Ardea
Official journal of the Netherlands Ornithologists' Union

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Deelder C.L. & Tinbergen L. (1947) Waarnemingen over de vlieghoogte van trekkende Vinken, Fringilla coelebs L. en Spreeuwen, Sturnus vulgaris L. ARDEA 35 (1-2): 45-78
Observations of the flying-height of migrating Chaffinches (Fringilla coelebs L.) and Starlings (Sturnus vulgaris L.). During the autumns of 1941, 1942 and 1943 the flying-height of migrating Chaffinches and Starlings was measured. Most observations were made on a flat meadow, alongside the dunes, south of The Hague in the Netherlands, at a distance of ca. 700 m from the shore. Moreover, some data were collected in the treeless Zoetermeerpolder, some 12 km from the sea, east of The Hague. The height measurements were made with the aid of a binocular, provided wit a scale inside and a graduated angle outside (fig., 1). As the body-length of both species is known, it was possible to compute their distance from the observer from their virtual length in the visual field. 'This distance could be converted into height by application of the elevation angle. Height-observations which have been estimated with the naked eye are provided with an asterisk in ,the figures. When comparing two series of observations, a difference in the averages (= - ) was considered to be significant only if it was as great as or greater than 3 its standard deviation (d). d was calculated according to the ford = , in which x and y represent the standard errors of the averages (x and j) of each series. Each of these deviations again was calculated from x = . In this formula n represents the number of the observations and x any observation of the series. Observations. The following factors (of which nos. 1 and 2 are the most important) were found to exert an influence on the flying-height: 1) Wind force. Strong wind causes a lower flight than light wind. This finds a very distinct expression when the wind force, alters in the course of a morning. Both species descend when the wind increases and rise when the wind decreases: 2) The angle between migrating- and wind-direction. The greater this angle, the higher the, flight This is seen very clearly when, during any day of migration, the wind changes its direction. Those observations on the action of (1) and (2) that were made under comparable, circumstances are collected into the figures 2 5. These graphs only contain average values; individual observations and standard deviations are to be found in figs. 6 and 7. In figs. 2-5 the lower curve refers to SSW-WSW winds. As the birds are migrating to the SW at the observing place this curve is considered to represent headwind. The middle curves have been drawn for crosswind and the top triangles for backwind. 3) The intensity of daylight (?). Chaffinches are flying higher in the early morning than later on. We have occasion to suppose that this is caused by the low intensity of light, as the difference occurs as well on mornings when wind force does not increase. More observations about this factor are required. 4) Visibility. In misty weather; Starlings and Chaffinches fly lower than under the same conditions without fog. 5) Precipitation. In driving rain both species are flying lower than under normal circumstances. Drizzling-rain causes no effect. 6) Scenery. Chaffinches are flying much higher above a bare, treeless ground than above a wooded area. Starlings make no difference in the two types of landscape. Both species, however, often fly at great height, when they begin to cross a sea or lake. 7) Size of the troop. Starlings often fly higher in great troops than in small units. 8) Birds of prey. Low flying Starlings and Chaffinches immediately rise when a bird of prey appears. Above a wood, however, Chaffinches often fly into the trees, which seems not to be done by Starlings. Discussion. As already stated, wind is the most important factor governing the flying height. We tried. to explain the behaviour towards this factor in the following way: both species dislike too small a flying height during migration and prefer a zone of ca. 50200 m. At the same time, however, they try to make their speed with regard to the earth as great as possible, probably until a certain maximum is reached. As the force of the wind increases with height, birds can lower the influence of this factor by decreasing their flying-height. The way this is effectuated by Starlings and Chaffinches is evident in conditions of headwind and side-wind. (It should be borne in mind that in the latter case the birds compensate drifting off by directing their body-axe between the angle of the , wind and the migrating direction. Consequently crosswind diminishes their speed in regard to the earth.) Both at side wind and at crosswind the birds compensate an increase of wind-resistance by lowering their flight. Now it is evident that crosswind of a certain force decreases the flying-speed of the birds to a less extent than a headwind of the same force. Accordingly the birds fly lower under the latter conditions. In order to investigate this dependence the relation was plotted between flying-height and the decrease of flying-speed is caused by the wind. The latter factor was calculated in the following way. The speed of the birds in regard to the earth (ve) was determined by composing their speed ,in regard, to ,the air (va) and the velocity of the crosswind. The difference of ve and va was thought to represent the counteraction of the wind. As va is insufficiently known a whole set of values approaching to the normal value of ve was tried (ve as measured with light wind). This procedure, however, did not influence the end result. From ,the graphs of fig. 10 and fig. 11 it is clear, that both species, when experiencing an equal counteraction by the wind, fly lower with crosswind than with headwind. In other words: the counteraction of the wind is not the only factor which regulates their behaviour. Most likely wind-velocity itself has an influence too. When Starlings and Chaffinches are migrating with the wind behind them they are able to give way to both their desires, namely to get a high speed and to fly rather high. Some of our observations seem to indicate that in an increasing backwind the birds lower their flight. So we supposed that the birds are afraid of getting too high a speed. More observations, however, are required to decide this. The great dispersion of individual observations in one series (figs. 6 and 7) shows that the acceptable speed and height vary between rather broad limits. It is possible, moreover, that this dispersion is a consequence of the fact that the birds are not able to estimate these elements as exactly as our instruments. In order to explain the shape of the curves of figs. 25 it was tried to correlate the behaviour of the birds with some simple factors. Calculations showed, however, that the angular speed at which the landscape is moving under them is not held at a constant value. Neither are the birds flying with a constant speed in regard to the earth, nor are they seeking a constant value of wind-resistance. Until now, we can only conclude that these curves show how the average Starling and Chaffinch try to find the compromise between their two desires. In comparison to Chaffinches, Starlings seem to have a less pronounced dislike for a low flight and to attach a greater value to a high speed. This is in accordance with their straight flight, which allows them to fly lower than the Chaffinches do with their more undulating flight. Before us, Lorenz (1933a) treated the question of flying height. He found that great birds (crows, geese, gulls) migrate low with a wind ahead and high with a backwind. Lorenz supposed that these big birds fly high with the wind behind them on account of two motives: 1) It is known that they use their wings as planes, just like aeroplanes, and so are receiving constantly during flight an upward pressure which eliminates gravitation. This pressure dependents on the speed with regard to the air. Now aeronautical experience shows that in gusts of wind the velocity of the air more quickly increases than it decreases afterwards. In a backwind such a sudden increase may cause a temporary decrease of the speed of the birds in regard to the air (as a consequence of. their inertia) and a decrease of upward pressure as well. Consequently. they will suddenly fall for a moment. They therefore will prefer flying at a considerable altitude in order to be able to avoid dangerous consequences. 2) They are afraid, as a consequence of their great speed in regard to the earth, to fly into dangerous air-conditions depending on surface structures (e.g. fields of turbulence). Furthermore Lorenz supposed that big birds fly low in a foul wind: 3) in order to profit by upward air-currents caused by surface structures and 4) -to a less extent- in order to profit by the wind-decrease close to the earth. These interpretations hold only Partly for the behaviour of the Chaffinch and the Starling: 1) When flying with a back wind the risk of falling in gusts of wind is only very slight, as it is well known that these small species can permit themselves to eliminate the upward pressure on their planes for a considerable time without losing much height. They do so by folding their wings during each normal upward movement (cf: Zimmer 1943): and, moreover, during the pauses of action that form a constituent of normal flight. Consequently the diminution of upward pressure in a gust of wind, which will only last for a fraction of a second, cannot be a reason for flying at an altitude of 100-200 m. 2) It may be expected that dangerous air conditions caused by peculiarities of surface structure extend to higher levels when the wind force increases. Our observations, however, indicate that both Starling and Chaffinch lower their flight in an increasing backwind. So we cannot assume that this factor is important for these species. 3) Upward air-currents develop themselves above hilly or wooded terrains and are lacking above flat grounds .(thermal currents excluded). Nevertheless, when flying with headwind both species go lowest above the latter terrains. So profiting by upward currents cannot be of primary influence. Furthermore upward currents must extend to a higher level in strong winds than in weak ones. If the birds would seek these upward currents, they would go lower with a decreasing wind. With the Starling and the Chaffinch, however, the opposite is the case. Finally the effect of upward currents must be equal at headwind and at crosswind. Nevertheless Starlings and Chaffinches fly with crosswind at other levels than with headwind. 4) As already mentioned, in our opinion profiting by the wind-decrease close to the earth plays an important role in regulating the flying height of Starlings and Chaffinches. We can be brief about the other factors that regulate the height beside the wind. Because the birds fly low in misty weather, we may be sure that they want to have a good look at the earth. We do not know exactly, why the Chaffinches fly high in twilight. Perhaps they express in this way their distrust of an environment without distinct outlines or perhaps they are afraid of any bird of prey, which could approach without being perceived quickly enough. It is unknown to us why a driving rain has its effect, whereas the influence of birds of prey is comprehensible from the lack of space which the low flying birds experience close to the earth. For the problem or orientation it is of interest that the flying height influences the reaction on difference in. the structure of the landscape. Chaffinches in particular are very sensitive to those differences. They have a dislike for treeless polders, lakes and seas and avoid such environments by changing their migrating direction when reaching the border of these (Van Dobben & Makkink 1933, Daanje 1933, Tinbergen 1941a, Vleugel 1943). Now it has been stated that their reaction is weaker when flying high than when flying low. The same conclusion can be drawn from Kramers's observations (1931). Not only Chaffinches but also Starlings are less sensitive to unfavourable environments when they fly high. It is curious that the reverse of this role holds as well when passing over unfavourable environments Chaffinches rise to a higher level, see above. As the flying-height is regulated by the direction and the velocity of the wind, it is evident that the reaction of the two species on coastlines in the last resort is dependent on these two factors. This dependence may be of great importance for the course of the migration of Chaffinches along and above the North Sea, as Deelder will show in a separate paper. Finally the flying-height determines the distance which the birds are able to overlook during migration. Some data are given and in combination with detail-observations on the behaviour of migrating Chaffinches and Starlings near The Hague (Tinbergen 1941a) our figures show that both species already react on the presence of the North Sea when this has a virtual height of less than 50' in the visual field.


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