bird migration || migration across the alps
TRANSCRIPT
Migration Across the Alps
B. BRUDERER and L. JENNI!
1 Environmental Conditions for Migratory Birds in the Alps
Climate and habitat in the Alps are governed by altitude. An altitudinal change of 200 m corresponds to a shift in temperature of 10 or to a N-S shift in the vegetational zones of about 200 km. Rich broad-leafed woods and wetlands are rare. A bird resting in the Alps has to endure low temperatures, accentuated by wind chill, and important changes in temperature. Frequency and amount of precipitation are increased at the windward side of the main Alpine chains. In the dry valleys between the chains limited surfaces of Mediterranean and steppe vegetation occur. For migrants on stop-over the chance of meeting harsh conditions and unsuitable habitats is increased compared with the lowlands. On the other hand, locally better conditions may be offered in the Alps by the delay in vegetation phenology and insect availability as well as by the spots of drier climate in some valleys. Some migrants preferring seeds of coniferous trees may find optimal conditions in the subalpine forests.
The weather conditions are also less predictable and less uniform in the Alps than in the lowlands. Geostrophic winds with limited directional deviation from the course of the mountain ridges are funnelled between the ridges, on the local scale and over the larger distance between the Alps and the Jura. Winds across the ridges are slowed down below the tops. The diurnal cycle of warming and cooling ofthe differently exposed slopes induces diurnally changing air currents along the slopes and valleys. Rising moist air leads to condensation and accumulation of clouds on the windward slopes, while on the downwind side clouds disappear. I n tense insola tion of rocky slopes may ca use the accum ula tion of high clouds and local thunderstorms. The progress offrontal systems is slowed by friction along the Alps; high cloud barriers and prolonged precipitation may prevent any crossing of the Alps and sometimes cause important losses of migrants trapped in the Alpine valleys. In anticyclonic conditions inversions, inducing haze or fog layers, may build up in the valleys and basins (Eichenberger 1966; G.A. Gensler pers. comm.). For a migrant bird on the wing the most important weather features are probably (1) the funnelling of the winds between the Alps and the Jura; (2) the frequent wind changes from one slope to another; and (3) more clouds and precipitation in the mountain area.
1 Schweizerische Vogelwarte. CH-6204 Sempach. Switzerland
E. Gwinner (Ed.) Bird Migration © Springer-Verlag Berlin Heidelberg 1990
Migration Across the Alps 61
2 Migratory Directions in Central Europe and Along the Alps
2.1 Questions and Data Base
The general directions of nocturnal migrants crossing central Europe during autumn aim mainly at the Iberian peninsula or southern France. For birds crossing the area covered by the radar stations in Fig. I, the directions leading to these (intermediate) goal areas are between 210 0 and 250 0
, centered around 230 0•
A smaller proportion of birds, especially of long-distance migrants, might cross the Mediterranean Sea E of Iberia, th us heading more S. SE directions will be very rare, as most of the migratory divides are E or N of the area considered. For nocturnal long-distance migrants crossing Switzerland, ringing recoveries show mainly directions around 220 0 (Baumgartner and Bruderer 1985); the recruiting directions of a wide variety of species can be found in Figs. 6 and 7. Thus, according to the ringing recoveries, the main bulk of night migration would approach the Swiss Alps from the NE and meet them at a small angle.
The questions are, whether the actual approach directions towards the Swiss lowlands and the Alps correspond to those expected, and whether the birds proceed in the same direction or adjust their course to cross or avoid the Alps.
The data and methods used were tracking radar observations during autumn 1987 and 1988 at several sites in southern Germany and Switzerland (B. Bruderer unpublished work) together with older data on nocturnal migration (Steidinger 1968; Bruderer 1975, 1977, 1978, 1981, 1982; Rusch and Bruderer 1981; Baumgartner and Bruderer 1985; Liechti and Bruderer 1986). We tried to separate topographical and meteorological factors by the comparison of simultaneous observations at different sites (Fig. 1). We observed the tracks (Figs. la and 3) in order to determine the resulting movements of the birds under the influence of winds. Our goal was to recognize the underlying behaviour of the birds by calcula ting the headings (F ig. 1 b) or by selecting birds flying in low wind speeds, where wind drift would be compensated (Fig. 2). We checked whether birds above the surrounding relief(i.e. > 1500 m above radar) behave differently from those at low levels « 1000 m above radar). The mean vectors and modes (if appropriate) of all the distributions (Table I) supported the visual comparison of the diagrams.
