Bridges, tehcnical comparison
We have compared data from three comparable bridges around Baltic sea and also collected data
from other places, like Confederation bridge in Canada. Technical overview of bridges is shown in
table below.
As general remark, Öresund bridge and Raippaluoto bridge are cable stayed bridges with high
pylons and supporting cables. Those structures are considered, in certain weather conditions, more
risky to birds than lower bridges. The risk to birds increases especially if bright lights are used
during foggy nights. Night migrating land birds may have risk of collision. That can be mitigated by
proper lighting design.
Öland Bridge and Confederation bridge are cantilever bridges. Öland bridge is clearly lower than
Confederation bridge.
Öland bridge is oldest, almost 40 years old, and we have not found any data if there were bird
monitoring before construction or after that. The impact of bridge to migrating birds is estimated
very low level. Birds successfully fly over it, and the only impact is that their flight route becomes a
little bit longer, but only 1-2 kilometers which means no critical impact at all.
When Raippaluoto bridge was constructed, the main targets of concern were more economical and
sociological- There was no bird monitoring. The number of migrants through that strait are much
smaller than in other places, but never the less some birds pass the bridge, and no critical problems
have been detected.
Confederation bridge in Canada is the longest, and from there are some notes that birds avoid flying
straight over. As mentioned in the earlier papers, most birds still finally pass the bridge without
problems. The bridge is much higher than Öland bridge, although it’s also a cantilever bridge. We
have not found any information of continuous monitoring of birds.
The Öresund bridge between Sweden and Denmark is a well known case, where monitoring of
different environmental aspects has been wide. There has been bird monitoring before and after
construction, and also radar has been used to monitor flying routes and altitudes. According to the
studies (Nilsson & Green 2002) most birds pass over the bridge. It is estimated that 0,01-0,05% of
passing birds collided with the bridge structures, mainly during foggy nights, when there was bright
lighting. Due to negative effect of lighting, the problem was mitigated by developing special
program for night lighting. During bad visibility conditions, lighting is reduced. The impact was
clearly positive, less birds have collided after implementation.
Conclusion: The reference bridges have different construction types and the volume of migrating
birds also differ. As the data from Denmark and Sweden clearly show, there is no significant
negative impact and bird mortality is low. Using low type of bridge construction, the impact can be
reduced even more.
Table: general data of reference bridges.
Öresund bridge
Öland Bridge
(Kalmarsund)
Type
Cable stayed
Cantilever bridge
bridge
Lenght
7845m
6072 m
Highest point
204 m
about 50 (no
exact data found)
Clearance below 57 m
36 m
Opened
July 1, 2000
30 September
1972
Raippaluoto
bridge
Cable stayed
bridge
1045 m
82,5 m
26 m
27. august 1997
Confederation
bridge, Canada
Cantilever bridge
12900m
less than 80 (no
exact data found)
60 m
May 31, 1997
Flying altitudes of birds
Generally flying altitude of bids is not strictly species specific, but more or less group specific. For
example, many raptors, geese and granes are flying relatively high during migration, and many
smaller birds are flying lower. Also habitat influences flying altitude, many ducks and seabird fly
low or just over water when they are crossing sea, but take more height when crossing continent.
And they also fly higher during night than during day.
Migration altitude can also be different than normal flying altitude near breeding or resting areas.
Many night migrating long distance migrants fly even at several kilometers altitude when migrating,
but normally they move at less than hundred meters altitude.
The flying altitude of birds is probably more or less the same at all Baltic coasts. So we can assume,
that those altitudes, which have been observed at Suur-Strait more or less reflect the normal flying
altitudes of those species. So some species are flying low also in Sweden and Denmark. But despite
that, the birds don’t fly against bridge (or other structures or cliffs etc). They can dodge them. As
earlier said, exceptions are those situations, when for example the pylons of high bridges have
heavy lightning and during foggy nights birds are “trapped by light” and continue flying around
tower and finally hit structures.
From Kalmarsund there are some notes about flying altitudes of birds. Pettersson has estimated in
one of his studies that the average flying altitude of seabirds during day at south Kalmarsundwas 20
meters, and in his other study 109 meters (Pettersson 2005, 2009). During the night average altitude
was 84 meters. Pettersson has aslo estimated that when passerine birds migrate over sea during
night, their flying altitude is 200m-1200m. But, these are average numbers, there is a lot of flocks
which fly much lower (see figure).
Flying altitudes of birds migrating during daytime at Kalmarsund
Flying altitudes of birds migrating during night time at Kalmarsund
So the lowest altitudes of seabirds are at the same level as Kalmarsund bridge, or windmills. But
there are only exceptional collisions. Sea birds either pass windmills left or right, of try to fly over,
as they do when they reach bridge. Petterson (2005) has in one of his studies showed this behavior
with common eider. Before wind mills, eiders fly at about 20 m altitude before windmills, and at
50 m when passing between windmills. Collisions against windmills were very rare, although they
have moving rotors, so probably collisions against stable bridge structures of Kalmarsund bridge are
even more rare.
Litterature:
Pettersson, J. 2005: Havsbaserade vindkraftverks inverkan på fågellivet I södra Kalmarsund. Lund
universitet.
Pettersson, J. : Flyttande små- och sjöfåglar - en förstudie med local radar I Kalmarsund.
Naturvårdsvärket 2006.
