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Correlations between weather and migrationMartin D. Williams
Hemmingsen (1951) notes that, in north China, weather variables such as temperature and air pressure show periodic fluctuations, with typical intervals between highs and lows of the variables of five to seven days. The numbers of migrants seen at Beidaihe are strongly dependent on these fluctuations in weather, and some birds, e.g. cranes, have ap-parently evolved migration strategies which involve them waiting for optimum migration conditions before undertaking the stage of their journey which passes Beidaihe.
These optimum conditions typically include winds with a northerly component, especially after a cold front has passed east over the area, and may lead to spectacular ‘waves’ of visible migration.
Thousands, or tens of thousands, of birds may also occur during ‘falls’—arrivals of grounded migrants, typically with very little visible migration. These falls apparently mainly coincide with falling air pressure.
The periodic changes in weather thus serve to stimulate migration, and migration waves and influxes are fairly frequent, though may differ strongly in the numbers of birds in-volved. However, stable, anticylonic weather can result in little evident migration, even though skies are normally clear (i.e. birds would not experience navigation problems). It may be that this is partly because winds are light and variable, and birds prefer to wait for a wind with a more consistent northerly com-ponent; also, the typically warm weather during these conditions will help ensure that food is still available.
Weather stimulating migration waves As mentioned, waves of visible migration were sometimes noted after the passage of cold fronts—i.e. in classic autumn migration weather (Elkins 1983). Typically, it becomes increasingly hazy prior to the arrival of a front, it may be warm, and there is little mi-gration (there may be good numbers of grounded birds; see below). The arrival of the front is marked by the sky becoming overcast and, as the front passes over, heavy rain or snow may fall, and the wind swings towards west to north, and becomes fresh or strong. The rain or snow eventually eases, and the visibility becomes good or very good, and the sky eventually clears (sometimes, the wind may not freshen until the cloud has passed to the east).
An excellent example of a cold front which stimulated a migration wave passed Beidaihe during 14th and 15 October 1989. The front arrived late on 14th, and it was raining, with a fresh north wind, in the early morning of 15th. Observations from the Jin Shan Hotel, eastern Beidaihe, produced over 200 unidenti-fied ducks heading south over the sea, but few other passing migrants while it was raining. However, as soon as the rain eased at around 09h00, Grey Heron migration was evident; 1417 were recorded by observers at the east coast of Beidaihe from this time to 11h30. Two White Spoonbills Platalea leucorodia and 135 Great Cormorants Phalacrocorax carbo were also of note during the morning. There was something of a lull around midday, but with the sky clearing and the wind from the north (cold), the afternoon produced most of the day’s totals of 21 Black Storks Cico-nia nigra, 26 Eurasian Sparrowhawks Accipiter nisus, 26 Northern Goshawks A. gentilis, 427 Common Buzzards Buteo buteo, 1167 Daurian Jackdaws Corvus dauuricus and 2693 Rooks Corvus frugilegus or Car-rion Crows C. corone.
The following day, the wind remained north-erly, fresh, in the early morning (became light by 08h00, and westerly from mid-morning onwards), the sky was clear, and the wave continued, with 52 Eurasian Sparrowhawks, 41 Northern Goshawks, 740 Common Buz-zards, three Saker Falcons Falco cherrug, 33 Great Bustards and 2303 Rooks or Carrion Crows.
Another cold front arrived on 7/8 November 1990. It cleared Beidaihe by dawn on 9 November; during the day, the sky was clear, the visibility excellent, and the wind moderate north-northeast in the early morn-ing, becoming northwest by mid-morning, and westerly by mid-afternoon. The day was especially notable for birds of prey, with 13 species including three White-tailed Eagles, Haliaeetus albicilla, six Eurasian Black Vul-tures Aegypius monachus (in a party, flying north), 190 Upland Buzzards, four Rough-legged Buzzards Buteo lagopus, one Greater Spotted Eagle, one Steppe Eagle Aquila ni-palensis and one Imperial Eagle Aquila heliaca. There were also 135 Oriental White Storks, four Black Storks Ciconia nigra, 491 Common Cranes (441 were seen from 15h00-17h00), ten Red-crowned Cranes and 14 Great Bustards.
The next day, the sky was again clear, and the wind north-northeast at first. But the wind soon became light, and variable after 10h00, becoming southerly by midday, by which time only rather low numbers of migrants had been seen; the only cranes were 20 Commons and five which were unidentified.
But, soon after 28 Commons flew north, a flock of 85 Common Cranes flying south at 12h40 marked the start of the best crane pas-sage in recent years (with birds seen all flying south). The day’s crane totals were 2728 Common 328 Hooded, 135 Red-crowned, and 111 Siberian Cranes, six White-naped Cranes Grus vipio and 396 unidentified cranes.
