nestlings unusually hungry (examples are given for two species of antbirds), or if an additional nestling is placed in the nest (an experiment with a tanager is described).3In four out of five cases, birds of four kinds in which the clutch consists almost invariably of two eggs succeeded in rearing three nestlings.4In numerous species of hummingbirds, manakins, cotingas, American flycatchers, Icteridae, etc., the male does not help to feed the nestlings; yet these birds nearly all rear broods of two, which in the same habitats is the prevalent size among species in which both parents attend the nest. If the latter were rearing as many young as they could properly provide for, we should expect the unmated females to lay clutches only half as big. Among flycatchers, when the female alone feeds the nestlings, their period in the nest is not significantly longer than at comparable nests of species in which both parents feed.5Also in nidifugous species, of which the chicks pick up their own food under parental guidance, clutches are far smaller in the Tropics than in the North.6On the basis of the time spent away from the nest by 18 incubating finches of 9 species, it is estimated that on the average 4 hours per day suffice each parent to find all the food it needs, preen, bathe, and perform other necessary activities. This would leave each parent, of the tropical species, about 8‐5 hours to devote to the feeding of the nestlings; both together would have 17 hours per day. Even allowing 5 bird‐hours for each nestling, the two parents together could attend 3; yet 7 of the 8 tropical species considered regularly lay sets of only2eggs.7Even when the climate appears to be favourable through much or all of the year, the breeding‐season of many tropical birds is short and the number of broods small. The fact that some individuals breed successfully at seasons when most members of the same species are resting from reproduction, points strongly to the conclusion that these birds do not attempt as many broods as they might successfully rear. Neither in the size of their broods, nor in the number per year, do tropical birds in general appear to rear as many offspring as their own powers and the environment would permit. Their rate of reproduction seems to be adjusted to their average annual mortality rather than pushed to the limits of their strength.8Two species of Tyrannidse,Myiozetetes similisandM. granadensis,are similar in appearance and habits and often build their nests in the same trees, but differ in their rate of reproduction. In the midst of their ranges, it does not appear that the more prolificM. similishas any advantage over its congener.9In a hypothetical species which with a clutch‐size of two keeps its range occupied at a high or optimum density, the fate of a three‐egg mutant is considered. It would be difficult for the more fertile genotype to displace the well‐established two‐egg strain, unless some catastrophe caused a severe reduction in the density of the population. Such catastrophes are rare in the humid Tropics.10The situation is radically different in a species expanding into a new area, or in one increasing rapidly in numbers after a great reduction in density of population. In these cases intraspecific competition is at a minimum, and the more prolific strains will tend to predominate. At high latitudes, recurrent drastic reductions in density as a result of great cold, famine, or disasters during migration, are followed by periods of free expansion; and under these circumstances it is likely that the rate of reproduction will be held to near the maximum which combined internal and external conditions allow. Lack's views appear to fit the facts in relation to the northern birds which he investigated, but seem to be in accord neither with observations nor theory when applied to the birds of humid tropical areas, where ecological catastrophes are at most r

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02293.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02294.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Summary.Some evidence is given to show that a small population ofElanus cæruleushas adapted itself to the desert environment of Masira Island by (a) substituting fish for rodents as staple diet, (b) nesting in caves in lava‐crags in default of suitable trees, and (c) having a much smaller bro

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02295.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02296.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1949.tb02297.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY