Dr Terry Mabbett focuses on the types of tree selected by rooks for their nesting colonies, and on how this has changed and is still changing in an increasingly dynamic treescape.

THE decline and disappearance of key tree species is becoming an increasingly frequent occurrence. It happened to English elm at the hands of Dutch elm disease (DED) in the 1970s and is now on the cards for common ash, courtesy of Chalara ash dieback disease. We wring our hands and lament our losses, but with little concern for the wildlife dependent on trees across the British Isles. Trees are our natural climax vegetation and the bedrock on which everything else is built. Swathes of wildlife have co-evolved with Britain’s native and naturalised trees and have come to rely on them. So, when a major tree species undergoes a sharp and significant decline, closely associated wildlife has to adapt, or decline and eventually die.

Wildlife most commonly associated with trees in the eyes of the public is the bird population. Trees provide food, perches, night-time roosts and, most crucially, safe and secure nesting sites. None are more dependent on trees than rooks, highly gregarious corvids which breed in tree colonies called rookeries.

Male rooks search for live twigs of pencil-width dimension which can be broken off the branches. They bring the bounty back to the rookery where females weave the twigs into large cup-shaped nests. Three to five eggs are laid, usually in March, with an incubation period of 16 to 18 days, and fledging birds leave the nest some time in May, around 30 days after hatching.

The rook’s nesting habit is in stark contrast to other common corvids like carrion crow and magpie, which build a solitary nest in complete isolation from others of the same species.

Rook behaviour has been a popular field of study in the natural history arena, with detailed literature stretching back to the 1920s and beyond, showing how our forebears spent days, months, years, indeed lifetimes, observing and documenting every move made by these birds.


Forestry Journal: Rooks hedging their bets in choice of tree species as nesting sites – English oak centre stage and common ash on the right in this small English West Country woodland.Rooks hedging their bets in choice of tree species as nesting sites – English oak centre stage and common ash on the right in this small English West Country woodland.

Rooks are highly sociable birds roosting together in large numbers, often with other corvids including jackdaws in Essex and Cornwall and occupying combined rookeries/heronries alongside grey herons in Northern Ireland.

Roebuck, who carried out a comprehensive study of rooks in the East Midlands (Leicestershire, Rutland, Nottinghamshire and Derbyshire) during the 1920s and ’30s, claimed a wide range of trees were used by rooks for nesting, listing elm, ash, oak and horse chestnut as the most frequently used deciduous trees, and Scots pine as the rook’s favourite evergreen. These were generally the most frequently occurring tree species in the areas studied thus indicating rooks’ choice of tree species was largely down to availability.

Larch and holm oak were also used by rooks, with a high hawthorn hedge in Nottingham perhaps the strangest choice. Around the same time, A.W. Boyd recorded a similar situation in the county of Cheshire with rooks’ nests often found in the most frequently occurring trees, which he cited as oak, ash, beech, elm and sycamore. Nests were also recorded in horse chestnut, birch, alder, black poplar and a range of conifers. 

The East Midlands study showed the relative exposure of a site appeared unimportant, with rookeries regularly found on bleak windswept hillsides in Derbyshire where the violent swaying of trees risked egg breakage and ejection. Altitude in itself was not a limiting factor in rooks’ choice of site, though at some point limitation of altitude on tree growth and tree height will clearly affect tree suitability for nesting rooks.

Forestry Journal: Rooks are highly sociable corvids with multiple nests in a single tree – in this case English oak.Rooks are highly sociable corvids with multiple nests in a single tree – in this case English oak.

Irrespective of tree species, provisos for selection were there being enough potential sites to accommodate the entire colony and trees sufficiently tall so that nests could be built on branches at least 10 feet from the ground. The distribution pattern of trees comprising a rookery was extremely varied and in some instances came down to a solitary tree. In situations where more than one tree was used, it could be a line of trees but often with considerable gaps between individuals. 

However, a clump of trees or a small plantation was preferred and where a large wood was selected the rookery was always located on the woodland margin and never deep inside the wood. The reason for this and the apparent preference for hedgerow trees around fields is the rook’s requirement for arable land as a feeding ground. Rooks are often associated with human settlements, nesting near farms, villages and open towns, but not in large, heavily built-up areas. When disturbed, rooks hover over the rookery to guard the living contents of their nests.


English elm was always the favoured tree for rookeries in London and its environs up until the 1970s, when DED struck and virtually all elm trees were felled. By the early 1980s, mature standard elms had essentially disappeared from the London treescape. Kate M. Hall  said: “Of all trees to the rook mind, none beat the common [English] elm for [nest] building purposes. It is high and commands a good view of the surrounding country and building materials are at hand. With much clamour they pull down the old nests of the previous year and make new ones of the young and supple elm twigs, which they break off with their strong beaks.”

Kate M. Hall listed Richmond Park, Wimbledon Common, Streatham Common and Hampstead Heath as sites around London where rookeries were present at that time (circa 1900). By implication they were all associated with elms and we know for certain that at least one rookery on Streatham Common was in elm trees because of the photograph in her book.

Forestry Journal: Rooks select the tallest trees in the area to provide a high and all- encompassing lookout point, presumably to spot any potential danger.Rooks select the tallest trees in the area to provide a high and all- encompassing lookout point, presumably to spot any potential danger.

The answer to why rookeries were no longer present in Central London in 1900 may lie in W.H. Hudson’s Birds in London (1898). It says rooks along with other corvids were expelled from the inner area of London during the second half of the 19th century, often by wanton destruction of the trees that were home to their rookeries.

Many landmark places in London carry ‘rookery’ in the name, though that does not necessarily mean rooks nested there in trees. According to W.H. Hudson, there were once many colonies of nesting rooks in London including large ones in Greenwich Park, Kensington Gardens and Gray’s Inn Gardens. However, the word ‘rookery’ was also used to describe an area of slum dwellings and there were clearly plenty of those in London back down the ages. Medieval Englanders clearly looked upon the massed ranks of rooks’ nests with accompanying mess and noise as slum areas.

Today’s largest remaining collection of Ulmus procera (English elm) and related Ulmus species is in Brighton and Hove (Sussex) on the south coast of England. In Preston Park (north Brighton) is a line of 150-year-old Wheatley (Jersey) Elms (Ulmus minor ‘Sarniensis’), the tree being a variation of the Cornish elm (Ulmus minor). There is no sign of a rookery in the elm trees at Preston Park, although the area now occupied by the park’s ‘Rockery’ was originally called the ‘Rookery’ due to rooks which nested in some of the taller trees in a woodland which occupied the site. Preston Park’s Rockery was constructed between 1934 and 1936.

My own recollections of rookeries in north-east Middlesex and South Hertfordshire in the 1950s and ’60s chime with this feeling that elms were the trees of choice for rookeries. Elm was most abundant as a tree of the wayside rather than the woodland, very common as a hedgerow tree around arable fields and therefore ideal for rooks by providing tall secure nesting sites adjacent to good feeding grounds. Furthermore, English elm was invariably found at the woodland margin, rather than deep inside the woodland – perhaps another reason why rookeries were always located at the edge of woodlands rather than inside.

So what happened to all those rookeries in English elm trees when DED struck in 1969 and millions of elm trees were subsequently felled? By 1978 it was calculated that out of 17.1 million elms across a selection of English counties, 10.6 million were dead or dying and 5 million had been felled since the start of the epiphytotic (epidemic); DED was still the major tree health concern in Britain after nine years.

By the mid ‘70s rooks were using considerably less elm trees for nesting. In 1930, 90 per cent of rooks nesting in Oxfordshire used elms, but by 1975 this had fallen to 66 per cent. Similarly, Wynne recorded 90 per cent of rook nests on the Isle of Wight in elms whereas only 37 per cent were found in elms in 1978 by Vernon and Sage. It was the same story further west in Gloucestershire with 78 per cent of rooks’ nests in elms in 1934 but only 58 per cent in 1975.

In response to the widespread loss of elms, rookeries had substantially shifted to common ash and English oak by 1996. Like the English elm, mature specimens of these native high (climax) forest trees are sufficiently tall for the birds to feel safe and the upper branching dense enough to support a large number of nests in close proximity. Over the same period of time, conifer usage by rooks in Scotland decreased.

Forestry Journal: Mid April 2016 and this English oak tree is not showing any clear signs of bud burst and indicating a tardy spring. However, both male and female rooks are in attendance suggesting that the eggs, traditionally laid in March, have already hatched.Mid April 2016 and this English oak tree is not showing any clear signs of bud burst and indicating a tardy spring. However, both male and female rooks are in attendance suggesting that the eggs, traditionally laid in March, have already hatched.

It would be reasonable to expect resulting disruption and upheaval to the rook’s long and well-established nesting habit would impact negatively on the population. However, a nationwide census of nesting rooks carried out in spring 1996 by the British Trust for Ornithology estimated a population of 1.27 million nesting pairs. This was 40 per cent higher than a count made during 1975–77 but less than the number recorded in 1944–46. The mean size of rookeries was larger in 1996 compared to 1975–77, especially in England. Authors of the 1996 report say increases may have stemmed from additional foraging opportunities provided by new outdoor piggeries and landfill sites, increased roadside carrion, and higher stocking rates on grassland.