To interprete the directional distributions it is important to be aware that the numbers of birds tracked at the different altitudes are not proportional to the density of migration, because we tried to track sufficient birds at all altitudes for a statistical comparison. The quantification of migration is not done by tracking single birds, but by fixed-beam counting or systematic scanning of the sky (Bruderer 1971, 1980). In order to allow a rough quantitative judgement of the directional behaviour at different levels, we recall that (according to Bruderer 1971) on average 50% of nocturnal migration is below 700 m above the Swiss lowlands, and 90% is below 2000 m. As the passes and lower ridges of the Alps are about 1500 m higher than the lowlands, most birds have to climb to cross the Alps. In fine weather with following winds or above fog layers the main mass of migration may be found above 1000 m above ground level (AG L), while in bad
Obiervallon periods
1.8.- 30 .8 .
2 31.8 .- 22 .9 .
3 22 .9 .- 9 . 10 .
4 9 . 10.- 30 .10.
.......
N: Nuremberg
R: Regensburg
S: StuHgart
A: AugsburO
F: Flaach
P : Peyerne
100 lim
6°
6°
10° 12°
,l A
Track directions autumn 1987 A
10°
\ 2
~ N
.... : ...
Headings autumn 1987 B
Fig. 1. A Distributions of track directions and B headings in southern Germany and in the Swiss lowlands during four periods of autumn 1987. At N the exact location of the radar is indicated by a point between the distributions from the four observation periods. In the other cases the circle marks the radar site. The letters P and F indicate the nearest village: N, R, A. and S are larger towns in the area. For statistics see Table I
Migration Across the Alps
N: Insel Nuremberg 7"
M: Merenschwand
E: Elzel
L: Lauerz
B: Blumensleln
K: Kappelsn
P: Pay erne
H : Hahnenmooa
C: Col de 18 Crol.
A: Algie
Track directions
winds < Sm/s
63
Fig. 2. Distribution of track directions at two flight levels (cf. Fig. 3) in weak winds « 5 m/s). The distributions are expressed as percentages of flights towards each direction (smoothed by running means over three direction classes of 50 ; the end points of the single vectors are connected); E. Land B are based on 10 0 -classes without smoothing. It is supposed that under weak winds drift is completely compensated. at least below 1000 m AGL. For statistics see Table I
weather and/or opposing winds nocturnal migration above the lowest few hundred meters may be very weak (see also Bruderer and Jenni 1988). Thus. in Fig. 1 high flying birds are overrepresented. In Figs. 2 and 3 we separated two flight levels. emphasizing the directions below 1000 m AGL. These so-called low flying birds are about 70% of all migrants in Fig. 3 (all winds) and about 50% in Fig. 2 (weak winds); the fractions of high flying birds (above 1500 m AGL) may represent about 20 and 30%. respectively.
2.2 Approach Directions in Southern Germany
In weak winds « 5 m/s; Fig. 2 inset "N" and Table I) the directions of nocturnal migration in southern Germany are around 230 0 to 235 0 below 1000 m AGL. The migrants passing north of the Danube valley will usually not reach the Alps. but pass N of the Jura. At higher levels the directions (headings and tracks) are usually more S. giving evidence of a southward aiming part of migration. which will approach the Alps. Birds heading SE are rare. however. and mainly confined to period 1. Leading lines met at small angles. as the Suebic Alb near "S" or the Rhine valley near "F". induce minor deviations of about 100.
Tab
le 1
. C
hara
cter
isti
c di
rect
ions
at t
he d
iffe
rent
loca
tion
s. M
ean
vec
tors
in t
he f
irst
row
. max
ima
of d
istr
ibut
ion
(the
mod
e o
fth
e sm
ooth
ed d
istr
ibut
ions
0
,
.;:,.
acco
rdin
g to
Fig
. 2)
in
the
seco
nd r
ow o
f ea
ch s
ite.
(n =
n
um
ber
of
bird
s p
er d
istr
ibut
ion)
All
win
ds
All
win
ds
Win
ds <
5
m/s
Loc
atio
n P
erio
d A
ll h
eigh
ts
< 1
000
m
> 1
50
0m
<
100
0 m
>
15
00
m
Tra
ck
(n)
Hea
d
Tra
ck
(n)
Hea
d
Tra
ck
(n)
Hea
d
Tra
ck
(n)
Hea
d
Tra
ck
(n)
Hea
d
Nu
rem
ber
g
30.7
.-30
.10.
87
209
(896
9)
225
220
(463
0)
228
196
(228
7)
220
232
(132
7)
233
213
(795
) 21
6 23
5 22
5 24
5 22
5 22
5 23
5 23
5 24
0 22
5 21
5
30.7
.-31
.8.8
7 19
4 (2
983)
22
7 20
0 24
0
31.8
.-22
.9.8
7 19
2 (1
485)
23
1 ?
