Warblers Caught on Holt Island 2020 – 2023

The numbers of birds caught at any site can vary between years for a number of reasons, including weather effects; variations in effort that is put into catching the birds; long term climate change; vegetation changes and more recently the pandemic. Between 2020 and 2023 we have tried to keep the level of effort as constant as possible, but there were minor issues affecting this due to flooding and bad weather. The pandemic also had an effect on when we could or could not go out.

In 2023 wide strips were cut in the large reed bed at the western end of the Island linked by a path. This allowed us to put nets up in the middle of the reed bed for the first time. This resulted in an increase of the numbers of Reed and Sedge Warblers caught in the early part of the season. Later on, when the reeds grew back, it became too difficult to get in to this area.

By way of comparison with ringing data, Dr Tim Reed undertook a Breeding Bird Survey in 2022 and 2023. This helps give us an idea of the number of pairs of warblers that might have bred on the Island. Because of the survey method, this is normally a lower estimate than the actual number of pairs for some species.

The table below gives the new birds caught each year. It does not include any re-trapped birds.

Cetti’s Warbler

In the early part of the breeding season in each year at least one, or possibly two males were singing regularly.

Only in 2022 was there evidence of successful breeding with four juveniles being caught. However by June 2023 they were not heard regularly and none were caught suggesting they had left the site. No young were caught in 2023.

Reed Warbler

The numbers of new Reed Warblers caught in 2022 showed that very few juvenile birds fledged, this was reflected in the numbers caught in 2023 with fewer being caught. The numbers of adults would not have been so high in 2023 had the new areas in the reed bed not been available at the beginning of the breeding season. In 2023 the numbers of young caught was similar to last years with 20 caught each year. These lower numbers were also reflected in the Breeding Birds Census.

Sedge Warbler

Sedge Warblers tend to inhabit the edges of reed beds whilst the Reed Warblers use the entire reed bed. In 2023 the only Sedge Warblers caught were on the edges of the cut areas of the main reed bed. Once they regrew it was not possible to monitor whether any of the pairs were successful in breeding, so the picture was rather incomplete. One bird from previous years was re-trapped and one juvenile was caught.

Blackcap

In 2023 the numbers of Blackcaps were lower across the UK, it is not known why. The ringing figures suggest a gradual reduction in numbers over the last four years. Only 14 juveniles were caught in 2023, exactly half the number caught in 2022. The Breeding Bird Survey indicated at least seven pairs in 2023.

A male Blackcap ringed on Holt Island as a first year in May 2022 was caught twice at Jews Gate in Gibraltar by local ringers in October 2022. Two were caught on the Island in November 2023, indicating possible overwintering in this area due to climatic warming. More generally Blackcaps seem to overwintering further north than they used to.

Chiffchaff

Chiffchaff numbers seem to have been stable throughout the period. They are caught between March and December so are present most of the year. However a lot of the birds caught are migrating, using Holt Island as a feeding station, particularly in the autumn. They breed on the Island but from the Breeding Birds Survey only three pairs bred in 2023. We have had some birds caught away from the Island (”controls”) with one being caught in the autumn at Beachy Head.

Willow Warbler

No Willow Warblers were caught in 2023. In 2022 we happened to be catching at a period when there were many Willow Warblers and Chiffchaffs passing through. These stopped for a short while to feed up before then moving on. A few were seen and heard during the autumn of 2023.

Garden Warbler:

In most years one or two pairs attempt to breed on the Island, this year was no exception. But while last year they were successful this year they were not. No Juveniles were caught in 2023.

Thanks to Dr Tim Reed for editing and amending the note.

Julian Limentani

December 2023

photos © Sue & Julian Limentani