Aarhus University Seal / Aarhus Universitets segl

No. 60: Breeding success of Oystercatcher, terns and gulls in the Danish Wadden Sea

Bregnballe, T., Thorup, O. Jensen, P.E., Nielsen, R.D., Pedersen, K.T. & Laursen, K. 2015. Breeding success of Oystercatcher, terns and gulls in the Danish Wadden Sea. Aarhus University, DCE – Danish Centre for Environment and Energy, 84 pp. Technical Report from DCE – Danish Centre for Environment and Energy No. 60. dce2.au.dk/pub/TR60.pdf

Summary

Background and aim

Breeding performance can be an important bottleneck in the life cycle of waders, gulls and terns and may influence where individual birds decide to breed in subsequent seasons. The assessment of breeding success and knowledge about the causes of breeding failure is therefore of great value when attempting to identify reasons for changes in local and regional population size. The parameter 'breeding success' can also be applied as an early-warning system to detect changes in the ecosystem or assess human impact, because it is sensitive to changing conditions in the environment. Knowledge about breeding success and the main factors affecting it is also important when discussing management options aimed at improving the breeding conditions for the species.

In the trilateral collaboration between the Wadden Sea countries (NL, D, DK) it was recognised that the lack of knowledge on 'breeding success' was an important gap in the monitoring programme of the Wadden Sea. It was therefore decided that breeding success should be incorporated into the set of parameters that should be monitored within the Trilateral Monitoring and Assessment Program (TMAP). The plan was to fully implement a system for monitoring breeding success of coastal birds in all three Wadden Sea countries from 2010 onwards.

The plan for monitoring breeding success was developed during the TMAP revision. According to this plan monitoring in the Danish part of the Wadden Sea should provide information about breeding success of Oystercatcher Haematopus ostralegus, Black-headed Gull Larus ridibundus, Herring Gull Larus argentatus, Sandwich Tern Sterna sandvicensis and Arctic Tern Sterna paradisaea on islands (not necessarily the same island for all species). The monitored parameter for breeding success in the International Wadden Sea should be the number of young fledged per breeding pair, for the selected species and study sites.

This report is a compilation of the results from the monitoring of breeding parameters and breeding success of Oystercatchers on Mandø during 2010-2011, of terns on Mandø during 2008-2011 and of terns and gulls on Langli mainly during 2009-2013.

Breeding success

Oystercatchers breeding on Mandø saltmarsh produced 0.26 fledged young per breeding pair in 2010 and 0.01 young per breeding pair in 2011. This difference was mainly caused by a higher loss of clutches early in the breeding season in 2011. A total of 46% of the clutches initiated in 2010 survived until hatching whereas only 6% of the clutches initiated in 2011 hatched. Most losses of clutches were caused by predation and flooding by sea water. The fledging success measured in 2010 reached 76%.

Common Terns and the Arctic Terns breeding on Mandø performed poorly during 2008-2011. It was estimated that the Common Terns produced fewer than 0.5 fledged young per breeding pair in 2008, less than 0.3 in 2009 and 2010 and no fledged young in 2011. The success of the Arctic Terns was estimated at a maximum of 0.1, 0.3, 0.2 and 0.1 fledged young per breeding pair in 2008-2011. The low breeding success was mainly due to losses of egg clutches. The cause of the extensive losses of eggs was not identified, except for a few cases where colonies had been partially or completely flooded by sea water. Few chicks hatched and their survival was low. The causes of chick mortality could not be determined. There were some indications that human disturbance had caused at least some of the losses of eggs and chicks.

Common Gulls bred with extremely low success on the island of Langli in all years between 2008 and 2013. The counts of large young indicated that as few as 1-2 young fledged per 100 breeding pairs. It appeared from the detailed studies in 2009 that most losses took place during incubation and were caused by predation of eggs, mainly by Herring Gulls.

Lesser Black-backed Gulls bred with high success on Langli in 2008-2010, with very low success in 2011 and with low-moderate success in 2012-2013. The pattern was almost the same for the Herring Gulls which bred with high success in 2008-2010 and 2013, with very low success in 2011 and with low success in 2012. The reasons for the low breeding success could not be identified.

Arctic Terns that attempted to breed on Langli had poor breeding success. The observations suggested that none of the breeding attempts during 2008-2013 resulted in successful production of fledged young.

Sandwich Terns apparently bred with high success on Langli in 2006 and in 2008-2010 at least until the chicks became 5-12 days old. Later visits confirmed that chick survival was high during these years. Hatching success was reasonably good when studied in 2009.

Evaluation of methods

The monitoring of breeding success of Oystercatchers on the saltmarsh of Mandø successfully applied the recommended standard methods to determine hatching and fledging success. Breeding performance was recorded with high precision and the main causes of egg losses were identified. Furthermore, the monitoring could be carried out at a fairly low cost.

Attempts were made to monitor hatching success of terns breeding on Mandø by marking individual nests. This method was changed in subsequent years because the marking attracted the public and the other methods used gave only rough measures of breeding success.

The monitoring of hatching success on the island of Langli by following individually identifiable nests was very costly in terms of manpower. Furthermore, the Danish Nature Agency found that this method required an unacceptable high number of visits to the breeding areas. As an alternative a combination of data collected during ringing of chicks and counts of young near the age of fledging was used. This only provided rough estimates of year-to-year variation in the production of fledglings.

Future monitoring

According to the plan for monitoring of breeding success in the International Wadden Sea, Denmark should provide information about breeding success of Oystercatcher, Black-headed Gull, Herring Gull, Sandwich Tern and Arctic Tern on 1-2 of the Wadden Sea islands.

For Oystercatcher we recommend that future monitoring of breeding success covers breeders on Mandø saltmarsh by use of the same methods as in 2010 and 2011. Two other potential study sites could be included.

The Black-headed Gull colony on Langli has become very small in recent years and we do not recommend a full survey of this colony. Furthermore, monitoring of breeding success in the colony in ‘Sneum Klæggrav’ on the mainland coast - having 90% of the breeders in the Danish Wadden Sea – would disturb the colony to an extent where it would influence the overall production of fledglings in the colony.

For Herring Gull we recommend that breeding success on the island of Langli is monitored using a combination of standardised ringing and counting of chicks.

Sandwich Tern is no longer breeding in the Danish part of the Wadden Sea but may return in coming years.

Due to recent declines in breeding numbers of Arctic Terns on Langli and Mandø there are at present no sites in the Danish Wadden Sea that are suitable for monitoring breeding success of this species. Large colonies (with 150-200 pairs) are currently only found on Koresand and on the roof of a building in the harbour of Esbjerg. It would be a logistic challenge to monitor breeding success in the colony on Koresand and monitoring of the colony in Esbjerg Harbour would not provide information which is representative for the breeding conditions in natural Wadden Sea habitats.