The question is, where will the rookeries which were relocated into common ash after the DED epiphytotic now go? Common ash is succumbing to Chalara ash dieback on an intense and widespread scale, with hundreds of thousands of trees now being felled up and down the land. Our local rookery in South Hertfordshire, which occupied elm trees until the 1970s, moved into mature Scots pine in the garden of a neighbouring Victorian house, and is still there today to provide a safer and hopefully long-term haven. And there lies a caution and warning to tree provenance purists who claim Scot’s pine is not native to southern England and is therefore the wrong tree in the wrong place.

Lines of fast-growing hybrid poplars planted by farmers around their fields as windbreaks appear to have become a popular alternative by providing tall trees near feeding grounds in arable fields in the same way that hedgerow elms clearly did centuries ago. Despite the catastrophic consequences of Dutch elm disease some five decades ago and more recently Chalara ash dieback, rooks appear sufficiently versatile to exploit Britain’s still substantial and varied tree resources as sites for their communal nesting habit.


Of all the British native corvids, including carrion crows, jackdaws and ravens, rooks are the most sociable, living and breeding in large groups. Other corvids like the carrion crow show variable behaviour in this respect. They are often seen as solitary birds or in pairs but socialising at other times except during the breeding season when they build solitary nests, frequently in oak trees of the hedgerow and at the woodland edge. Accepted wisdom is that birds in larger groups are less healthy and therefore more stressed because diseases and parasites are more easily spread. However, very recent research carried out at Anglia Ruskin University appears to have found the exact opposite.

Research was conducted by Dr Claudia Wascher and her colleagues on captive carrion crows. Birds in larger social groups excreted less parasites in their droppings than those with fewer social interactions. Such birds were healthier and showed reduced stress levels. The theory is that closer social bonds reduce stress to make the crows less susceptible to parasites. On this basis rooks should be the healthiest and least stressed of all British native corvids, despite losing their favoured tree nesting sites.


Boyd, A.W. (1934) ‘Trees used by nesting rooks in Cheshire’. British Birds Volume 27: 72.

Burns P.S. (1957) ‘Rook and Jackdaw Roosts Around Bishop’s Stortford’. Bird Study, 4:2, 62-71, DOI: 10.1080/00063655709475872.

Coombs, C.J.F. (1961) ‘Rookeries and Roosts of the Rook and Jackdaw in South-West Cornwall. Part II. Roosting’, Bird Study, 8:2, 55-70. DOI: 10.1080/00063656109475989

Elkington, J. (1978) ‘A tree-less Britain’. New Scientist 78: 72-74

Hall, Kate M. (1908) Nature rambles in London. Hodder and Stoughton Publishers London.

Hudson, W.H. (1898) Birds in London. Longmans Green & Company.

Mabbett, T.H. (2014) ‘English elm in black and white’. Forestry Journal March 2014. Page 46-47.

Mabbett, T.H. (2014) ‘Bon voyage Scotland but don’t take our native pine’. Forestry Journal September, 2014. Page 70-71.

Marchant, J.H. and Gregory, R.D. (1999) ‘Numbers of rooks Corvus frugilegus in the United Kingdom in 1996’. Bird Study Vol 46 Issue 3.

Nicholson, E.M. and B.D. Nicholson, (1930) ‘The rookeries of the Oxford District’. Journal of Ecology 18: 51-66

Osborne, P. (1982) ‘Some

effects of Dutch Elm Disease on nesting farmland birds’. Bird Study, 29:1, 2-16, DOI: 10.1080/00063658209476733

Roebuck, A. (1933) ‘A survey of rooks in the Midlands’. British Birds 27:4-23

Sage, B. and Vernon, J.D.R. (1978) The 1975 national survey of rookeries Ornithological Section, Fieldwork Review 1975:6-7. Bird Study 25: 64-86.

The London Literary Gazette and Journal of Belles Lettres, Arts and Sciences (1822) Saturday 5th January 1822. ‘The Rookery’, pages 340-341.

Vernon, J.D.R. (1976) Survey of the rookeries of Avon County. Bristol Naturalists Society.

Wascher C. A. F., Canestrari, D., Baglione, V. (2019) ‘Affiliative social relationships and coccidian oocyst excretion in a cooperatively breeding bird species’. Animal Behaviour, 2019; 158: 121 DOI: 10.1016/j.anbehav.2019.10.009.

Witherby, H. F. (ed.) (1943) Handbook of British Birds, Volume 1: Crows to Firecrest. H. F. and G. Witherby Ltd. pp. 17–22.

Wynne, J.F. (1932) ‘The rookeries of the Isle of Wight’. Journal of Animal Ecology, 1: 168-174

Yapp, W.R. (1934) ‘The rook population of west Gloucestershire’. Journal Animal Ecology, 3: 77-80.

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