Breeding Ecology of Great and Arctic Skuas
on Handa Island
Handa Island Skua Monitoring Programme
Final Report 2013
Duncan Leckie and Claire Smith
Contents
1.0 Summary ........................................................................................................................................... 2
1.1 Outputs ......................................................................................................................................... 2
2.0 Methods ............................................................................................................................................ 3
2.1 All Island Count ............................................................................................................................. 3
3.0 Results ............................................................................................................................................... 4
3.1 Population ..................................................................................................................................... 4
Great Skua ....................................................................................................................................... 4
Arctic Skua....................................................................................................................................... 5
3.2 Breeding Statistics ......................................................................................................................... 6
Great Skua ....................................................................................................................................... 6
Arctic Skua....................................................................................................................................... 8
4.Ringing ................................................................................................................................................. 8
4.1 Re-sightings and Recoveries ......................................................................................................... 9
5.Discussion .......................................................................................................................................... 10
6. Funding ............................................................................................................................................. 11
7. Acknowledgments............................................................................................................................. 11
8. References ........................................................................................................................................ 12
APPENDIX. Map of Handa showing skua territories 2013 ................................................................... 13
Suggested Citation
Leckie, D. Smith, C (2013) Handa Island Skua Project Final Report 2013, Unpublished report
to SWT, SNH, JNCC, SOC, and the Seabird Group
Contact Details
Claire Smith: [email protected]
Duncan Leckie: [email protected]
Photo Credits
Cover Photo: Four-day-old Arctic Skua chick by Duncan Leckie
Figures and Maps Credits
Duncan Leckie, Claire Smith & Thomas Plant
1
1. Summary
2013 saw the eleventh continuous year of the Handa Island Skua Monitoring Project. The
productivity of Great Skuas (Catharacta skua) and Arctic Skuas (Stercorarius parasiticus) was
determined, and chicks were ringed with BTO and Darvic colour rings. This year the AllIsland Count of Great Skuas was conducted on the 25th, 26th, 28th June and counted 135
apparently occupied territories (AOT’s) (this includes the nests within the Study Sites). A
major 45% decrease from the previous count in 2010 (Smith et al, 2010).
2013 proved to be a disastrous year for Great Skuas on Handa. The productivity of Great
Skuas was monitored by focusing on a sample of 48 pairs across two study Sites with 24
pairs in each Site; this is below the average of 75 pairs monitored between 2004-2012,
which usually accounted for a third of the breeding population. The size of the study sites in
2013 was consistent with previous years, but far fewer pairs settled in the areas. Over both
study sites productivity was 0.14 chicks fledged per pair, with Site 1 showing higher
productivity (0.25) than Site 2 (0.04).
All of the Arctic Skua pairs were monitored, with a total of 10 pairs breeding on the Island,
and an additional pair holding territory. Twelve chicks survived until the end of the study
period giving a productivity of 1.2.
Eight Arctic Skua chicks were fitted with BTO and white or black Darvic rings (with black and
white lettering). A further four chicks were fitted with BTO rings only (only three of which
successfully fledged). Within the study sites seven Great Skua chicks were ringed with both
BTO and black Darvic rings (white lettering). A further three were ringed and colour-ringed
located out with the study Sites
Throughout the season, both Great and Arctic Skuas with colour rings were observed on the
Island. A colour-ringed Arctic Skua that fledged form Handa in 2009 successfully reared one
chick.
The Great skua club-site in the north-east of the island continued to be used by some birds
and the second club in I5 held up to 35. Swaabie Loch regularly held over 50 birds and is the
preferred bathing site rather than Hill Loch.
1.1 Outputs
A summary of the breeding results were provided for the annual Handa Rangers report with
the full report being posted on the Handa Island Skua Project website:
(http://www.handaskuas.org).
Census and productivity data were also uploaded to the website of the JNCC seabird
monitoring programme: (http://jncc.defra.gov.uk/page-1550).
Previous reports of the Handa Island Skua Project, and other research and monitoring
results are available at: http://www.handaskuas.org/research.
2
2. Methods
Claire Smith and Duncan Leckie began to map Great Skua nests on the 25th and 26th of May.
Duncan Leckie monitored continuously until the 25th of August when all fieldwork was
complete and nests had either successfully fledged or failed. Andrew Ramsay of the
Highland Ringing Group visited on two separate occasions to assist with the finding and
ringing chicks. Out of those occasions only one chick was ringed. Duncan was regularly
assisted by Paul McDonald, Handa Island Ranger and fellow volunteers Krista Kokina, Adam
Butler, Frances Kenward and others.
