Survey of five gull breeding sites

The Eolien en Mer SIG initiated its first study in 2021. The project involved multi-year monitoring of five natural gull nesting sites. The aim is to show the trends in the number of nests and the production of young, particularly in relation to the future construction of the Dieppe Le Tréport offshore wind farm.

From one year to the next, monitoring of the birds in these colonies should reveal whether they are staying on already occupied sites, whether they are dispersing or abandoning the sites and whether they are moving to new breeding sites.

Background: herring gulls in Normandy and on the Caux cliffs

The herring gull is the most common breeding seabird in Normandy, with colonies found all along the Seine-Maritime coast, particularly on the Caux cliffs (map below).

However, the development of the colonies is highly disparate, with an increase in numbers in urban areas and a strong decline in colonies in natural areas (map below).

From the sample of sites represented by all the reserves of the Normandy Ornithology Group (GONm), the number of breeding pairs of herring gulls diminished from 7700 in 1998 to 4700 in 2010 and down to only 1200 in 2019. This suggests that the total breeding population on natural sites, which numbered almost 18,000 pairs in 1998, must now number no more than 3000.

The steep cliffs of the Caux coastline are nearly 110 kilometres long and inaccessible enough to allow a small population to have always remained there, even at the beginning of the 20th century when the species had almost completely disappeared from France.

Since then, almost all herring gulls that breed on the Pays de Caux coastline build their nests on the inaccessible or almost unreachable cliffs. Until a few years ago, a large proportion of them nested on the shingle strands in some areas at the foot of the cliffs. The gradual disappearance of these shingle clusters has led to the almost total disappearance of the gulls breeding in this type of environment.

Survey and monitoring by the Éolien en Mer GIS

In the prefectural decrees of 28 February 2019 authorising the construction of the Dieppe Le Tréport wind farm, the Conseil National de la Protection de la Nature (National Nature Conservancy Council – CNPN) asked the Éolien en Mer GIS to identify and monitor the development of several natural herring gull breeding sites.

Several colonies on the Caux coast are mentioned in the decrees:

The three colonies proposed by the CNPN are located approximately 20, 80 and 100 km from the future Dieppe Le Tréport offshore wind farm (map below).

As the GONm is already in charge of monitoring most of these breeding sites, the Éolien en Mer GIS commissioned this organisation to carry out the study.

Localisation des sites inclus dans l’étude

Literature study of historical development

Firstly, a literature study of the historical development of the colonies will determine the level of saturation of sites in terms of available space, taking the limit of morphological changes to the cliffs into account. If the shingle strand disappears and/or if large landslides occur, the number of available sites may decrease.

Monitoring of censuses and production of sites

Over at least 8 years, adult and especially young birds will be counted and ringed. From the rings, we will be able to determine the links between natural and urban colonies, the production of young and feeding grounds.

Monitoring of birds at these three colonies, together with the monitoring planned for Dieppe, Le Tréport and Criel-sur-Mer, should highlight any dispersal behaviour, abandonment of sites, resettlement on new breeding sites or staying on already occupied sites from one year to the next. These movements between sites will be related to possible changes in access to food, disturbance due to the wind farms and the distance from them.

Historical monitoring of herring gulls breeding on the Caux cliffs

The number of gulls nesting on the Caux cliffs increased greatly between the late 1970s and the late 1990s. However, the number of breeding pairs almost halved between the 1998 and 2009 censuses, from 12,575 to 7,416 pairs. This trend has continued and in 2021 there were only 2,409 pairs on the Caux cliffs.

This very steep decline in the number of breeding pairs can be linked to several factors:

In 1986, 24% of gulls nested on the banks at the base of the cliffs, whereas in 2021 this figure was 3.9%.

The situation of the herring gull on the Caux cliffs (nesting in the natural environment) is catastrophic and, in common with what is happening elsewhere in Normandy, especially in the GONm (Normandy Ornithological Group) reserves of Chausey and Saint-Marcouf, the drop-off in numbers is particularly spectacular.

The three sectors selected by the CNPN have been affected by this same decline and there seems to be no way of counteracting this long-term trend.

The only positive point is the recent stabilisation at a low demographic level; however, examination of the curves shows that while there have already been some surges, the underlying trend has not been reversed.

Thank you to the GONm’s many volunteer and salaried observers for this time-consuming collective fieldwork!

Results of 2021 annual monitoring

In anticipation of the ringing campaign, the three colonies were surveyed in order to locate the nests where captures could be planned.

It was found that, unfortunately, the three colonies no longer hosted breeding herring gulls, as the birds had abandoned the shingle banks. There are several reasons for this desertion: the disappearance of shingle banks (as at Berneval-Le-Grand), regular marine submersion of shingle banks (in the case of Cap Fagnet and Cap d’Antifer) and egg predation. The birds have moved to other, safer sites such as cliff faces or urban areas.

The ringing project therefore could not be undertaken and will not be undertaken on these colonies.

Regarding the production of young, the colonies show a poor to average rate for the number of fledglings per nest (between 0.31 and 0.79). Continued monitoring of chick production will allow us to see the change in the quality of the environment.

Given the results of the 2021 monitoring, the Scientific Council of the Éolien en Mer SIG has therefore decided to continue with the census and monitoring of the production of young at the Bracquemont, Berneval, Belleville, Cap Fagnet and Cap d’Antifer colonies. However, the urban colony of Le Havre will be ringed in order to monitor a “control” colony in comparison with the colonies closer to the future Dieppe Le Tréport wind farm.

Results of 2022 annual monitoring

The year 2022 saw a vast epidemic of Avian Influenza which caused the death of hundreds of herring gulls on the Normandy coast and in particular at the colonies targeted by the monitoring. The GONm has therefore decided not to carry out ringing of gulls in the region, in order not to increase the risk of the virus spreading within the colonies and to the farms, and of transmitting the virus to operators.

Regarding the production of young, i.e. the number of fledglings per nest, the colonies show poor rates (all below 0.2). These results are also partly related to the avian influenza epidemic.

In order to compare the evolution of the colonies on the cliffs and in the city, the numbers of the colonies in Dieppe and Le Tréport will be included in the study in 2023. An assessment of the pre-construction period of the Dieppe Le Tréport offshore wind farm will be carried out.