240
22.9
.-9.
10.8
7 22
8 (1
671)
22
1 24
2 (8
96)
221
209
(418
) 22
0 23
3 (1
56)
231
215
(153
) 21
7 24
0 21
5 25
0 21
0 23
5 24
0 25
5 ?
~
9.10
.-30
.10.
87
222
(286
4)
220
240
225
Aug
sbur
g 1.
8.-3
0.8.
87
189
(233
5)
219
210
(903
) 22
7 17
2 (8
22)
209
233
(366
) 23
0 20
5 (1
98)
216
? 24
0 24
0 24
0 17
0 25
5 23
5 24
5 23
0 22
5
Reg
ensb
urg
31.8
.-22
.9.8
7 18
0 (2
034)
22
9 19
5 (9
62)
233
175
(559
) 22
6 22
9 (3
20)
233
224
(97)
23
2 17
0 23
0 22
0 23
5 17
0 22
5 23
0 23
5 ~
?
Stu
ttga
rt
22.9
.-9.
10.8
7 22
7 (1
826)
22
5 24
1 (9
05)
226
220
(499
) 22
6 24
8 (2
01)
245
225
(274
) 22
8 24
0 23
5 25
0 24
0 23
5 23
5 25
0 24
5 23
5 22
5
Fla
ach
9.10
.-30
.10.
87
219
(205
3)
216
225
(100
8)
219
215
(554
) 21
5 22
2 (5
73)
218
215
(242
) 21
6 to
225
215
230
225
195
205
225
225
~
225
to .. '"
Pay
erne
31
.8.-
26.9
.87
222
(153
7)
234
Q.
(!>
,.., 24
0 24
0 (!
> .. '"
Pay
erne
23
.8.-
25.9
.86
228
(497
6)
230
236
(168
1)
235
2 I3
(1
876)
22
4 23
4 (8
94)
236
223
(547
) 22
7 ::>
Q
.
+ 1
.9.-
30.1
0.87
24
0 24
0 24
0 24
5 22
0 23
5 24
0 24
5 23
0 23
5 r '-
Mer
ensc
hwan
d 2.
8.-2
0.9.
82
224
(257
9)
223
227
(125
7)
225
220
(651
) 22
0 22
8 (9
64)
226
228
(431
) 22
6 (!
>
::>
240
230
235
230
250
235
235
230
250
225
2.
K:Jp~len
20.8
.-12
.9.8
8 22
4 (1
425)
23
2 2
"
(768
) 2J
8 24
0 m
24
0 l4
0 ft
ul
9.8.
12
.10.8
0 23
8 (4
626)
22
3 (2
432)
24
0 l5
0
Lauc
r,
9.8.
15
.10.
80
214
14
948)
23
2 (2
277)
24
0 0
Blu
men
stei
n 5.
9.-2
4.10
.8)
230
1332
4)
250
(214
3)
250
260
liah
ncn
moo
s 27
.7.-
11.9
.75
217
1317
2)
222
225
(174
1)
228
230
230
230
m
Co
l dc
Ia (
"l"Qi~
1.8.
-2).
9.8
8 22
9 (4
)13)
lJ
2
233
(271
1)
231
235
230
m
230
Mon
the\
' 10
.8.-
14.9
.88
226
(220
9)
lJ2
12
3 (6
7))
231
l35
23
0 21
5 '"
,... (l
45)
222
239
(422
) lI
S
230
245
197
(118
5)
2)7
(1
76
)
1I0
2<0
190
(150
2)
2JJ
(
(26)
0
230
III
11
(81
) 26
1 12
93)
25O
27
0
211
(720
) lI
S
227
(121
3)
lJO
23
0 2
)0
21S
(80)
) 23
1 2
))
/177
81
'"
l30
m
227
(104
6)
117
220
(560
) m
23
0 21
5
2<0
224
2<0
0 lJ4
25
0
lJ4
2<0
246
250
230
226
m
220
lJ2
22
9 lJ
O
, 22
9 m
11
5 m
(97)
22
5 lJ
O
(59)
(] (
5)
119
0)
(l0
))
22.