All Arctic Skua pairs were monitored and a sample of 48 Great Skua nests (APPENDIX) was
monitored across 2 study Sites. The size of the study sites in 2013 was consistent with
previous years and nest finding effort was increased.
In previous years, Study Site 1 has been consistently less productive than Study Site 2, which
contains fewer breeding pairs of Greater Black Backed gulls. See Jones (2003) for a detailed
methodology and Smith & Jones (2004) for a detailed explanation of selection of study Sites.
The length and breadth of all eggs were measured to the nearest 0.5mm using callipers and
egg volume calculated using 0.00048 x length x breadth2 (Coulson, 1963). Lay-dates were
estimated for all nests of known hatching date, by subtracting the gestation periods of 28
and 25 days for Great and Arctic Skuas respectively (Hamer, 2001; Phillips, 2001).
Birds present at the club and bathing Sites were counted opportunistically throughout the
season. Swaabie Loch was monitored on a weekly basis; notes were made on the number of
individual birds with one hour, any additional species and the weather conditions.
2.1 All Island Count
Due to weather conditions the All-Island Survey was carried out over three days 25th, 26th
and 28th of June, following standard methodology as described in (Walsh et al. 1995).
The survey was carried out after study site territories had been mapped and vantage points
were carefully located as not to double count the territories with the study sites. The AOT’s
were mapped out and totalled after the survey was complete. This number was then added
to the number within the two study sites to gain the all island count.
3
3 Results
3.1 Population
Great skua
The All-Island survey was conducted on 25th, 26th and 28th June during which 135 AOTs were
recorded, a major 45% decline from 249 AOTs recorded during the previous All-Island count
in 201o (Smith et al 2010). The breeding population has not been this low since 1997 when
there were 140 pairs (Jones, 2003).
300
Great Skua AOTs
250
200
150
100
50
0
1964
1974
1984
1994
2004
Year
Figure 1: Number of Great Skua apparently occupied territories on Handa Island since 1964.
The Club Site within Study Site 2 was well utilised throughout the season with a maximum
recording of 35 individual Great Skuas on the 17th June and a seasonal average of 24
individuals. Swaabie Loch, which is used as the bathing Site for mainly Great Skuas, was
monitored on a weekly basis, with a maximum recording of 91 on the 11th July. It had a
seasonal average of 49 individuals. Other species including Great Black Backed Gull (Larus
marinus) and Herring Gull (Larus argentatus) used the loch on a regular basis. The last count
of the season on 15th August recorded no Great Skua activity.
Recruitment
At least 4 birds ringed as chicks on Handa attempted to breed on the island in 2013. These
were aged nine, six and four, the oldest returning to breed for the third time. A further two
colour-ringed birds aged 6 were recorded at the bathing site, so breeding status could not
be determined. No birds colour-ringed as adults between 1989-1993 were recorded
breeding on Handa in 2013.
4
Arctic Skua
A total of 10 Arctic Skua nests plus one AOT were present on Handa in 2013.
The number has declined in recent years from a stable number in the region of 30-36 pairs
between 1985 and 2003 (Figure 2). The majority of the nest sites were located in the
southeast corner of the Island, above the beach of Traigh Shourie. The other Sites were
scattered from the village up to Puffin Bay (APPENDIX).
45
40
35
30
25
20
15
10
5
0
1964
1969
1974
1979
1984
1989
1994
1999
2004
2009
Figure 2: Number of Arctic Skua apparently occupied territories on Handa Island since 1964.
Colour Phases
Of the 22 Arctic Skuas either breeding or holding territory 15 (68%) were dark phase and 7
(32%) were light phase adults. Pairs were composed of four dark/dark pairs and seven
light/dark pairs.
Recruitment
The light phase Arctic Skua, colour ring E3, on territory F8A1 successfully reared one chick.
The four colour re-sightings of Arctic Skuas (G8 and I3 (White/black, 2008), E6 and E2
(White/black, 2009) in 2012 (Cunningham & Smith, 2012), failed to return to the Island and
breed this year.
5
3.2 Breeding Statistics
Table 1: Breeding statistics of Great and Arctic skuas on Handa 2013.
Sample sizes are in parentheses.
Breeding
pairs
Mean lay
date
monitored
Mean
Mean
alpha egg
volume
1
Hatching
Fledging
Chicks
2
clutch
success
success
fledged
size
(%)
(%)
per pair
(ml)
25/05/2013
80.38
1.5
81.08
16.2
0.25
(23)
(20)
(24)
(24)
(24)
(24)
22/05/2013
77.8
1.8
54.75
2.4
0.04
(16)
(23)
(24)
(24)
(24)
(24)
24/05/2013
78.93
1.6
67.09
8.9
0.15
(29)
(48)
(48)
(38)
(48)
(48)
753
22/05
79.27
1.80
84.74
41.94
0.65
10
4 June
43.68
2
85
60
1.2
2013
(10)
(10)
(10)
(10)
(10)
(10)
Arctic skua
04/06
45.14
1.80
70.93
63.23
0.78
Great skua
24
Study Site 1
Great skua
24
Study Site 2
Great skua total
48
2013
Great skua mean
2003 – 2012
Arctic skua total
mean
19.3
2003-2012
1
For both species we used the egg with the greatest volume (Furness, 1987).
Great skuas were considered fledged if still alive after 40 days; Arctic skuas were considered fledged if still
alive after 28 days.
3
Exception: 202 pairs monitored in 2003
2
Great skua
Lay dates
The average lay date of 24 May was the latest for five years. The first survey visit is timed to
ensure the majority of birds are incubating but the late settling of birds was easily observed
at the study sites this year.
Table 1 summarises the breeding statistics for the Great and Arctic skuas. The productivity
of Great Skuas on Handa Island was considerably lower in 2013 when compared with the
average productivity of years 2004 to 2012. Productivity for the Great skuas in 2013 was
0.15 which is the second lowest since 0.07 chicks fledged per pair in 2005. Productivity was
considerably higher in Study Site 2 than Study site 1. The second club site that has
6
established in site 2 regularly held up to 35 birds which placed predation pressure on
neighbouring territories.
Causes of Failure
In Site 1 only one egg was addled, seven predated, and two died hatching. 20 chicks were
never found all of which were <10 days. A further three disappeared after 10 days. There
were three Great Black-backed Gull nest located in Site 1, within several of the pellets, Great
Skua chick beaks were found.
In Site 2, one egg was addled, fifteen predated and three died hatching. 17 chicks
disappeared <10 days assumed predated, although not confirmed. Two of the territories
had smashed eggs on the nest and scattered fragment around the territory. A further four
disappeared and one found dead after 10 days.
This is the lowest incidence of addled eggs recorded during the study. Clutch size was the
lowest recorded since this study began in 2003 and hatching success nearly 20% lower, this
is largely accounted for by the high number of predated eggs in study site 2.
Table 2: Great skua mortality factors, showing proximate causes
of breeding failure on Handa Island in 2013.
Egg stage
Chick stage
Cause of
breeding failure
2013
(%)
Addled
Predated
Died hatching
Total egg loss
2.5
27.8
6.3
30.6
Chick loss <10 days
69.8
Chick loss 10+ days
Total chick loss
15
84.8
Table 3: Great skua mortality factors by study site in 2013
Cause
Egg
Site 1
Site 2
All
Addled
1
1
2
Predated
7
15
22
Died Hatching
2
3
5
17
3
20
Disappeared <10 days
3
14
17
Found Dead >10 days
0
1
1
Disappeared >10 days
3
4
7
Chick Predated <10 days
7
Diet
The diet of the Great Skua was not monitored this year. Casual observations were made in
territories. A wide variety of animal remains were found including Rabbit (Lepus curpaeum),
Crow (Corvus corone), Common Gull (Larus canus), Kittiwake (Rissa tridactyla) (both adult
and chick), Razorbill (Alca torda) (both adult and chick), Guillemots (Uria aalga) (both adult
and chick), unidentified fish species and Great Skua chicks. Several Great Skua chick beaks,
still with their egg tooth, were found in pellets across the island. The Arctic Skua diet
consisted of Rabbit, Sand eel, other fish species and Arctic Tern chicks (Sterna paradisaea)
(found uneaten at F7A1). Great Skuas were regularly see taking guillemot and kittiwake
chicks from the cliffs.
Arctic Skua
Causes of Failure
Three eggs were predated, three chicks were never found, and two chicks were found
predated on their own territory, one of which had been ringed on the previous occasion.
Ravens were observed being chased off territory G2A1 by the adult Skua, during the next
nest check one of the eggs were missing and there was no sign of chick activity. A rat was
also seen in a territory prior to an egg disappearing.
4.0 Ringing
12 Arctic Skua chicks were fitted with BTO rings four of which were also fitted with white
Darvic rings (with black lettering) and another four were fitted with black Darvic rings (with
white lettering). The remainder were fitted with BTO rings only. Ten Great Skua chicks were
ringed with both BTO and black Darvic rings (white lettering) and an additional chick with a
BTO ring only. Five great skuas were ringed within the study site and an additional three
birds out with the study site, this indicates that productivity was low across the island and is
the lowest number of Great skua pulli ringed since 2005.
A1
(a)
A1
(b)
Figure 3: The style of Darvic rings deployed on Great skua (a) only and
Arctic skua (a) and (b) pulli on Handa in 2013.
Since annual monitoring started in 2003, a total of 1088 Great Skuas have been fitted with
BTO rings and 751 colour-ringed. 169 Arctic Skua chicks have been fitted with BTO rings and
107 colour ringed (Table 4).
8
Table 1: The Numbers of Great and Arctic Skuas Ringed since 2003
Year
Great skua
Arctic skua
BTO
Colour
BTO
Colour
2003
106
0
27
0
2004
117
50
29
0
2005
22
22
12
12
2006
47
47
2
2
2007
121
88
13
13
2008
188
63
5
5
2009
138
138
24
24
2010
111
111
16
16
2011
113
112
14
14
2012
115
111
15
13
2013
10
9
12
8
Total
1088
751
169
107
4.1 Re-sightings and Recoveries
Great skua
Within the colony
Only five Great skuas ringed as chicks on Handa were re-sighted back on the island in 2013,
this is 75% less than in the previous two years and reflects the poor season of these birds,
three attempted to breed, including a nine year old returning for the third year, a six year
old and four year old also held territory. An additional two six year olds were recorded at
the Swaabie loch so it was not possible to record their breeding status.
A 2012 fledgling was found dead within its natal territory in Study site 1.
Away from the colony
Only three colour-ringed skuas were reported away from the Colony in 2013 and none of
these on migration. A 2007 birds on Coll, 2008 on Canna and 2009 on Skye all during June.
Arctic Skua
A four year old Arctic Skua (E3) successfully reared one chick. This was not one of the four
prospecting colour-ringed Arctic skuas seen in July and August 2012. These birds were not
seen on Handa in 2013.
9
25
2011
20
2010
2009
15
2008
2007
10
2006
2005
5
2004
0
2010
2011
2012
2013
Figure 4: Number of Great skuas colour-ringed as chicks returning to Handa 2010-2013
5 Discussion
The number of Great skua AOTs has declined 45% from 249 in 2010. However, observations
in 2011 and 2012 noted the density of the population and that all available habitat was
being used (Smith et al, 2011; Cunningham & Smith, 2012). Numbers of chicks ringed across
the island were consistent between 2010-2012 as was the number and density of pairs
within the study sites, compared to 2013 so this decline has occurred since 2012 not 2010.
March 2013 in the UK was colder than any of the winter months and the coldest for the UK
since 1962. Overall this was the coldest spring for the UK since 1962 (marginally colder than
spring 1979), and the fifth coldest in a series since 1910 (metoffice.gov.uk) Skuas were
therefore migrating into bad weather and arriving in poor condition. This was associated
with a negative phase of the North Atlantic Oscillation and cliff nesting birds on Handa also
had a delayed start to the season, approximately 3 weeks late. Seabirds across Scotland had
a poor breeding season this year (RSPB, 2013). 2013 breeding figures for other skua colonies
from JNCC are not available for comparison at the time of writing.
Site 2 was affected by high numbers of non-breeders at the club Site in I5 (highest recording
was 35 individuals). Nest Sites which were located around the club Site failed before the
eggs hatched. With some areas with high territory density, the risk of chicks wondering into
another pair’s territory is high and could result in predation (Jones, 2003). Site 2 traditionally
has the higher productivity due to the number of Great Blacked-back gull territories, this is
not the case. The same number of Great Black-black Gull nests were in Site 1 as 2012 (F5, E4
and C3) and no nests were located in Site 2.
As the all island count was carried out this year more of the colony was covered and this
increased effort should have picked up more colour-ringed birds if they were present.
10
Therefore the low number is assumed to be a true reflection of the low number of returning
birds present this year.
Unfortunately this year a rat population has re-established itself, this would not explain the
high number of early fails as the rats would be mainly concentrated around the coast.
However this cannot be ruled out completely as a single arctic egg disappeared a few days
after a rat was spotted close by.