Number of birds migrating and staging near Öresund and Öland bridges
It’s generally known that Öresund between Sweden and Denmark, and Kalmarsund between
Swedish mainland and Öland island are important migration flyways for birds and also important
wintering places for many water birds.
It’s estimated, that during migration about 100 000 000 birds migrate through area between Öresund
and Falsterbo cape at south-west Sweden (Nielsson & Geen 2002). During special migration study
September-October 2001 at Öresund, over 100000 birds were count, representing 90 species. Dead
birds were collected from the bridge and identified. Also, a radar study was carried out to study the
migration routes of birds. As shown in next picture, many birds (mainly land birds) passed straight
across to Denmark, but many birds also flew over bridge.
General flight directions (yellow arrows) in the area around the Öresund bridge according to the
radar study. Note: bridge is ‘Bro’
Tracks of 16 bird flocks seen from Lernacken and followed by the radar. Yellow arrows: Barnacle
Geese. Blue arrows: Greylag Geese, Red arrows: Wood Pigeon.
The number of dead birds was relatively low. All found dead birds are shown in next table. Note,
that no water birds were found, most of birds are night migrating passerines. Nielsson and Green
estimated that 0,01-0,05% of passing birds collide with the bridge structures.
Number of dead birds found on the Öresund bridge, September – December 2001.
Stenfalk Falco clumbarius
Rörhöna Galinula ochropus
Morkulla Scolopax rusticola
Gråtrut Larus argentatus
Havstrut Larus marinus
Stadsduva Columbia ”domestica”
Jorduggla Asio flammeus
Hornuggla Asio otus
St hackspett Dendrocopus major
Trädlärka Lullula arborea
Sånglärka Alauda arvensis
Ängpiplärka Anthus pratensis
Gärdsmyg Troglodytes troglodytes
Rödhake Erithacus rubecula
Koltrast Turdus merula
Björktrast Turdus pilaris
Rödvingetrast Turdus iliacus
Taltrast Turdus philomelos
Trädgårdssångare Sylvia borin
Svarthätta Sylva atricapilla
Lövsångare Phylloscopus trochilus
Gransångare Phylloscopus collybita
Kungsfågel Regulus regulus
Blåmes Parus caeruleus
Sept 1 – 15 Okt 16 -31 Okt
1
1
1
1
1
1
2
2
1
1
2
1
5
12
1
4
5
1
2
10
63
52
Nov
Dec
2
1
1
4
1
1
1
12
1
4
21
1
1
1
10
9
1
1
17
4
Total
1
1
1
4
1
1
4
1
3
1
17
10
3
125
1
1
4
37
1
2
2
2
28
13
Skata Pica pica
Kråka Corvus corone
Bofink Fringilla coelebs
Bergfink Fringilla montifringilla
Grönsiska Carduelis spinus
Gråsiska Carduelis flammea
Gulsparv Emberiza citrniella
Dvärgsparv Emberiza pusilla
Sävsparv Emberiza schoeniclus
Småfågel NN Passeres spp.
TOTAL
6
1
1
1
3
2
1
1
23
1
1
5
127
1
1
1
4
1
141
5
1
From Öland bridge no monitoring data is found, but a lot of waterbirds migrate throught strait. We
know that huge number of eiders pass Kalmarsund, Sweden. Almost 1 000 000 eiders pass Öland
Island during migration, and many of them use Kalmarsund and must pass the bridge. For example
on 5.4.1992 130000 birds were recorded to have passed Beijerhamn, just a few kilometres south of
the bridge (Lindell 2002: Sveriges fåglar).
1
1
10
3
1
1
2
1
5
6
295
How far of other bridges are the nearest staging areas?
It has been said, that Suur-Strait differs from Öresund and Kalmarsund because many birds are also
resting so near bridge. I measured distances from nearest Natura-areas to the middle of Öresund
bridge and Kalmarsund bridge, results are in table.
Öresund bridge is practically very near Natura area, because the west end of bridge is at
Saltholmen.
Nearest Natura 2000 areas, birds directive:
Natura code
Öresund bridge
DK002X110
DK002X111
Kalmarsund bridge
SE0430002
SE0430173
SE0330054
SE0330109
SE0330083
SE0410053
Name
Saltholm og
omliggende hav
Vestamager og havet
syd for
Falsterbo-Foteviken
Lommaområdet
Beijershamn och
Svansholmarna
Eckelsudde
Ottenby
Kristianopels skärgård
Distance (to middle of
bridge
about 3 km
about 13 km
about 13 km
about 14 km
about 7 km
about 27 km
about 50 km
about 50 km
Many of these Natura areas are important breeding sites, but also important wintering areas for
many seabirds and wader. Also, a lot of birds rest out site these areas near bridges, as can be seen
from next picture.
Picture: wintering waterbirds near Öland bridge
How bird and seal studies were used at Natura-assessment?
Both bird and seal study from Suur-Strait was used as bases of Natura-assessment. The data from
bird study show the number of birds, migration routes and altitudes clearly. Seal study showed the
migration route and time of ringed seals. The only difference is, that in Natura –assessment the
conclusion about impacts significance is not the same as in special studies. Comparison to
published literature and experiences from other bridges don’t show as critical impacts as said in
Suur-Strait bird study. Also the published data about other seal species and ringed seals in Saimaa
does not show that bridges cause significant impacts to seals, if mitigation measures and best
practices are used during construction.
The experiences from Öresund bridge and Öland bridge can be used as reference to Suur-Strait. No
significant negative impacts to population levels have been found there.