A third example of a cold front stimulating migration is one which passed Beidaihe over 13th and 14 November 1986, clearing the area by the morning of 14th. Though rather late in the autumn, 14 November produced a fair mi-gration wave, with totals including 277 Oriental White Storks, 43 Upland Buzzards Buteo hemilasius, three White-tailed Eagles Haliaeetus albicilla, 111 Common and 61 Red-crowned Cranes, 31 Great Bustards, 344 Daurian Jackdaws and 1042 Rooks or Car-rion Crows.
Hemmingsen’s records indicate that he similarly observed migration waves after cold fronts had passed, e.g. ‘the big goose climax in the autumn of 1943 [ca. 2400 geese; also ca. 1000 cranes] came after a strong East storm which brought with it the temperature fall, but on the day of the maximum migration the weather was calm and thus appeared mild.’ (Hemmingsen 1951).
Fast moving cold fronts as on 14/15 October 1989, 7-9 November 1990 and 13/14 November 1986 mainly occur in late autumn, from mid-October onwards (they may move from northwest China to the southeast of the country in only two or three days). They are not the only weather features to give rise to substantial migration wave con-ditions at Beidaihe, though it appears waves mostly occur as the air pressure rises, i.e. on the eastern flanks of high pressure cells, where winds have a northerly component (the opposite of the case in spring, when migration waves are associated with the western flanks of high pressure cells). An example is the wave on 12 September 1986, when totals in-cluded 2957 Pied Harriers, 152 Japanese Sparrowhawks and 915 Oriental Pratincoles. Though no active front was noted, weather conditions were as for the arrival of a high pressure cell: the air pressure rose from 11th to 12 September; the minimum temperature fell from 22°C over 10-11 September to 15°C over 11th-12th; visibility improved from less than 8 km with increasing haze on 11th to over 15 km on 12th; the sky was partly cloudy on 11th, clear on 12th. The wind on 12th was northwesterly, force 2, in the early morning and became westerly, force 2, for much of the day from mid morning onwards.
A wave similarly occurred with rising air pressure on 29 October 1986, when totals included 359 Oriental White Storks, 590 Bean Geese, two White-tailed Eagles, 412 Common, 35 Red-crowned, nine Hooded, two White-naped and 49 Siberian Cranes and 13 Great Bustards. Though the weather remained fine the next day, the wind was southwest from mid-morning onwards, becoming force four to five by early afternoon, and little mi-gration was evident, with totals of just three Oriental White Storks, three Bean Geese and 11 Common Cranes (no White-tailed Eagles or cranes other than Common). The weather synopsis for this day is much as for crane wave days in spring, so the low numbers of migrants presumably reflect the strong influ-ence of wind direction on crane migration.
Often on wave days, the sky is clear or partly cloudy and there is no rain. But water-fowl, especially, may pass in numbers during rain (they are better insulated, and so experi-ence less heat loss, than most land birds). The most notable example was a wave of of geese, ducks and waders on 29 October 1987, when totals included 2150 Bean Geese and 10,500 Northern Lapwings Vanellus vanellus. It was overcast, calm in the early morning; there was a fresh northeast wind from around 07h30 and rain began around 09h00, lasting until 15h30. Though some migration was noted in the morning, it was not until around 11h00 that birds began passing in numbers, with large flocks of geese and Northern Lapwings heading towards the southwest over the southern coast of Beidaihe (most were seen from the Diplomatic Personnel Guest House).
We have found that, as on 30 October 1986, days with southwest winds produce little visi-ble migration. This agrees with the hawk trappers who told Hemmingsen (1951) that few birds pass on days with southwest winds.
A hawk trapper we spoke to said northwest winds are best for raptor migration at Beidaihe, in agreement with our observations. Such winds will stimulate migration as they have a component in the migration direction; as noted above, they may produce the highest raptor numbers at Beidaihe as the west com-ponent drifts birds eastwards from a more westerly route. Our observations indicate that cranes, and perhaps Oriental White Storks, geese and other birds, prefer northeast winds—i.e. winds close to, or exactly in, the migration direction.
The weather which prompts visible migra-tion may result in large ‘clearouts’ of migrants present in the area, which can be all the more obvious as there may be falls as cold fronts approach (see below). A good example was 15 October 1989, when, although atten-tion largely focussed on visible migration, there were no records of Red-flanked Blu-etails (37 were logged on 14th), only one Black-browed Reed Warbler Acrocephalus bistrigiceps (26 on 14th) and no Dusky Warblers Phylloscopus fuscatus (13 on 14th). Several species had been fairly com-mon before this date, but were recorded in far smaller numbers afterwards, e.g. the Black-browed Reed Warbler, the Radde’s Warbler, the Red-throated Flycatcher and the Chestnut-flanked White-eye.
