Scarce passage migrant that has bred.

Sedge Warblers are badly affected by droughts in the Sahel zone of the southern Sahara, where they winter. 

The 1973 Breeding Atlas did consider breeding as possible from five of the tetrads covering Stevenage, the 1992 Atlas confirmed breeding from the tetrad covering Aston and, the 2012 Atlas considered breeding as possible from the tetrad covering Norton Green.

In the 1950s birds were seen at the old sewage works site, Roaring Meg.

The records for the 1960s and 70s are: one singing at Boxbury Farm, next to Box Wood 18 July 1965; one seen at the Pond in Watery Grove 8 June 1967; one at Box Wood 22 September 1968; one at Monks Wood 13 May 1969 and 3 May 1970: Box Wood 9 May 1970; one at Watery Grove 12 and 16 May 1971; two seen behind the former John Lewis Warehouse (now Costco) in Gunnels Wood Road August 1976; up to four at Fairlands Valley Lakes 2 May 1977; Watery Grove during the 1977 breeding season; Fairlands Valley Lakes 5 May 1979; Astonbury 9 May 1979.

During the 1980s and 90s the records were: one heard at Watery Grove May 1980; one ringed at Box Wood July 1980; two pairs seen feeding young at Ridlins Mire 14 June 1981 and, four males singing there on 12 July; a bird present in Watery Grove during the 1985 Breeding Season; one at Fairlands Valley Lakes 28 May 1989; one heard calling from a small reedbed at Fairlands Valley Lakes during May 1994; one seen on spring migration in Ditchmore Lane in 1995.

They were not recorded again until one was at Fairlands Valley Lakes on 6 May 2002.

Between 2010 and 2019 the records are: one Fairlands Valley Lakes 27 April 2010; one along the River Beane at Aston 16 August 2013; one at Norton Green Tip 7 September 2016 and 21 April 2017; one at Fairlands Valley Lakes 30 April 2018.

Since 2000 the records are; one at Fairlands Valley Lakes 21 April 2020; one at Fairlands Valley Lakes 22 and 28 April 2021; one at Fairlands Valley Lakes 16 September 2021; one in a Fairview Road Garden 17 September 2021; one at Fairlands Valley Lakes 30 May 2022; one at Fairlands Valley Lakes 29 April 2023; one along the River Beane at Aston 29 April 2023; one at Fairlands Valley Lakes 29 April 2024; one at Fairlands Valley Lakes on 16 August 2025.

Scarce passage migrant.

Because of lack of suitable habitat, Reed Warblers are scarcely seen at Stevenage.

Between 1979 and 2015 there were 10 records: five passage birds ringed at Box Wood between 12 August and 23 September 1979; two passage birds ringed at Box Wood on 1 July and 1 August 1980; one at Astonbury on 14 May 1989; one at the now demolished Angotsmead House in Meadway on 7 April 1996; a singing bird in a garden in 1997; one at Fairlands Valley Lakes on 26 April 1997; one in Ditchmore Lane on 29 May 2008; one in the Old London Road behind the Magistrates Court on 20 and 21 May 2009; one at Norton Green Tip on 28 April 2010; One at Fairlands Valley Lakes on 29 July 2015.

With no records between 2015 and 2021 there has since been an increase in records with: one at Fairlands Valley Lakes on 28 and 29 April 2021; two at Fairlands Valley Lakes on 2 May 2021; one at Fairlands Valley Lakes on 7 May 2021; one at the River Beane, Aston on 14 June 2021; one at Fairlands Valley Lakes on 7 June 2022; one at Fairlands Valley Park on 27 May 2023; one at Fairlands Valley Lakes from 9 to 16 June 2023; one at Fairlands Lakes on 25 and 26 July 2023; one at Norton Green Tip on 3 September 2023; one singing in a Fairview Road Garden on 18 May 2025.

A ring of a Reed Warbler ringed at Thrybergh, South Yorkshire on 23 August 1990, was found at Stevenage on 22 May 2011. Whilst Reed Warblers can live for over 10 years, as there are no details of how the ring was found it cannot be assumed that the “ringed” bird was over 20 years old.