The reduced numbers of Arctic Skuas in the colony leaves the remaining pairs vulnerable to
attack and predation by Great Skuas. Four pairs are isolated from the main cluster of nest
located on the dune system above Traigh Shourie where pairs have been observed working
together to defend from any threat. It is interesting to observe the location on where each
species nests; the Great Skuas prefer the longer grasses and heathers quite far from the
path where the most disturbance occurs. Whereas, the majority of, the Arctic Skuas on
Handa tend to nest in the shorter grasses and closer to the path. This could be the result of
displacement and the Arctic Skuas having a higher tolerance for disturbance.
The Arctic Skua population had a much higher productivity, this could be due to the low
number of great skuas exerting less predation pressure and the fact that when they arrived
on the island to breed the food source had arrived and was readily available for their
utilisation.
6 Funding
Scottish Wildlife Trust provided subsistence costs for Duncan Leckie and supported the
project by accommodating Duncan Leckie, Claire Smith and members of the Highland
Ringing Group.
7 Acknowledgments
We are grateful to Dr Jean Balfour, the Handa Island Management Committee and Scottish
Wildlife Trust (SWT) for their continued support of the project and allowing us the incredible
privilege of working on Handa. To Andrew Ramsay, Nigel Richards and the Highland Ringing
Group, for training, advice and supplying BTO and colour rings. Thank you to Thomas Plant
for creating the map. To Paul McDonald, Handa Island Ranger, for continuous moral support
and great company, along with fellow island volunteers Krista Kokina, Adam Butler, Frances
Kenward and others, always willing to help find elusive chicks and assist with ringing. To
Sven Rasmussen for supporting the project, thank you. Thanks also to Paul and Roger for
trips to the mainland. The project relies on the continued support and generosity of
everyone above. Finally thank you to all the short term volunteers who helped in the field
and provided entertainment and meals!
11
8. References
Coulson, J. C. (1963). Egg size and shape in the Kittiwake (Rissa tridactyla) and their us in estimating
age composition of populations. Proceedings of the Zoological Society of London, 140,
pp.211-227.
Cunningham, L., & Smith, C. (2012). The Breeding Ecology of Great and Arctic Skua On Handa Island
2012. Unpublished Report to SWT, SNH, JNCC and The Seabird Group .
Hamer, K. C. (2001). Catharacta skua Great Skua. BWP Update, 3(2).
Jones, T. (2003). The Breeding Ecology of Great and Arctic Skuas on Handa Island 2003. Unpublished
Report to SWT, SNH, JNCC and The Seabird Group .
Jones, T., Smith, C., Williams, E. & Ramsay, A. (2008) Breeding ecology and diet of Great Stercorarius skua and
Arctic skuas S. parasiticus on the west coast of Scotland. Bird Study 55: 257-266.
Jones, T., Green, R. & Smith, C. (2008) Handa Island Skua Monitoring Programme Final Report 2008.
Unpublished report to SWT, SNH, JNCC, SOC, The Seabird Group.
Jones, T., Green, R. & Smith, C. (2009) Handa Island Skua Monitoring Programme Final Report 2009.
Unpublished report to SWT, SNH, JNCC, SOC, The Seabird Group.
http://www.metoffice.gov.uk/climate/uk/summaries/2013/spring
Phillips, R. (2001). Stercorarius parasiticus Arctic Skua. BWP Update, 3(1), pp. 25-41.
RSPB. (2013). News [Online] Available at: http://www.rspb.org.uk/news/353892-scotlands-seabirdsin-need-of-lifeline-after-difficult-breeding-season [Accessed 24 September 2013].
Smith, C., & Jones, T. (2004). Handa Island Great and Arctic Skua Monitoring Programme Final
Report 2004. Unpublished Report to SWT, SNH, JNCC and The Seabird Group.
Smith, C, Culshaw, V, Turnbull, V & Jones, T (2010). Handa Island Skua Monitoring Programme Final Report
2010. Unpublished report to SWT, SNH, JNCC, SOC, The Seabird Group.
Walsh, P. M., Halley, D. J., Harris, M. P., del Nevo, A., Sim, I. M., & Tasker, M. L. (1995). Arctic Skua
Stercorarius parasiticus/Great Skua S.skua. Seabird Monitoring Handbook for Britain and
Ireland . Peterborough: JNCC/RSPB/ITE/Seabird Group.
12
APPENDIX Map of Handa showing study territories 2013.
13