Download Impacts of Gull predation at Loch Leven NNR

Impacts of Gull predation at Loch Leven NNR
SNH position statement – revised June 2010
Background
Breeding birds
Large gulls (lesser black-backed and herring gulls) on St Serf’s island, Loch Leven
NNR were regularly culled by SNH up to 1997. In 1997 the cull was ceased because
of possible impacts of the cull on the East Scotland lesser black-backed gull
population and particularly the Forth Islands SPA. After this, large gull numbers
increased rapidly until 2001 when numbers stabilised at ~2250 nesting adults.
Black-headed gull numbers showed a sharp decline in 2006 to 1800 nesting adults
from a relatively stable 5 year count averaging 4500 and in 2007 deserted the site.
Black-headed gull numbers nationally have seen a 63% decline in numbers between
1993 and 2004.
Analysis of annual duck nest counts, suggest a very shallow decline over the last 40
years in nesting pairs but this is not statistically significant and is within the range of
natural fluctuations. Commissioned research and analysis of SNH data showed
duck productivity, as measured by the number of duck broods and brood size,
showed no decline after the gull cull ceased.
However in 2007, following a poor year in 2006, there was a catastrophic failure in
breeding. No ducks appeared to nest, the black-headed gull colony deserted and
there was an almost complete failure of breeding by the large gulls. Subsequent fox
control took 5 foxes and so much of the catastrophic failure was considered to be
due to fox predation. In 2008 numbers of breeding ducks and large gulls started to
improve on the previous year although it was found that there were still a number of
foxes on the island which were subsequently dealt with. Monitoring for foxes in
spring 2009 found no evidence of their presence.
Scientific research undertaken:
•
Commissioned survey looking at the impacts of large gulls on the breeding
duck population
•
Annual Duck Pairs and Brood counts
•
Walkover survey to establish presence or absence of nesting duck species.
•
Annual Gull counts
Ecology of St Serf’s duck colony
Intensive bird studies in the early 1970’s conducted as part of the International
Biodiversity Programme demonstrated the relationship between the duck and gull
colonies. All duck species had more hatching success when nesting in the gullery
than outside. This difference was statistically significant in the tufted duck and in the
combined data for all species. A preference was shown for ducks to seek the
“protection” of a black-headed gull colony above that of a large gull colony, but both
were seen as preferable to no “gull protection” at all. It presumably occurred
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because the gulls, whilst not themselves serious predators on hidden duck clutches,
kept out other species like jackdaws that were known to predate duck eggs.
SNH management of St Serf’s
Concerns have been expressed by other parties regarding the SNH approach to
management of the large gull colony; the argument being that the gulls were
decimating the number of ducklings and also pushing out the black-headed gull
colony from their traditional nesting areas.
In light of these concerns, a review was conducted by SNH Reserve staff and SNH
specialist ornithologists during August 2006. Research findings and SNH count data
for St Serf’s and national bird data were reviewed and it was concluded that there
was not a compelling case to re-establish a gull cull. The basis of this was:
• There is no significant downward trend in brood size or numbers of broods
over the last twenty years indicating no change in predation levels since gull
numbers started to increase on the loch. Removal of gulls might only result
in increased predation by herons, pike, brown trout, otters, and mink
achieving the same net impact.
• The four year average (data from one year was not available) of 400 nests in
the early 1970’s compared to a recent five year average of 325 in the most
recent counts. This decline of 75 nests over the 35 year period is not below
the Site Condition Monitoring threshold of 25% acceptable natural fluctuation
although it does suggest a trend of a shallow decline.
•
We did not find the argument compelling that controlling large gull numbers in
the vicinity of the black-headed gull colony would increase numbers of blackheaded gulls. Black-headed gulls are very flighty in selection of sites for
establishment of breeding colonies and are not particularly faithful to specific
areas. Although there has been some reduction in colony size there is still
plenty of suitable alternative habitat on St Serfs. In addition, numbers of birds
have dropped off in colonies elsewhere on the island out with areas used by
large gull colonies. In conclusion, black-headed gull numbers may not rise as
a result of large gull control as there is a national downward trend in numbers
and also there is still plenty of available habitat not being utilised on St Serfs.
Following further complaints about the SNH approach, and in light of the catastrophic
failure of the breeding colonies in 2007, SNH Reserve staff and ornithology
specialists conducted another review in November 2008. Two members of the
Scientific Advisory Committee were also involved in the process.
This second review concluded:
•
Foxes, are probably to blame for the catastrophic breeding failure of ducks
and gulls in 2007/8.
•
The crash in both the duck and black-headed gull nesting colonies do not
reflect a failure throughout Scotland and therefore weather is not likely to be
the main factor.
•
A cull of large gulls, which offer some protection to nesting tufted ducks from
egg predation by jackdaws, may do more harm than good.
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•
It is accepted that there was a contraction of the black-headed gull colony as
a result of large gulls invading into their colony, however there still remains
adequate nesting habitat for black-headed gulls away from the large gull
area.
•
The large gull population is stable and evidence indicates may have reached
a self limiting state.
•
There is no limit on availability of suitable black headed gull nesting habitat
and so there is no justification for partial cull to increase availability for this
species.
•
To encourage black-headed gulls to re-establish a colony, decoying in
conjunction with tape play-back will be attempted.
Position statement
1. Gull predation did not appear to have resulted in a decline in the
breeding population of tufted ducks on the site up to 2006. The
catastrophic decline from 2007 onwards appears to have been due to
large numbers of foxes on the island.
2. SNH will keep its policy on large gull culls under constant review. If
there is evidence that the effects of the gull colony are having an
adverse impact on the duck and/or a re-established black-headed gull
colony, SNH will re-consider lethal control. For example, if calculated
productivity from pairs data falls below the threshold of 25% acceptable
natural fluctuation over a 5yr period.
3. Given that the black-headed gulls are no longer present, a cull of large
gulls, which offer some protection to nesting tufted ducks from egg
predation by jackdaws, may do more harm than good.
Operational Requirements
4. In the first instance, SNH will focus its effort on; i) control of foxes, rats,
jackdaws, mink and crows, ii) habitat restoration of the areas of wetland
which form part of the duck colony and iii) other works to encourage
black-headed gulls to nest on the island.
5. Regular monitoring of fox, jackdaws and other pest species will
continue.
6. Detailed monitoring of nesting ducks will be undertaken in 2009 to
assess whether there has been any recovery since foxes were
eradicated from the island.
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