230
(222
) 23
2 0
(484
) 23
6 m
'" ... • g. , >
0 ~ 5- • > -. ~ ~
66 B. Bruderer and L. Jenni
7' 50 km
Location of radar
> 1500 m AGL
Fig.3. Distribution of track directions including all wind conditions. Same representation as Fig. 2
The influence of wind results in drift, compensatory reactions, differential migratory activity (pseudodrift) or selection of flight levels in order to avoid unfavourable winds. The fact that the headings in Fig. I b are much more uniform than the tracks in Fig. la indicates that there is more drift than compensation above southern Germany. Only relatively weak winds (up to 5 m/s) are fully compensated by birds flying lower than 1000 m AGL. At higher levels or with higher wind speeds only partial or no compensation takes place (F. Liechti unpublished work). The distributions of track directions in Fig. la differ more between different periods at the same site than at different sites during the same period (except "P", see below). A comparison of the wind situations (weak winds from all directions in period 3, rather strong westerly winds during the other periods) suggests that the influence of winds prevails over that of topography above the hilly landscape of southern Germany.
Under the frequent and often strong westerly and northwesterly winds most birds are drifted S towards the Alps (see track directions in Fig. la). The wind conditions at the moment of passage have a paramount effect on the flight paths of nocturnal migrants, inducing a large variation in directions from day to day. The proportion of SE heading birds (Fig. I b) is very small and only relevant in period I; it is small even at the easternmost point, at Regensburg. Thus, the tracks pointing Sand SE are in most cases caused by drifted birds and not by birds heading in these directions;pseudodrift of populations wintering to the SE is negligible in the area considered, at least after period 1. Pseudodrift towards S
Migration Across the Alps 67
may account for the limited fraction of migrants flying southward with southerly headings under all wind conditions.
2.3 Directions in the Area of the Alps
Birds at low levels and in weak winds (Fig. 2 and Table 1), approaching the Swiss lowlands from the area of the Danube valley and S of it, are able to proceed in the same directions unless deviated by high mountain ridges. Directions to the right of the main direction (230°) will lead towards the southern flank of the Jura; those to the left lead towards the border of the Alps or tangentially along it. Birds arriving at a ridge below its top may climb above or follow the ridge. The higher the ridge top relative to the flight level, and the less a ridge deviates from the bird's previous course. the more likely it is that the bird will follow the ridge: At "B" the low flying birds meet a high ridge (1500 m above radar) at an angle of about 30°; they are completely deviated (Fig. 2). At "L" they meet a ridge (1000 m above radar) at an angle of about 90°; some birds are deviated to the left or to the right. while most of them climb to cross the ridge (Fig. 2). At "A" the birds fly along the valleys joining at this point. At the Alpine passes "H" and "C" the axis of the passes is close to the preferred direction; accordingly the funnelling along the ridges is very distinct (including a notable fraction of reversed migration).It is even more pronounced when all winds are included (Fig. 3). Climbing and crossing of ridges is generally reduced in opposing winds and nearly stops when the mountain tops are in clouds (Fig. 3; Liechti and Bruderer 1986). The tendency of migrants to follow mountain ridges met at small angles relative to their primary direction (even under calm conditions) is certainly one of the major reasons for the funnelling of migration between the Jura and the Alps; this results in extremely constant directions around 224 ° under all weather conditions near Geneva (Ba umgartner and Bruderer 1985). It is, however, not readily understood why birds even far off the mountain ridges maintain their track directions under westerly winds. In period 2 with strong westerly winds, the headings at "P" are extremely concentrated along the axis of the lowlands, indicating no compensation (Fig. I b); the tracks (Fig. la) unlike those at "N" and "R" are scattered but not deflected in a particular direction by the strong westerly winds. The absence of cross winds due to the funnelling of winds between the Alps and the Jura is one of the reasons for this constancy of tracks and headings; however, the difference relative to the results from the flat country in Germany persists when birds flying under equally strong southwesterly winds are considered in both areas. The birds in Germany shift their headings slightly S of 230° and are drifted ESE. The birds at "P" maintain their headings and fly with increased air speeds. but with low or even negative ground speeds along the lowlands. In spite of the low migratory activity under such conditions. this example shows that there is a directing influence of the mountain ranges in addition to the funnelling of the winds (B. Bruderer unpublished work).
Birds approaching the area of the Alps at high altitudes (higher than 1500 m above the lowlands) show a wider variation of directions. often resulting in
68 B. Bruderer and L. Jenni
different cohorts. This is most conspicuous at "K", where one group of birds follows the Jura (which is below their flight level) in the same way as the low flying birds (245 0
); the other group has (probably) crossed the Jura with a mean track of about 220 0 (corresponding to the mean directions of high flying birds above southern Germany). At "M", most distant from the Alps and the Jura, the same tendencies are slightly indicated; the mean direction, however, equals that at low levels (see also Bruderer 1978, 1982). At the ridges bordering the Alps and at the main chains of the Alps, the birds flying above the ridges show two or even three directional tendencies: (l) maintaining the (supposed) primary direction; (2) following a leading line; (3) flying westward, possibly towards the lowest part of the horizon, the Lake of Geneva basin.
