Biodiversity

A QUICK TOUR OF THE BIODIVERSITY OF CORSTORPHINE HILL

By Ian Moore, Friends of Corstorphine Hill committee member with responsibility for wildlife liaison, and author of photoblog “Chilloutdoors”.

BIRD FACTORY

In 2006 I began systematically recording the wildlife of Corstorphine Hill, visiting on an almost daily basis. I was immediately struck by the abundant bird life. As a rule of thumb, I judged there to be about twice as many species, and four times as many birds compared to ‘typical’ countryside. In the peak year of 2016, 45 species bred on or in close association with the hill, comprising an estimated 750 pairs.

So, why so many birds?

A simple answer can be arrived at by looking at what we do to make our gardens bird-friendly. We put out food and we hang nest boxes. Something to eat, somewhere to breed; this is all birds ask of their environment, and they are remarkably tolerant of human intrusion.

FLORA

I began to survey the flora of the hill. I was astounded to find 43 species of trees and 26 species of shrubs. To date, I have identified 125 species of flowering plants and there are about a dozen grasses on which I am still working!

The latter include such exotic-sounding herbs as Perforate St John’s Wort, Hemp Agrimony and Climbing Corydalis, while the list incorporates a remarkable 19 species that are recognised as ‘marker plants’ for ancient woodland, such as Wood Anemone, Pignut and Dog Violet.

A vast array of fungi can be found throughout the year (but particularly in autumn), and these include some of the more distinctive species like Artist’s Fungus, Black Bulgar, Common Stinkhorn, Death Cap, Earth Star, Fly Agaric, Jelly Ear, Lawyer’s Wig, Parasol Mushroom and Razor Strop.

So here is part of the answer.

There is an unprecedented diversity of flora (taking a liberty with the classification of fungi), and this is at the root of a plentiful and varied food supply. Not only in spring and summer when birds are breeding, but also in winter. Redwings come for the berries of Holly, Crossbills for the seeds of Scots Pine and Norway Spruce, Siskins for Larch, and Redpolls for Alder and Birch. During ‘mast years’ hundreds of Woodpigeons gorge upon the seeds of Beech.

INVERTEBRATES

The richness of flora is also the basis for a copious population of invertebrates, most notably insects and spiders. Many birds are at least part-carnivorous, and this is most evident in spring when adult Starlings load their beaks with hatching St Mark’s Flies, and in early autumn when passage migrants such as Spotted Flycatchers, Swifts and House Martins stop off to feed on airborne insects.

The invertebrate population sustains many resident species, from Goldcrests to Jackdaws to Great Spotted Woodpeckers, along with Swallows and four species of warblers that come from Africa to breed: Blackcap, Chiffchaff, Whitethroat and Willow Warbler.

HOLES, HEIGHT & HIDEAWAYS

The second key plank in explaining the avian abundance is found in the trees – and this is from the perspective of nesting rather than food. 13 of the bird species that breed on the hill are hole-nesters. These range from Treecreeper to Stock Dove, and from Great Tit to Tawny Owl.  For holes, it is necessary to have old trees. They lose branches and they grow cracks, folds and crevices. Using a method called Mitchell’s Rule it is possible to age a tree from its girth; some of the larger Oaks and Beeches may be three hundred years old (noting that ‘Beechwood House’ on the south side was built in 1780). In 2021, a single ancient beech tree was home to Blue Tit, Great Spotted Woodpecker, Jackdaw, Kestrel, Mistle Thrush, Nuthatch, Stock Dove and Woodpigeon – a nature reserve in its own right! Large, tall trees are also essential to provide nesting sites high in the canopy for birds such as Buzzard, Carrion Crow, Mistle Thrush and Sparrowhawk.

This aspect – of suitable breeding sites – extends to the shrubs and undergrowth, most notably the extensive patches of gorse, brambles and willowherb. This dense, almost impenetrable habitat provides protected nesting for as many as 16 species of birds, such as Dunnock, Greenfinch, Song Thrush and the comparatively scarce Whitethroat, as well as hosting a locally important population of Long-tailed Tits.

ANIMAL SOUP

The exceptional biodiversity of the hill is reflected in other classes of animals, such as butterflies and mammals.

