摘要:

SummaryThe results of an expedition to Endau, an isolated mountain in eastern Kenya which retains a cap of mist‐forest, are described. The avifauna of the forest was found to be remarkably poor despite apparently favourable conditions, and a number of forest species found in similar isolated residual hilltop forests were absent. Notably no representative of the genusTurduswas found. Possible explanations of these anomalies are discusse

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03678.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

摘要:

SummaryObservations were made with radar from the Shetland Isles in 1960. During the late autumn a regular broad‐front movement passed westwards over Shetland from Scandinavia about midnight. Its front evidently extended by dawn from the Outer Hebrides towards the Faeroe Islands. The movement consisted of fast‐moving bright radar responses.During the later hours of the night the responses from the overnight passage movement generally, though not invariably, almost totally disappeared. Their disappearance seems to have been due to the birds having descended below the radar beam.The radar display always filled up again with rather faint, evenly distributed, responses in a short period around dawn, over a wide area of the south Norwegian Sea. The evidence indicates that these responses were from the same species as those observed on overnight passage, and that the sudden reappearance of responses around dawn was due to a sudden gain of altitude by these birds.The responses then assumed directions of movement which were not randomly distributed around the compass. Continuation of flight southwestwards or westwards on the overnight heading was infrequent, even in southeasterly winds. Movement in directions between W. and N.N.W. was very rare after dawn. Off Shetland three main directions of movement were recorded, E.S.E.–S.E., N.–N.N.E. and S.–S.S.W.Ascents at dawn and re‐orientation behaviour have been established as taking place over both the northeastern Atlantic Ocean and the North Sea. Ascents with at least one direction of reorientation have been observed also off the Outer Hebrides, off eastern Scotland and north of East Anglia.Overnight passage was recorded from Shetland on 22 out of 48 nights and the dawn ascent was recorded on 33 mornings. Directions of movement after dawn were determined on 26 mornings, on only two of which were the directions random. On 15 of the 24 mornings when re‐orientation occurred the wind at 900 m. above Lerwick was not greater than 15 knots.In 1961 ascent, without re‐orientation, was recorded from Iceland Redwings and Wheatears off the Outer Hebrides by Lee (1963). The possibility that birds of Icelandic origin were responsible for any of the southeastward directions of movement observed around Shetland in 1960 could be excluded for a combination of reasons. The possibility that warblers, flycatchers, chats, or finches drifted to Fair Isle and Shetland could have been responsible for the dawn ascents and re‐orientation off Shetland was also excluded. The radar responses, the detailed analysis of the radar data, the correlation of radar observations with field observations on Fair Isle and North Rona and other circumstantial evidence have strongly suggested that thrushes of the genusTurdus(especiallyTurdus musicusand probablyT. merulaandT. pilaris) were responsible for the overnight passage movements, dawn ascents and re‐orientation behaviour after dawn. It is shown that Shetland falls within the regular migratory path of Scandinavian thrushes in the autumn.Neither dawn ascents nor re‐orientation have yet been observed in Scandinavian thrushes in spring, but dawn ascent by Icelandic Redwings has been recorded in the spring on one occasion.The occurrence of dawn ascents was a rather better indication of the occurrence of migratory passage than the actual detection of movement during the night. On 12 mornings out of 48 a dawn ascent occurred although no overnight passage had been detected. The frequency of arrival of thrushes off Shetland was less in both moderate and strong southeasterly winds than in light winds, and the density was only very slightly higher in moderately strong, compared with light, southeasterly winds.While the immediate stimulus for the dawn ascent is undoubtedly sight of the sea beneath them, its purpose remains uncertain. The fact that it occurs not at dawn but as much as 11/2 hours before sunrise in the later part of the migration season is also unexplained.Depending on the method of analysis re‐orientation took place around Shetland in 1960, E.S.E. on 11 out of 15 (or 16 out of 24) mornings, S.S.W. on 9 out of 15 (or 19 out of 24) mornings and N.N.E. on just over one‐third of the mornings. Re‐orientation in two or in three directions was nearly twice as common as re‐orientation in only a single direction. Marked re‐orientation by thrushes took place in the lightest of winds, of any direction. Re‐orientation does not occur only after more or less severe drifting in easterly winds.The directions of movement after dawn were not simply down‐wind or into‐wind. While two‐thirds of the E.–S.E. movements were more or less into‐wind, most of the N.–N.E. movements, about half of those S.–S.W. and one‐third of the E.–S.E. movements were across the wind. N.–N.N.E. re‐orientation occurred only in light southeasterly winds and when the density of birds was high. In northeasterly winds movements were mainly down‐wind S.–S.W. Unfortunately westerly winds were very infrequent in 1960.The directions of re‐orientation were more regular and exact in their occurrence than the observed variations in the aspect or the strength of the wind, but thedirectionof the wind may play some part in determining which direction is mainly adopted.Although only one direction of re‐orientation, S.S.E., appeared to have been observed in the southern North Sea it is suggested that some of the responses travelling S.S.W., apparently on a continuation of the overnight heading there, were in fact on a re‐oriented S.S.W. course as off Shetland and the Outer Hebrides.In northwestern Europe the southeastward re‐orientation in thrushes is evidently an adaptation enabling those of the population which find themselves out over the eastern Atlantic to regain the western seaboard of Europe. Comparison is made with the situation on the American Atlantic coast.Migratory birds generally continue flying on the same course after dawn as before it. Scandinavian thrushes are the only birds known to make marked changes in direction after sunrise, but it is not very likely that thrushes could goal‐navigate after dawn while incapable of doing so during the night. It is argued that simple re‐orientation is the basis of the behaviour observed. This behaviour appears to be a simple “escape” response, not directed towards a localized goal, and on average it tends to bri