2.4 Conclusions and Discussion
Three (overlapping) groups of nocturnal migrants approach the area of the Swiss Alps from southern Germany: (1) The birds which under weak or following winds cross the area of the Danube valley and S of it, and proceed with directions close to their supposed primary directions around 235 0
, being deviated by the funnelling effect of the Alps and the Jura to reach Geneva with directions around 2240. (2) The birds with more southerly directions, originating further N, which seem to prefer higher flight levels and are prone to cross mountain ridges. (3) The huge mass of birds drifted towards the Alps and the Jura by the frequent westerly winds (including also individuals of groups 1 and 2). Considering group (3), those at low levels (a) either shift their directions first WSW and later SW (because of the protection against cross winds or additional leading effects of the mountain ranges) or get intercepted at single ridges, enter a valley, zigzag through the mountain ranges, and cause important concentrations of migrants at Alpine passes; a limited fraction of these birds may also pass through N -S directed valleys and cross the Alps towards Italy; group (3) birds at high levels (b) are either drifted across the ridges or concentrated along the highest Alpine chains.
The cohorts flying westward away from the Alpine chains at high altitudes, the extreme concentrations of high flying birds at "B", and those flying parallel to far off mountain ranges call for additional explanations. Birds released in dark nights often fly towards the brightest part of the surroundings (e.g. a valley, a white cloud, a white walL the moon or another light source; B. Bruderer unpublished work; Bruderer and Neusser 1982). Free flying birds under hazy or misty conditions often change their flight paths to approach the light dome above a town (B. Bruderer unpublished work), or are attracted by lamps in the fog. These data suggest the following hypothesis: Nocturnal migrants might tend to avoid flying towards dark areas like the unlighted flank of a mountain; instead, they would prefer to fly towards lower, brighter parts of the horizon. Other interpretations might involve a preference to fly above a known habitat (even at night) resulting in avoidance ofthe mountain areas, or unknown effects such as detecting nearby mountain areas by infra-sound or other atmospheric features.
Migration Across the Alps 69
3 Which Birds Cross the Alps?
Almost all species migrating regularly through Switzerland in autumn have been observed in the Alps (lenni and Naef-Daenzer 1986). However, several species seem to be underrepresented in the Alps, especially soaring birds and species dependent on wetlands (Bruderer and Winkler 1976; Sutter 1955; Thiollay 1966, 1967). In this section. we first examine whether flight capabilities and habitat requirements correlate with the proportion of birds crossing the Alps.
Radar studies suggest that birds crossing the Alps either fly southward at high altitudes or approach the Alps at low altitudes on tracks determined largely by wind drift, thus, deviated from their innate direction of migration; of course. intermediate behaviour between these two extremes exists. At the border of the Alps. the migrants show a great variation of behavioural reactions. It is possible that this variation as well as the altitudinal segregation are related to certain properties of the birds. In the second part of this section, we try to identify the high flying birds crossing the Alps and examine, both at the inter- and intraspecific level, whether there are correlations between crossing the Alps and certain characteristics of the birds, i.e. (1) origin and direction of migration and (2) energy reserves.
3.1 Flight Capabilities
Soaring raptors have been frequently observed to abandon crossing alpine passes under strong head-winds when raptor species migrating chiefly by flapping flight still pass (Thiollay 1967; H. Schmid pers. comm.). Therefore we might expect that crossing the Alps depends on flight capabilities.
Indeed, there is a negative correlation between the relative frequency of 11 raptor species in the Alps and their proportion of soaring during migration (Fig. 4). For Buteo buteo, one of the most distinct soaring species, the few individuals flying in the Alps occur early during the migration season. This is due to the fact that the climbing rate when soaring decreases by one-third over the migration season because of decreasing lift in thermals (Schmid et al. 1986).
Among passerines as well as among waders/waterbirds, flight techniques are similar and flight speed in isometric birds depends on size (Rayner 1988). Wing-beat pattern and frequency from radar-tracked birds allow us to distinguish waders/waterbirds (continuous wing beats) from passerines (wing beats alternating with pauses) and two separate size classes in each group. With tail winds, there are no differences in the proportions oflarge and small birds among passerines or waders/waterbirds at different flight altitudes in the Alps. With southwesterly winds, however. when birds are flying lower and are drifted towards the Alps. the proportion of large birds increases with altitude both in passerines and waders/waterbirds (Bloch et al. 1981). This indicates that large birds can cope better with opposing winds and are able to fly higher under adverse conditions. A comparison of the proportion oflarge and small passerines during the same period in late autumn between sites "K" and "C" shows that with
70
I/)
CD I/) I/)
as Co CD c::: '0.