Weather leading to influxes Influxes of migrants appear to mainly coincide with fal-ling, or low, air pressure (low pressure tends to inhibit migration: Nisbet and Drury 1968), sometimes as a cold front approaches. Though it may not rain, or even become cloudy, when the pressure falls, the visibility falls as the air becomes increasingly hazy (humidity is inversely correlated with air pressure).
There was a substantial fall as the 15 October 1989 cold front approached. Numbers included 68 Red-flanked Bluetails, 35 Black-browed Reed, 45 Dusky and 18 Radde’s Warblers and 395 Pallas’s Leaf-Warblers Phylloscopus proregulus on 13 October. Other falls associated with low pressure in 1989 included 210 Red-flanked Bluetails and 285 Pallas’s Reed-Buntings Emberiza pallasi on 24 October and 80 Red-flanked Bluetails, 148 Dusky Thrushes Tur-dus naumanni eunomus, 302 Rustic Buntings Emberiza rustica and 290 Yellow-throated Buntings E. elegans on 27 October.
Hemmingsen (1951) apparently also re-corded falls of migrants as fronts approached: ‘Days marked down as special “migration days” were e.g. 11th September and again 5th October and both these occurred before a storm from E or NE.’
The largest documented fall at Beidaihe was observed by Wilder and Hubbard. Wilder (1924b) rather casually relates that ‘On Sep-tember 10 the Siberian blue chat [= Siberian Blue Robin] (Larvivore cyane [= Erithacus cyane]) was in the fields and on the grassy hillside among the small pines in thousands, and the brown flycatcher (Muscicapa lati-rostris) in almost equal numbers. The next morning the former but not the latter had all disappeared, and other forms had come in on the wings of a rainy northeaster.’ It may well be that this is another example of a fall as a cold front (the ‘rainy northeaster’) ap-proaches, and the passage of the front prompted the departure of the Siberian Blue Robins.
An influx of ducks followed the passage of the front: ‘Green wing teal [Common Teal Anas crecca], pintail [A. acuta] and other ducks were in great numbers on the 12th at Peitaiho.’ The largest duck influx of recent autumns coincided with the passage of a cold front. Sleet and snow fell as the front moved over during the morning of 27 October 1986, and in the afternoon ducks on the sea off Ea-gle Rock included 2140 Common Teal, 222 Gadwall Anas strepera, 25 Eurasian Wigeon A. penelope and 90 Northern Shoveler A. clypeata.
Species systematically treatedGeoff J. Carey, Daniel G. Duff, Martin D. Williams and Xu Weishu
Around 345 species were recorded during the surveys.
Summaries of the autumn records of La Touche and Hemmingsen are given, and augmented where appropriate by records of other observers—records cited from Wilder and Hubbard (1924) sometimes refer to He-bei province and Beijing, rather than specifically to Beidaihe—and by spring re-cords of La Touche and Hemmingsen. Cheng (1987) is also referred to; notes on status for a species cover its known range within China.
Note that survey periods and intensities dif-fered. The dates were: 1986—20 August to 20 November; 1987—18 August to 30 November; 1988—8 September to 18 November; 1989—14-29 September and 5-7 October (only selected records are in-cluded from these periods) and 8 October to 16 November; 1990—19 August to 22 October.
The 1987 data is from Palfery (unpubl.); 1988 data is mainly from Hornskov (1989), sometimes together with records from the Earthwatch survey from 8 October to 16 November (there was considerable overlap in records during this period, especially of birds recorded from the Lotus Hills); 1989 data from 14-29 September and 5-7 October is from Jørgensen (unpubl.); 1990 data to 22 October was supplied by J.H. Christensen (in litt.); other data is from logs kept by MDW and GJC.
Abbreviations used are as follows:
for species listed in Collar et al. (1993): (CR)—critically endangered; (EN)—endangered; (VU)—vulnerable; (CD)—conservation dependent; (NT)—near threatened.
for references: Ch—Cheng (1987); COE—Williams (1986); H—Hemmingsen and Guildal (1968); Ho—Hornskov (un-publ.); LT—La Touche (1920, 1921); Sh—Shaw (1936); WH—Wilder and Hub-bard (1924).
for localities at Beidaihe: LH—Lotus Hills; Re—Henghe reservoir; Se—seafront along the southern coast of Beidaihe (from Legation Point west to near the Lotus Hills); SF—the (Henghe) Sandflats, an estuarine area on the northern edge of town; YH—Yanghe estuary (ca. 5 km south of town). Please see next webpages, from: Species systematically treated
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