Trees being felled at the Thames Water site. Photograph: Janet Pearce

A utility company is in hot water after residents spotted trees being felled during nesting season.

Keen swimmer Janet Pearce quizzed the people felling trees on a Thames Water site near the Castle Climbing Centre and the West Reservoir, and not far from Woodberry Wetlands nature reserve.

She was told that Hackney Council had given permission and that there were only “old nests” without eggs  in the trees.

The workers said they were being “really careful”.

Another passerby also asked the workers to stop.

Pearce has asked for evidence and raised her concerns with Thames Water.

“Why are they doing it in May?” she said. “It’s really important. It’s ironic that it’s near a nature reserve opened by Sir David Attenborough.

“Birds are disappearing at a rate of knots, but we keep doing this and ignoring the fact that it’s nesting season.”

She added: “I’m aware that it’s disastrous for the bird population.”

According to the RSPB, the main nesting season runs from February to August.

Tamara Micner also challenged the water company about the timing of its work.

In response to her complaints, Thames Water tweeted that “the works taking place have been surveyed and set up appropriately and also approved by the local authorities”.

It added: “The survey was completed to ensure no wildlife is affected and the job is completely legal.”

A Thames Water spokesman said: “Following discussions with nearby residents, we agreed to fell some trees on the site which were blocking sunlight in the gardens of neighbouring properties.

“We worked with the local authority on this, but when we later became aware of nesting activity we immediately stopped the work.”

He said a work plan was first raised in January and was “awaiting approval from the local authority”.

The work will now resume after nesting season, not before October.

Thames Water has been approached for further comment. Hackney Council has also been approached.