1.0
t;j 0.5 c::: o c::: ~ ... o Co o is.
.F.tin .F.sub
.F.col • A.nis
• P.hal • C.aer
• C.cya
0.1 0.2 0.3
• P.api
M.mig
0.4
proportion of soaring
B. Bruderer and L. Jenni
Fig. 4. Proportion of soaring in II raptor specieS' during migration (determined by tracking radar) vs relative frequency in Alps vs lowlands. The latter is the number of birds observed at two alpine passes (Col de Bretolet and Wasserscheide) divided by the total observed at Col de Bretolet. Wasserscheide and Fort l'Ecluse. the extreme end of the lowlands near Geneva) (data from H. Schmid. pers. comm.). n = I I. least squares correlation coefficientr = -0.84. P< 0.01. A. nis = Accipiter nisus; B. but = Buteo buteo; C. aer = Circus aeruginosus; C. cya = C. cyaneus; F. col = Falco columbarius: F. sub = F. subbuteo; F. tin = F. tinnunculus; M. mig = Milvus migrans; M. mil = M. milvus; P. api = Pernis apivorus; P. hal = Pandion haliaetus
opposing winds, there are 40% large passerines over the Alps but only 30% over the lowlands (n = 1509, chF = 18.1, P< 0.001); this confirms that large birds have a better capability to cross the Alps under opposing winds than do small birds.
3.2 Habitat Requirements
The number of captures per species at the alpine pass Col de Bretolet correlates with the abundance of the species in the breeding areas (estimations in five countries in the area of origin of Swiss migrants). The residuals ofthis relationship depend on the size of the bird, whether its migration is nocturnal or diurnal, and other factors (Jenni and Naef-Daenzer 1986 and below). This suggests that species of the same group (with respect to size, etc.) are caught roughly in proportion to their abundance at this alpine pass; here they normally do not rest.
An analysis of the residuals of this relationship with respect to habitat requirements reveals that species dependent on wetlands are underrepresented on Col de Bretolet (lenni and Neaf-Daenzer 1986). However, this could be due to differences in flight behaviour between wetland and non-wetland species which in turn could influence catching rates.
Yet, there is circumstantial evidence that habitat requirements might playa role. When night migrants land in the early morning at the alpine pass Col de Bretolet, species dependent on the general type of habitat found at this site (bushes) are caught in higher proportions (compared with the number of night
Migration Across the Alps
day I(day+night)
%
80
60
40
20
71
Fig. 5. Proportion of day captures (number of day captures divided by number of day and night captures) of night-migrating passerine species according to six habitat types. The number of species is indicated below the column. the range by a line. The types of habitats available at Col de Bretolet are marked by the shaded bar
captures) than species dependent on wetlands and forests (Fig. 5). This indicates that night migrants are prone to land in approximately appropriate habitats. The poor availability of wetlands in the Alps (the next large reed beds are 60 km further S ofBretolet) might therefore be a reason for wetland species to avoid the Alps or to cross the Alps non-stop with high energy reserves and at altitudes high enough to prevent capture.
3.3 Origin, Direction, and Altitude of Migration
Interspecific LeveL An analysis of the residuals from the expected number of captures at Col de Bretolet (see Sect. 3.2) with respect to direction of migration within Europe reveals that species migrating S or SE (120-180°) are caught in very low numbers because the valley leading to the Col de Bretolet is oriented southwest and acts as a funnel only for southwestern migrants. Species showing a migratory divide within central Europe are also caught in lower numbers than expected (Jenni and Naef-Daenzer 1986). Moreover, for passerine species not dependent on wetlands with generally southwesterly directions of migration, and no migratory divide within Europe, the residuals from the expected number of birds correlate with the mean direction of migration within Europe (Fig. 6). Species with directions around 210° are caught in higher numbers than expected from their estimated number of breeding pairs than are species with more westerly directions (around 230°).
72 B. Bruderer and L. Jenni
• 1.2 -: •
•• • • 0.6 • •
Ul • 'iii • :::l • •• I '0 • 'iii CD
0 ••• ... • • • -0.6 •
2000 2100 2200 2300 2400
direction of migration
Fig.6. Mean recruiting direction of migration versus residuals from the expected number of birds at Col de Bretolet (see Jenni and Naef-Daenzer 1986). The mean direction of birds migrating towards Switzerland is calculated from Swiss ringing recoveries (n> 10) or estimated from Zink (1973) for seven species. Included are 26 passerine species not dependent on wetlands. without a migratory divide within Europe. and which migrate generally to the southwest (> 180°); r= -0.45. P< 0.03
It is difficult to obtain data on the species-specific altitudinal distribution of migrants in order to test if a southerly direction implies a higher flight altitude for the species (but see Sect. 3.1). Wing-beat patterns do not allow us to recognize the species in passerines.