17 species of butterflies have been recorded, and there are indigenous populations of Comma, Green-veined White, Meadow Brown, Orange Tip, Ringlet, Small Copper and Speckled Wood, along with day-flying Cinnabar and Silver Y moths. Other moth species identified include Ghost Swift, Light Emerald, Peppered Moth and Poplar Hawk. Around 20 species of bees and wasps are present; currently three of the latter breed in the stock of a single rotting beech tree.

For a suburban site there is a remarkable mammal population, of about 15 species, which includes Badger, Common Shrew, Fox, Long-tailed Fieldmouse, Natterer’s Bat, Pipistrelle Bat, Rabbit, Roe Deer and Short-tailed Vole. Despite the lack of standing water, amphibians are represented by Common Frog and Common Toad, and actually manage to breed in damp spring seasons. It should be noted that the four species of birds of prey found on the hill owe their existence to several of the aforementioned birds and mammals!

ACCIDENT OF HISTORY

If the ‘first level’ answer to the question, ‘Why the great diversity of birds?’ is the diversity of plants, fungi and invertebrates, the ‘second level’ question is ‘Why the diversity of plants, fungi and invertebrates?’

A clue to this can be found by looking at the golf courses that border the nature reserve. Nice and green-looking (no pun intended), and a far preferable buffer than a housing estate, but pretty much a monoculture. Rarely are there any birdies on the fairways, whereas the adjoining zebra enclosure often has a mixed flock feeding.

The hill owes much of its biodiversity to what it has never suffered: the spraying of agrochemicals; herbicides, fungicides, pesticides.

It is probably as simple as that.

In contrast, the hill’s wildlife has been greatly influenced by human hand. Old maps and traditional lore suggest it was predominantly farmed for sheep before the advent of the main woodland in the 19th Century, and was certainly hunted over from the various estates (Beechwood, Clerwood, Craigcrook, Hillwood and Ravelston) which shared its territory. Many of the tree species are likely to have been introduced as part of a Victorian fashion for the arboretum, and more recently shrubs have escaped from adjacent gardens, not always to good effect.

NOT EVERYTHING IN THE GARDEN IS ROSY

In the time I have been watching closely, the hill has lost the following as regular breeding species: Collared Dove, Green Woodpecker, Kestrel, Linnet, Pheasant, Pied Wagtail, Rook and Yellowhammer.

Edinburgh Natural History Society records from the 1970s show Hawfinch, Spotted Flycatcher, Tree Sparrow and Wood Warbler all formerly bred on the hill.

Numerically, therefore, the hill has lost almost a quarter of its breeding birds in species terms.

The hill’s birdlife in particular, and biodiversity in general, is declining because of three factors.

First, you may notice that many of the lost species are primarily birds of what might be described as ‘rich farmland’. Throughout Europe, and indeed the world, farming practices have led to a catastrophic collapse in bird populations. The RSPB estimates that Britain has lost 44 million birds since 1966. In France one third of its birds have disappeared in the past 15 years. In North America the figure is estimated at a jaw-dropping 3 billion. When fringe species that live on ‘island’ reserves like Corstorphine Hill die of natural causes, there simply aren’t the populations in the surrounding countryside to replenish their empty territories.

The second factor is the relentless residential development taking place in Edinburgh. As old houses, hotels and hospitals are converted into flats, large areas of rambling gardens are lost, degrading the natural value of the ‘apron’ or green buffer zones and wildlife corridors that help sustain populations on the reserve itself.

And, finally, the third factor is an internal one, that of encroachment by invasive species. Salmonberry in particular is aggressively crowding out the ‘native’ flora, and could eventually blanket the entire hill, preventing even tree regeneration. Meanwhile, the precious gorse which sustains many classes of wildlife, is highly susceptible to damage from fires, started both carelessly and in some cases maliciously.

CONSERVING OUR HERITAGE

So, there can be no complacency. Friends of Corstorphine Hill works closely with the City of Edinburgh Council and other conservation bodies and concerned individuals to preserve as best possible this extraordinary place for its wildlife and responsible visitors.

An experiment in 2021, in which the boundary hedgerow alongside Clermiston Road was allowed to grow from its normally severely cut ‘fencing’ style, saw 43 birds’ nests built in a half-mile stretch. Once vacated, many of these nests were then repurposed by fieldmice to store rose hips for the winter.  The wildlife does not ask for much!

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