31/05 Thatcham Marsh LNR

Location: Thatcham Marsh LNR CES 2026.04

Session:   31/05/2026 06:00 to 11:00hrs

Present:   JHW, IW, RAD, JL, JA.

Weather: bright sunny, calm, hot.

Nets: 198M Reed Ride (site A) + 1 x12M separate rides set in scrub (site B) up from 06:30 to 10:30hrs.

Lures (none): 

Notes: The first cooler day after a very hot period. As can be seen below the total for visit CES 2026.04 is a little better than the 2025.04 visit, particularly for Reed Warbler. However, for the time of year the reed bed still seems quieter than usual - fewer Reed and Sedge Warblers singing and very few other species seem to be venturing onto the reed bed than in the recent past. To test these perceptions below are a couple of charts that plot the totals for most years the group has been in existence.

Provisors are:

·       In the early days until the 1980s ringing on the marsh was less structured, the number of nets erected or there locations not recorded.

·    Site names were general for much of the marsh until the 1980s when specific areas ringed were recorded including the number and location of nets erected.

·     Ringing was less regular and was alternated with other sites such as Hambridge Lake and at Thatcham south of the railway. On occasions this meant that ringing was at best sporadic and some years few visits were made due to poor weather and the alternation.

·      The number of active ringers (effort) has also been a factor; numbers peaked in the early 1970s at 18 members then dropping to four in the early 1980s before peaking at again in 2010 at 14 active members – currently there are usually between four and six ringers in attendance.

·    All records still in existence for Thatcham Marsh have been added to the data system back to 1973 prior to this there are gaps due to data not yet being entered or no longer in existence and which may account for the lower numbers for this period.

·    In the mid-1980s ringing on the Thatcham LNR was prioritised from April until the last warblers left late Oct – Early Nov and on every visit each net ride was recorded including species encountered therein. In 1993 were joined the British Trust for Ornithology’s Constant Effort Site (CES) scheme that runs from late Apr to early Sep when 12 “official” visit limited to the reed bed ride of 198metres of net are made. Ringing in between and outside these visits also follows this structure including any peripheral nets rides erected.

The first chart shows the yearly totals for the primary species ringed Reed Warbler and Sedge Warbler. From 1989 numbers (with exceptions) become more consistent as we adopted CES type ringing methods. With some exceptional years it shows a steady decline in numbers of both species from 1990 and as perceived by us

The next chart indications species diversity, again the overall trend is downwards. Some account for this may be due to the number of nets erected at any one time and  their location also the loss of reed beds due to gravel extraction etc, and the steady encroachment of scrub into the reed habitat over the period. Prior to the mid-1960s passing stream trains would sometimes throw their fires. Setting fire to the nearby phragmites reed that would spread over the reed beds and curtail the encroachment of scrub growth. This probably accounts for the survival of the extensive reed habitat along the Kennet valley to the late1960s when steam power was replaced.

Session Capture details.

Recaptures: (13)

Blue Tit ringed: 23/05/2026

Cetti’s Warbler ringed:11/08/2024, 29/06/2025

Chiffchaff ringed 02/04/2023

Reed Warbler   ringed: 03/07/2022, 11/06/2023 03/05/2025, 10/05/2025x2, 02/08/2025, 18/05/2025, 26/04/2026, 02/05/2026

Sightings: Less activity this session, still some species singing but some have gone quiet – Garden Warbler? The juvenile Robins and Blue Tits  also a couple of tit families in the scrub towards the end of the session show that species are now fledging including some summer migrants – Blackcaps etc. The change in the weather to unsettled meant that fewer insects were out and about and for the first time this year more Banded Demoiselles than Beautiful Demoiselles seen.