Intraspecific Level At this level the origin of migrating birds can be evaluated by ringing recoveries and by the geographical variation of morphological characters. Both methods are applied here. Two species provide enough recoveries to compare the origin of birds occurring in the Swiss Alps and in the Swiss lowlands north of the Alps during autumn migration: In Erithacus rubecula, a short-distance night migrant, birds occurring in the Alps during autumn migration originate from more distant areas (Scandinavia) while birds occurring in the lowlands originate predominantly from Germany and Czechoslovakia. The latter have a wider spread of directions and a more westerly mean direction than birds occurring in the Alps (lenni 1987). Also in Fringilla coelebs, a short-distance day migrant, birds occurring in the Alps originate from more distant areas and arrive with more southerly directions than birds from the lowlands (L. lenni unpublished work). Other species consistently show the same tendency but the small number of recoveries available does not give significant differences.
Among the breeding populations of the long-distance night migrant Sylvia borin there is a dinal increase of wing-length over Europe towards the north-east (Klein et al. 1973; L. lenni unpublished work). A detailed analysis of wing-length and feather-length (Berthold and Friedrich 1979; lenni and Winkler 1989) at nine different ringing sites in Switzerland shows that longer-winged and therefore northern populations are caught at night in the Alps and at a stop-over site S of
Migration Across the Alps 73
the Alps; this indicates that northern populations are more likely to cross the Alps (Jenni and Jenni-Eiermann 1987). In Sylvia borin, northern populations also migrate with more southerly directions over Europe than southern populations (Klein et al. 1973; Zink 1973; Jenni and Jenni-Eiermann 1987).
Do northern populations migrate at higher altitudes? On the alpine pass Col de Bretolet, birds are normally caught during the night in mist nets of8 m height. On foggy nights lamps are used to attract the birds. It is assumed that, in the fog, birds normally flying higher than 8 m are caught in addition. In the three most common species caught during the night (Erithacus rubecula, Ficedula hypoleuca, Sylvia borin, n > 400 each), significantly longer wings occur in the captures of foggy nights than in those of clear nights (wing-length corrected for seasonal trends by regression analysis). Northern populations of these species have longer wings. Hence it can be inferred that they migrate at higher altitudes. Furthermore. in Sylvia borin. the mean wing-length of trapped birds increases with head-winds which induce lower flight altitudes and apparently force even the larger northern birds to fly low enough to be caught (Jenni and Jenni-Eiermann 1987).
At the inter- and intraspecific leveL we conclude that passerines meeting the Alps at large angles are more likely to cross the Alps than birds approaching at small angles. At the intraspecific leveL the former are normally northern populations (i.e. from Scandinavia) migrating within Europa with more southerly directions and higher above ground than southern populations (i.e. from Middle Europe).
3.4 Energy Reserves
It is possible that higher energy reserves balance the harsher environmental conditions and risks met by birds during migration and stop-over in the Alps.
Interspecific LeveL For night-migrating passerines. we showed that species with high weight surplus over pre migratory weight in the lowlands north of the Alps. indicating high fat reserves. are caught in higher numbers during night at Col de Bretolet than expected (Bruderer and Jenni 1988). Using new data (fat scores. ranging from I to 5) of a greater number of species (day and night migrants). this correlation can be confirmed: there is no clear difference between day and night migrants.
The mean fat scores in the lowlands north of the Alps during the migration season of the species (lenni 1984), however, correlate with the mean recruiting directions of migration according to ringing recoveries (Fig. 7). This correlation prevents the variable fat score from contributing significantly to explain the variance in the number of birds caught on Col de Bretolet in a multiple regression analysis incorporating mean direction (see Sect. 3.3) and the measure of population size as other independent variables.
Thus, species arriving with more southerly directions in Switzerland have larger fat reserves and are more likely to cross the Alps than species migrating parallel to the Alps. For a given direction of migration, there are no clear
74
3.0
Q) ... o ~ 2.5 I
~
2.0
1800
• • • • • • • • ..
• • •• • ••
• ... • • • 1900 2000 2100 2200 2300
direction of migration
B. Bruderer and L. Jenni
Fig. 7. Mean recruiting direction of migrants versus mean fat score in the lowlands north of the Alps during the migratory season of the species concerned. Triangles Species crossing the Mediterranean and the Sahara; squares crossing very often the Mediterranean; dots wintering predominantly on the continent; n = 28. r=-0.80. P<O.OOI
differences in fat score between long-distance migrants crossing the Mediterranean and the Sahara, short-distance migrants crossing the Mediterranean regularly, and short-distance migrants wintering mainly on the continent; a migratory divide or specialized habitat requirements also do not have an effect.
This suggests, for the passerine species examined until now, that the larger energy reserves of species migrating S are not due to a fundamentally different destination or migratory strategy, but could be in response to the Alps. Further analyses are needed to test this hypothesis.
Intraspecific LeveL In Sylvia borin, S. atricapilla, Ficedula hypoleuca and Oenanthe oenanthe. birds flying higher above the alpine pass Col de Bretolet (attracted by lamps. see Sect. 3.3) are significantly heavier than birds flying low over the pass on clear nights (weights corrected for seasonal trends. size and time of day by multiple regression analysis; Bruderer and Jenni 1988). Other nightmigrating species show the same tendency.
3.5 Discussion
Species arriving in Switzerland with southerly directions cross the Alps in higher proportions and with higher energy reserves than species paralleling the Alps on migration. The former might correspond to the increased proportion of southward heading birds seen at high levels by radar. This general pattern might be influenced by differences in flight capability and habitat requirements in direct response to the difficulties imposed by crossing the Alps. Most birds are drifted towards the Alps with westerly winds, the most frequent winds in autumn. Therefore, we expect a differential reaction to weather and wind conditions according to species, amount of energy reserves or other yet unknown properties. either by refraining from migration under certain conditions or by different reactions at the border of the Alps.
Migration Across the Alps 75
lntraspecifically, the same general pattern shows up: Individuals arriving with southerly directions have more energy feserves and are more likely to cross the Alps. They originate from northern populations and apparently fly higher above ground. The analysis of Sylvia borin data from nine different ringing sites revealed that the endogenous differences in the population and age-specific migration program. found in the laboratory (Berthold et al. 1974), are not blurred by environmental factors. They are still recognizable in the field by the seasonal course, sequence. and energy reserves of the populations and age groups migrating through Switzerland (Jenni and Jenni-Eiermann 1987, unpublished work). Therefore. it is probable that crossing the Alps is facilitated by the endogenous migration program and the general migration strategy.
However. the question remains. whether higher energy reserves are built up proximately in response to the Alps or endogenously without exogenous stimulus. The findings at Falsterbo support the former: there, the northward reorientation of night and day migrants is thought to serve for fattening up as a direct response to the Baltic sea. a relatively small and flat obstacle of only 25-100 km (Alerstam 1978; Sandberg et al. 1988; Lindstrom and Alerstam 1986). It would of course be a much safer strategy to adjust the endogenous build up of fat reserves in direct response to perceived environmental barriers.
The ultimate causes of crossing or avoiding the Alps are difficult to assess. We merely mention some of the advantages and disadvantages. The disadvantages regarding climate, habitat and weather are listed in Section 1. They demand more energy and a better risk insurance. The advantages might be that the distance of migration is shorter for some destinations, the more so since the bird can tolerate drift by the frequent westerly winds. Energy expenditure would be smaller and migration more rapid. It might be an advantage to migrate more rapidly because of time pressure. food availability. competition and because first arriving birds might occu py better and nearer winter territories. We expect that the trade-off between disadvantages and advantages varies for different species and populations. Birds wintering S of the Sahara might be less dependent on the land mass of Iberia if they are able to put on fat as far north as central Europe or southern France and Italy: the distance from central Europe to western Africa (Switzerland - Lagos) is 10% longer over Gibraltar than directly. Many species moult completely in the winter quarters where they establish territories: first arriving birds might have an advantage. Therefore long-distance migrants should have more southerly directions (Fig. 7). Northern populations of long-distance migrants are especially under time pressure. Short-distance migrants migrating through Switzerland predominantly winter in southern France and Iberia and have no gain in distance by crossing the Alps. As seen by the time of migration (e.g. Sv/via atricapilla: Klein et al. 1973: Turrian and Jenni 1989). southern populations stay longer in the area of the breeding grounds before migration. wh ile northern populations pass through: to arrive early on the wintering grounds does not seem to be important. Short-distance migrants depend more than long-distance migrants on the temperature gradient running NE-SW over Europe during winter. Therefore it is understandable that they take more westerly directions.
76 B. Bruderer and L. lenni
Acknowledgement. This work was supported by the Swiss National Foundation for Scientific Research. grant Nos. 3.171-0.85. 3.161-0.81.
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