摘要:

SummaryRaptorial birds which depend on a small number of prey species, especially those living in open habitats in the tundra, semi‐desert and desert belts of the Palaearctic, undergo local fluctuations in numbers which are synchronous with the fluctuations in numbers of their main prey species. In this they contrast with the asynchronous or lagging type of predator‐prey oscillation which has received wider attention in the literature.Evidence from ringing recoveries shows that the species which fluctuate locally in synchrony with their prey tend to move greater distances from their birth‐place or previous breeding place than those with more stable populations. The former species also tend to be split into fewer sub‐specifically distinct forms than the latter. It is argued that the species which are subject to a fluctuating food supply have evolved, as an important adaptation, the ability to undertake more or less extensive ‘searching migrations’, which enable them to find and settle in areas of adequate food supply, often far removed from the previous breeding area. In the longer evolutionary aspect, this ability may be a factor promoting food specialization.The synchronous population fluctuations of raptorial birds with their prey are compared with the asynchronous or lagging oscillations of carnivor

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00232.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

SummaryThe total population of Eiders at the Sands of Forvie National Nature Reserve, Aberdeenshire, increased from about 3000 birds during 1961–63 to about 4800 birds in 1964–70. At the same time the estimated number of breeding pairs increased from about 1200 in 1961–63, to about 1800 in 1965–68 and to 2000 in 1970.The mean size of completed clutches was 4.4; the number of eggs in a clutch showed a significant decline during the season from an average of 4.7 to 3.4 in most years. The overall hatching success was 63%, but there was a decline with season as a result of increased losses to predators.In 1964 and 1969 laying was retarded, mean clutch‐size reduced, and the number of birds attempting to lay dropped by half, due to inclement weather just at the onset of laying. In 1969 the eggs produced were smaller than in other years, and hatching success was lower.The female incubates without feeding, and is required to store sufficient energy, in the form of fat, to last her through the 26 days in addition to producing her clutch of eggs. An hypothesis is advanced which relates the feeding efficiency of the paired female immediately prior to laying to her egg production and incubating pattern, and which offers an explanation of the decline in clutch‐size and hatching success evident during the season.In most years less than 5% of hatchlings survived to fledge; high survival was recorded in 1963, 1968 and 1970. Following the high production in 1963 the population increased in 1964 and the breeding stock in 1965 (females start to breed at two years old). The corresponding changes, following good breeding in 1968, were not so clearly detected because of unusually high adult mortality from oil pollution at the winter grounds in 1968 and the ‘late’ breeding year in 1969. The breeding stock in 1970, however, was higher than in any other year. This population continues to grow apparently as a result of its own reproductive output checked only by low productivity in most years and occasional high

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00233.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

The significance of habitat selection during nesting of the EiderSomateria mollissima mollissima

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00234.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

SummaryThe changes brought about by the introduction ofEucalyptusinto Africa on the ecology of birds are outlined, in so far as its contribution as a habitat, as a food‐source, and for other uses is concerned. Species known to breed in gums are listed, together with notes on habitat preference, and its limitations in comparison with the indigenous resources are discusse

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00235.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

SummaryDuring the northern spring of 1970 and 1971, wader departures were tracked by radar from saltpans on the coast of Ghana near Accra. All but one of the departures headed northeast, and none were directed along the coast. There was a statistically significant difference between the mean heading of April departures (28°± 10°), and May and June departures (41°± 14°). Projected on great circles, all headings were directed towards the known breeding ranges of the wader species wintering at the saltpans.In ten species, the dates of the first returning birds to the saltpans in 1966, 1968 and 1970 were about three weeks earlier than dates recorded in Senegal. It is likely that those reaching Senegal follow a coastal route, whereas waders wintering at the coast of Ghana and eastwards to Nigeria evidently make a crossing of the Sahara both in autumn and s

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00236.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

SummaryMigration over Puerto Rico was recorded by time‐lapse filming of the display of a long‐range surveillance radar on 40 days and 37 nights in the period 2 March‐29 May 1971. Moderate density movements occurred every night; low density movements occurred on most days. Many birds, primarily passerines, took off from Puerto Rico each evening at 20–45 minutes after sunset.Almost all birds flew to the west, NW or north. Birds were seen approaching from the direction of the Windward Islands and Venezuela, over Puerto Rico, and departing towards the Bahamas and eastern coast of the U.S. Uni‐ and multivariate analyses showed that the number of birds departing W‐N each evening was positively correlated with following winds.There is less night‐to‐night variation in the amount of migration at Puerto Rico than in eastern North America. However, this is apparently the result of less variable weather in the tropics, not the result of any lesser degree of meteorological selectivity by the migrants.The tracks of the birds were correlated with wind direction. Birds moved WNW‐NW with NE side winds but NW‐NNW with SE following winds. The tracks were rarely exactly downwind. The variance amongst the directions of individual birds at any given time was usually small and not correlated with cloud cover or magnetic disturbances. The estimated headings of the birds varied from day to day in a pattern suggesting adjustment of headings to compensate at least partially for lateral wind drift.In autumn many birds approach Puerto Rico from the north or even east of north; in spring few birds moved in the opposite directions. This difference in routes takes advantage of preva

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00237.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

摘要:

SummaryEider migration in southern Scandinavia in spring 1972 was studied simultaneously at three radar stations and ten field observation sites. The Eiders winter in Danish and West German waters, from which they set out on spring migration on courses ranging from SE to NE; those from the northern part of the wintering area flew SE, those from the south NE, while birds wintering in between these geographical extremes set out on intermediate courses around E.Radar showed that most Eiders fly over the island of Zealand and the peninsula of Skåne, but only exceptionally further north over Sweden. Few Eiders crossing land were registered by field observers and most evidently passed at high altitudes beyond the range of vision. After having crossed Skåne, on varying tracks both south and north of east, the Eiders descended and changed flight direction towards NE.Some Eiders from the northern part of the wintering area circumnavigated Skåne, passing south along the Swedish coast of the Sound, crossing the very narrow peninsula of Falsterbo, and subsequently following the south coast of Skåne eastwards. Eiders from southern Denmark and Germany were often deflected eastwards along the south coast.After having passed Skåne no significant land crossings were made. Upon reaching the province of Blekinge on a course towards NE, the Eiders were deflected E to ESE along the coast and later NNE along the west coast of Öland through Kalmar sound.Approximately 350 000 ducks migrated during daytime of the study period, 250 000 crossing the land of Skåne while 100 000 passed along the south coast. Almost 300 000 migrated north in Kalmar sound, and 17 % of all Eiders passed east of Gland over the open sea.Three distinct peaks of activity during the day were noted in Skåne. The first, at around sunrise, originated from birds resting in waters off Skåne. The second occurred about four hours later and probably consisted of birds which had departed from the wintering area in the early morning. A final peak at sunset was often recorded. Migration also took place during the night, but involved only one fifth of the total number.From combined radar and field counts the total Baltic Eider population is estimated to have been 700 000–800 000 in spring 1972, indicating about 300 000 breeding pairs in the

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00238.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00239.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00240.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY

ISSN:0019-1019    

DOI:10.1111/j.1474-919X.1974.tb00241.x

出版商:Blackwell Publishing Ltd    

年代:1974

数据来源: WILEY