A hare springs away over the swell of the hill as we drop down into the wood. On this bright morning, a mistle thrush flings its clear song on to the breeze. Wild garlic shines emerald beneath the trees and woodrush thrusts through fallen beech leaves. Other signs of spring: arrow-shaped lords-and-ladies, pale green flowers of dog’s mercury and, on a sheltered bank, the first primroses.

There’s a feeling of movement, of growth, of upward vitality. The thing that jars is that which is not alive: the twisted, distorted tubes of long-ago tree guards, redundant now that the trees have grown. They cling on, cloven in two, forced apart by bark, still attached to mossy stakes by black ties. Or tumbled and half buried in grass and soil, where they will remain for many years. In an act of guerrilla tidying, we gather a load of the split, broken, battered plastic and pile it under an old piece of wriggly tin. These pieces will still break down into microplastics, but removing them frees the struggling trunks.

This is just a small copse in Northumberland, but this scene is replicated across the country. In this last week I’ve seen: sand-coloured straps of plastic like giant tagliatelle wrapped round a hazel on a nature reserve; a roadside red-stemmed dogwood half-throttled as it tries to throw off its burden; and a top-heavy hawthorn hedge, its marching line of white tubes masking a barren base (to be stock-proof, a hedge needs to be dense right from the base).

Tree guards are used to protect new plantings from rabbits, hares, voles and an ever-expanding deer population. They are, in theory, reusable or recyclable, but few are collected once they are no longer needed, and probably damaging the tree and littering the countryside. Biodegradable alternatives being trialled by the Woodland Trust, the National Trust and the Tree Council are made from materials such as wool or cardboard.

I’m heartened by two Northumberland farmers. One does indeed collect the tubes and offers them for reuse, and these are snapped up on Facebook. The other is planting a mix of species within wooden post-and-rail cages without individual guards.

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The sweeping edge of the Pennines at Geltsdale is a cathedral for birdsong on a still spring evening. Everything from thrushes to curlew are calling from this diverse mix of heather moorland, resurgent scrub, rough grassland and pools of water around a rewilded stream.

The conditions on this large nature reserve are perfect for the rare hen harrier to thrive and conservationists hope that this year it will. But there remain fears that illegal persecution will continue to hinder the rare raptor’s recovery.

“There are loads of birds about but they are not lasting very long,” said David Morris, the RSPB area manager for Cumbria and north-east England. “It’s like the Bermuda Triangle for birds when they leave the reserve.”

Last spring, RSPB Geltsdale hosted two successful pairs of nesting hen harriers for the first time since 1999. But a satellite-tagged male bird was found shot dead on a neighbouring grouse moor and the tag of one of five fledglings stopped transmitting in a known hotspot for raptor persecution. Ecologists calculate there could be nine nests on the reserve if there wasn’t so much illegal persecution in the surrounding uplands.

According to 2023 figures released on Monday, the UK and Isle of Man hen harrier population has increased by 20% from 545 territorial pairs in 2016 to 691 pairs. In England, there were 50 breeding attempts in 2023, up from just four pairs in 2016, although numbers are still below the 749 pairs recorded in 2004.

Hen harriers are bouncing back – but illegal persecution is increasing too. Combined Natural England and RSPB data shows that 32 satellite-tagged hen harriers vanished or were confirmed as having been illegally killed in England in 2023 – the highest recorded number of hen harriers killed or to disappear suspiciously in one year.

Newly fledged birds continue to disappear over grouse moors and other shooting estates, and hen harriers remain absent from swaths of suitable habitat in England, including the Peak District and the North York Moors. According to the latest RSPB Birdcrime survey, 71% of confirmed incidents of birds of prey persecution occurred on land managed for game bird shooting.

Hen harriers are targeted because they are seen to consume large quantities of red grouse chicks, which grouse moor managers prize for the lucrative driven grouse shooting season.

Hen harriers’ preferred diet is often voles and on Geltsdale this year conservationists are hopeful that conditions are ideal for hen harriers to thrive. Their security is bolstered by the birds being satellite-tagged by the RSPB as well as Natural England, the government’s conservation watchdog.

“It’s looking like a good vole year,” said Morris. “We want to see a good year and the reserve sustaining four pairs free from any outside interference or illegal persecution. Everything is in the hen harriers’ favour this year. A lot of them have got satellite tags on them so we know there are birds about and we know where they are. We just hope people leave them alone. We’re not asking for much – just the law to be upheld and the police to take it seriously when it isn’t.”

The rising numbers of hen harriers has been welcomed by shooting interests as a sign that the government’s recovery plan – controversial “brood management” – is working.

Brood management enables shooting estates that host multiple breeding hen harriers to have chicks removed from wild nests, raised in captivity and released elsewhere. The process is aimed at preventing shooting estates from being inundated by hen harrier nests and therefore reducing the pressure on gamekeepers or others associated with the estates to potentially commit wildlife crimes.

Andrew Gilruth, the chief executive of the Moorland Association, said: “Fifty per cent of hen harrier habitat in the uplands is managed for grouse shooting yet grouse moors host 80% of their nests – a very significant conservation contribution.

“It is disappointing that the RSPB still can’t acknowledge the extraordinary success of Defra’s hen harrier recovery plan, which has increased the English population to a 200-year high in just five years. The RSPB may be frustrated that grouse moors have more harrier nests than all its nature reserves combined but either way, if it has found evidence of illegal activity it should do what everyone else does, take it to the police.”

At Geltsdale in May 2023, one nest was abandoned when the male bird provisioning it, called Dagda, was shot dead. The bird’s satellite tag led RSPB investigators to find the body on neighbouring Knarsdale moor. The postmortem concluded it died instantly or a short time after being shot.

Northumbria police have made no arrests relating to the incident and the RSPB has submitted a complaint about their investigation. It is understood that Northumbria police are currently considering the complaint.

A spokesperson for Knarsdale Estate said: “We take the protection of wildlife extremely seriously and were deeply concerned when we were made aware in May last year of a fatally injured hen harrier. To be clear, no one from the estate was involved in this incident and RSPB confirmed this in its Birdcrime Report issued in November 2023 stating: ‘for the avoidance of doubt, there is no suggestion that the landowner, agent or any employee was involved in any way.’

“The estate deplores any form of wildlife crime and has happily had satellite-tagged hen harriers present on and over its land for years without any issue, and we continue to have a healthy and diverse raptor population.”

A Northumbria police spokesperson said: “We received a report on 11 May last year that a hen harrier had been found dead in countryside in the Haydon Bridge area of Northumberland.

“It sustained injuries consistent with being caused by a firearm. Anyone with information should use the ‘report’ page of our website or call 101, quoting NP-20230511-1263.”

According to RSPB staff, because shooting estates are able to access the satellite-tag location data of captive-reared birds once they are released under the brood management scheme, the satellite tags that should deter illegal persecution in this instance can lead people who want to harm the birds straight to them.

The Scottish government recently passed a bill introducing licensing for grouse shooting in Scotland; the RSPB and other conservationists want similar legislation in England.