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03679.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

摘要:

SummaryOccasional observations were made during 1960–62 mainly around Quito and Guayaquil, to supplement the works of Chapman (1926) and Marchant (1958). Three species are mentioned for the first time from western Ecuador and one from the Amazonian side; 34 North American migrants were seen. Data are provided for several tropical species ranging up to the Sierra. The existence of a large mixed heronry on the Guayas is also mentione

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03680.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

摘要:

SummaryThe Gannet lays only one egg, and replacements are common. In 1962 in Scotland the Gannet was shown to be capable of hatching two eggs and rearing two young of the same age. If one of the brood of two was younger it did not survive. In striking contrast,S. leucogaster and S. dactylatrahardly ever manage to rear two young even though they normally lay two eggs.The results of artificial “twinning” showed that hatching success was as high for two eggs as for one, though the incubation period was two days longer, and that fledging success was 83% against 94% (of eggs hatched). This gave the parents of twins a 76% greater output than singles. The twins grew almost as well as singles, though weighing slightly less than a single of the same age, and taking on average four days longer to fledge. These figures refer only to twins of approximately the same age.The twin growth curves are compared in detail with normal single‐chick curves and with twin‐Shag growth curves, The results are discussed in terms of a possible causal mechanism related to feeding behaviour, since, though twin Gannets grow equally as well as each other, and, in the later stages, as well as singles, they are always slightly behind due to a lag which occurred early in growth when the actual amount of food consumed was extremely small.The results are thus apparently at striking variance with the theory that the Gannet rears the maximum number of young that it can feed; but it is still necessary to know how well the twinned Gannet young survive to breeding, and whether the rearing of an extra chick imposes a real strain on the adults (signs of which were beginning to show by the time the young fledged) which in long‐lived birds might offset the reproductive advantage of producing twins. It is suggested that, particularly in sea‐birds, this problem is worth exploring more systematically than has yet been done. Catastrophic years are thought unlikely to weed out twins, since to offset the 76% reproductive advantage would require such years to occur improbably often.Finally, the results are discussed in terms of the Gannet's social behaviour, in particular its well‐developed aggression, which is concerned with site‐defence, and affects the Gannet's food gathering power by requiring constant site attendance by one or other parent throughout the nestling period. In this sense, aggression may partially act as a population control mechanism. The relevance of these results to Wynne‐Edward's theory of animal population control mechanisms is b

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03681.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

摘要:

SummaryThe population of Golden Eagles was assessed in four different areas of the Scottish Highlands. The average area per pair varied from 11,400 to 17,884 acres, excluding country that was not used by the eagles.Eagle food in these areas consists largely of Red Grouse, Ptarmigan, Mountain Hares and Rabbits, and also dead Red Deer and dead sheep.The numbers and biomass of the four main species of living prey were assessed by representative counts in three main types of habitat; and of dead deer and sheep partly from counts but mainly from published data on stocking densities, mortality and weights.The annual food requirement from a home range was estimated at 249 kg. of meat, calculated from data on the food consumption of eagles, allowing for known breeding success and for the presence of some immature eagles and unmated adults; and this was compared with the total food potential in the four Study Areas, allowance being made for the inedible portions of carcases.The average food potential in all areas is greatly in excess of the requirements. In the three western areas living prey is very scarce, but amounts of carrion are large and the eagles in these areas depend mainly on carrion. Large differences in food potential between areas do not correspond with differences in eagle density. Two examples are cited where a drastic reduction in food supply had no effect on eagle density, which remains remarkably constant in spite of seasonal and annual fluctuations in food supply.Golden Eagle nest‐sites are used over many years, and the home ranges are big enough to supply a more than adequate food supply at all times of year. Evidence is provided of territorial or home‐range defence, which generally takes the form of display flights but occasionally involves more overt aggression. The home‐range size is fixed so high that a critical food level is probably very rarely re

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03682.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

摘要:

SummaryComparison of radar and visible passage in Norfolk during autumn and spring is made. With following winds migrants fly high, well above visual range. With strong opposing winds migrants fly below the radar horizon. It is possible to detect high flying migrants by making observations lying supine, thereby adding useful data.

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03683.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

摘要:

SummaryField work by the author on petrels began in 1936 and finished in 1957, many of the results of which have already been published. The Mottled Petrel was not studied in detail, as were the other species concerned, and this paper brings together those observations which were made on the Mottled Petrel and compares them with the other species studied. In the main the timing of the breeding season and the breeding behaviour of the Mottled Petrel appear to be much the same as for the Sooty Shearwater.

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03684.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03685.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03686.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1964.tb03687.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY