Do Buzzards eat Partridge?

In September, to great fanfare and publicity alarums, Chris Packham and George Monbiot released “A Peoples Manifesto for Wildlife”. This wide-ranging document claims to having no party political bias, but admits to being political and even controversial. The manifesto declares its intentions to form no less than 17 separate ministries for the management of the British countryside and seas. These 17 separate ministries will be administered by no fewer than 18 ministers (one of the ministries has two ministers). There is a ministry for trees and a ministry for hedgerows. Quite who gets the funding for a hedgerow with trees in it is left undecided. There are ministries for lead ammunition and one for wildlife welfare.  There is a ministry for the uplands and another for pesticides. Almost everything a farmer or land manager does will be impeded by one ministry or another. In a very British tartan-slippers-and-cardigan sort of way, this document has a whiff of the flavour of Stalin’s collectivisation of farmland.

News travels fast on Twitter and a number of arable farmers on my timeline, based in the East of England, clearly felt that they had been misrepresented by the Packham/Monbiot axis. Malc Parr tweeted a picture of a flock of Lapwing flying over one of his fields. Sure enough, a lurking @ecobadger bit the bait and got a bit acerbic. Malc followed it up with another photo of a cultivated field covered with cobwebs from millions of spiderlings, which had landed overnight and decorated the furrows with a gossamer mist:

Ecobadger wasn’t satisfied, so Rich Thomas tweeted this:

And still Ecobadger was not content. So Andrew Ward tweeted this:

And finally, Andy Roberts tweeted this:

This last photograph is remarkable, because it contains at least fifteen very pale plumaged Buzzards, all carefully scrutinising the freshly turned over ridges for soil invertebrates. They are doing exactly the same thing as the gulls in Andrew Ward’s photo – but slowly, more deliberately and without the noisy clamour. Clearly, there is a lot more life in these soils than some of the better known nature pundits would have you believe.

The conversation took a turn and a caveat from one of the more reasonable conservation professionals, who suggested that he was worried that farmers were taking these incidences as indicators that all is well in the environment and farming.

This conversation has exposed the two broad objections to farmers who suggest that things do not seem to be as bad as they are cracked up to be. The two pillars of official, received wisdom can be summed up in the following statements:

• That farmers are responsible for continued, widespread, wholesale environmental destruction.
• That large data sets are definitive and show serious declines in farmland wildlife – and that it is ‘agricultural intensification’ which is almost entirely responsible for those declines.

The allegations against “agricultural intensification” are widespread across all conservation NGOs; and official amongst Defra publications. Here, for example, is Defra’s  “Wild bird populations in the UK 1970 – 2016” document which details the indices for many species of UK birds. In particular, we should look at Figure 2 on page 10, which looks like this:

Without any question this shows a horrible decline in farmland birds to 40% of their 1970s numbers. Notice that horizontal curly bracket between 1970 and 1987 which clearly states: “Rapid changes in farmland management”. The link between declines of bird populations on farms, and farmers, is firmly established and taken without question by anyone in government or environmental NGOs. Not forgetting George Monbiot, Chris Packham, Bill Oddie, most of the Labour Party, Islington dinner parties, their followers, acolytes and every single eco-warrior in the country.

There is just one tiny little fly in this slick, comfortable explanation. And that is that there is very little direct evidence provided by countless studies on many species, that “agricultural intensification” (howsoever that is defined) is actually responsible for these measured declines. It is true that there are suggestions that the switch from spring to autumn sowing of cereals (for example) causes earlier crop growth and cover to alter bird nesting behaviour (such as making Skylarks to shift their nests closer to tramlines and thus being more prone to predation). But whilst suggestions are made, very little in the way of hard evidence is actually provided.

What cannot be doubted is that populations of some farmland birds have plummeted considerably since the 1960s: Turtle Doves are at 1/40th of their population in the early 1970s. Skylarks have dropped to about a third in the same period. Grey Partridge are at about 8% of their previous populations. Lapwing are at 1/3rd of their numbers in 1980. And so on.

And yet the photographic evidence provided by Andy Roberts, Andrew Ward and Malc Parr (amongst many other farmers) suggest that things are actually quite good for some species on some farms. This leaves us with the question: what is going on?

Look again at the screenshot of Figure 2 above and notice that the green line depicting the aggregate farmland bird index is labelled “All farmland birds (19)”. That is to say that the index is average of 19 species. The list of species which make up this index is divided into two parts:

Farmland Bird Generalists (7 species)

• Greenfinch
• Jackdaw
• Kestrel
• Reed Bunting
• Rook
• Woodpigeon
• Yellow Wagtail

Farmland Bird Specialists (12 species)

• Corn Bunting
• Goldfinch
• Grey Partridge
• Lapwing
• Linnet
• Skylark
• Starling
• Stock Dove
• Tree Sparrow
• Turtle Dove
• Whitethroat
• Yellowhammer

Those readers who are wide awake will note that this list does not seem to be exhaustive. In fact, three common farmland species which are missing from this list are Carrion Crow, Magpie and Common Buzzard. All of these three species have increased considerably in the last 50 years. And this brings us to the first possible objection to the idea that farmers are solely responsible for farmland bird declines, and which forms a broad hypothesis: The farmland bird index is biased because of the exclusion of three critical species from the index.

To examine this possibility, the graph below shows the combined indices for the specialist farmland birds shown above; and the same index incorporating the indices for the predators – Buzzard, Carrion Crow and Magpie.

The red curve is lower and flatter, reflecting the incorporation of the predators, but is otherwise a similar shape. Before the above hypothesis is dismissed, we need to dig a little deeper to see if there is a viable explanation for the plunging farmland bird index. To begin this process, we first need to understand some very basic features of some of the species involved:

The Buzzard is a raptor of open spaces and farmland. They hunt by soaring high and watching the ground below them, or they spend a lot of time perching on a gate post or telegraph pole watching the ground immediately below them for small rodents. They may also walk upon recently ploughed or mown fields to pick up invertebrates such as grasshoppers and worms. Anything small that moves on the ground is potential prey including nestlings of ground nesting birds.

The Carrion Crow is also a bird of open farmland. They are generally shy of human habitation and prefer isolated trees to watch the area. They will take nestlings of small birds from the ground or from hedgerows, as well as carrion and invertebrates.

The Magpie is a bird of mixed woodland and shrubs. They tend to frequent places of greater density of foliage than the previous two species – and so large suburban gardens are ideal for them. They will take large numbers of nestlings from small birds which also rely upon the denser foliage. They will also take invertebrates and carrion.

The individual indices for the above three predators look like this:

Whilst Carrion Crows have risen steadily in numbers from 1966 and show no sign of levelling out, Magpies and Buzzards have more distinctive curves. Magpie numbers rose quickly from the 1960s and then numbers levelled out in 1985 and have shown a small reduction in numbers ever since. This would suggest that Magpies have reached saturation in terms of the available habitat. Meanwhile, Buzzards were slower to pick up, remaining fairly constant from 1966 until 1991, when their numbers started to rise dramatically. There is no sign yet of the Buzzard population levelling out like the Magpies have done.

Buzzards and Grey Partridge

A well known attribute of Buzzards is that they eat the young of game birds whenever they can find them. To find out if the numbers of Buzzards affect the numbers of Grey Partridge, the Grey Partridge index was plotted against that of Buzzards to give the curved relationship shown below:

In the top left-hand corner, Buzzard numbers are very low and Grey Partridge numbers are high. In the bottom right-hand corner, the opposite is the case. Note that R² is given as 0.8496. This figure is a measure of how closely the data match the fitted curve. The closer this number is to 1, the better the fit. Conversely, a figure close to zero means that there is little or no fit. In this case the figure is approximately 0.85 and so the data match the fitted curve very closely. It suggests that as Buzzard numbers rise, Partridge numbers fall – and do with increasing rapidity until Partridge numbers fall to a very low level.

As every statistician will tell you ‘Correlation does not prove causation’. This statement is often repeated; but it has a flip side in that if there is causation between two variables, then there must be correlation.  It is this that makes correlation (and its related technique of regression like that used above) a very powerful tool for analysing data, particularly as a stimulus to further work to confirm (or dismiss) any hypothesis.

To test the possibility of these two events being linked, we can divide the story into two parts. The first part is to look at what happens to Partridge numbers in the period when Buzzard numbers were low but fairly constant, i.e. from 1966 to 1989; and then again when Buzzard numbers started to increase – from 1990 to 2017:

From 1966 to 1989 when Buzzard numbers were low, it is clear that the Grey Partridge index varies very widely. There is no discernible pattern in the data. Furthermore, the R² value is 0.1862, i.e. very close to zero. That is, there is very little association between Buzzard numbers and Grey Partridge numbers whilst the buzzard numbers are low. However, the picture is different as the Buzzard numbers begin to increase from 1990 onwards:

Here, as the Buzzard numbers start to rise, the Grey Partridge numbers begin to fall; and the Partridge decline is accelerated as Buzzard numbers increase to their current levels. In this case, the R² value is 0.8525 which is very close to 1.0000 and shows that the data points are all close to the fitted curve.

Carrion Crows and Skylarks

In a similar way, the effects of Carrion Crows upon Skylarks can be assessed by plotting the Carrion Crow index against Skylark index:

This set of data show a different pattern from the Buzzard and Grey Partridge.  For Carrion Crows and Skylark, there seem to be two distinct clusters and these have been  circled. The data points inside the blue oval are all points between 1966 and 1981. Those within the red oval are from 1982 to 2017. The blue group appear not to have a discernible pattern, whilst the red group seem to be linear. Once again, the data are separated according to year:

The lack of any trend for the early part of the relationship between Carrion Crows and Skylarks is obvious by inspection – and this is confirmed by the R² value which is 0.1156 i.e. there is little or no relationship.

However, from 1982 onwards the relationship is clearly linear and very tight, giving an R² value of 0.8536:

A reasonable interpretation of these two graphs is to say that Skylark numbers were unaffected by increases in Carrion Crow numbers until about 1982. After this, the numbers of Carrion Crows increased to the point where they started to adversely affect the numbers of Skylark.

Fluctuations in numbers of predators and prey have been studied by ecologists for many years, and various patterns of population responses have been described. This article summarises one study which describes the relationship between Canadian Wolves, Moose and Caribou. In essence, when numbers of predators begin to increase, it usually has a negative effect upon the population of the prey species.

In the two instances given above, the numbers each prey species (Partridge and Skylark) have begun to alter when the numbers of predators (Buzzard and Carrion Crow) increase beyond a critical point.

Buzzards, Carrion Crows, Grey Partridges and Skylark are all birds of open farmland. The first two species are avid predators of the nests and young of the other two species. It is biologically reasonable to say that the increasing populations of the predators cause decreasing numbers of their avian prey species.

By repeating this exercise for some of the other species in the specialist group of farmland birds, the results can be expressed in the following table:

Table 1 – Associations of predator and prey

The above table lists six out of the twelve specialist farmland birds whose massive declines have all negatively influenced the overall Farmland Bird Index. I have not yet examined in detail all of the birds on that list, or indeed on the indices for woodland and wetland birds, but I can say that the increase in these three avian predators is very strongly associated with the declines of many other species.

Why have the predators increased?

One of the great puzzles of this story is how or why Buzzards, Carrion Crows and Magpies have increased in numbers so much that they would appear to be depleting the countryside of so many smaller birds.

As far as I am aware, there are few published studies which have given the causes of the sudden increase in Buzzard population from 1991. However, there are at least two major changes to the British landscape which have occurred over the last 60 years which may give us some clues.

The first is the amount of public and private forestry planting which rapidly accelerated after WW2. From the 1950s to the 1970s, large areas were forested with coniferous plantations, all over the UK. As these matured, say to about 25 or 30 years old, they were tall enough to accommodate Buzzard nests. Buzzards like to nest at an average height of 11 metres[1]. However, this has to be well below the crown of the tree, suggesting a tree height of 30 metres or more, as this stock photo shows:

Buzzard nest in Larch tree (photo: David Boag, Getty Images)

By the 1990s, many of these plantations were reaching the required height for Buzzards. Thus, Buzzards were able to enjoy plenty of discreet nesting sites in the new man-made habitats.

Another change which took place in the British countryside, took place silently and invisibly. The use of DDT and other organo-chlorine pesticides as agricultural insecticide started in the post-war agricultural boom. By the early 60s, it was clear that there was a problem. Predatory and other birds were dying and disappearing from the landscape in huge numbers. As a result of some first-class work by Monks Wood Experimental Station, organo-chlorine pesticide residues and PCB levels were measured in the livers of many bird corpses found throughout the UK[2]. Birds were dying directly because of residues accumulated in their fat and livers. Eggshells were thinned and egg smashing behaviour by raptors was reported. Kingfishers and Herons were also amongst the many species which were affected.

The worst and most persistent of these organo-chlorine pesticides was DDT. After a lot of pressure from environmentalists and the public, its use was banned in the UK and the USA in 1972. Use of DDT has almost stopped worldwide, with some exceptions. DDT has a half-life in soils of approximately 17 years. In water it is said to be up to 150 years. If we take it that DDT in the environment was at a maximum in 1971, after which it was banned, then the quantity remaining in soils after the ban would have been approximately half by about 1989 or 1990. Perhaps coincidentally, this was also about the time when Buzzard numbers started to rise.

Whilst there were doubtless other factors, the perfect combination of lower pesticide residues and more nesting sites gave the Buzzard a serious boost, and numbers started to climb steadily until the present day. There is no sign of Buzzard numbers levelling out just yet.

Carrion Crow numbers have risen steadily since the 1960s and this demonstrates that they too may well have benefited from the same boom in new forest growth (Crows also like to nest high in conifers but will breed in many more scrubby areas at lower levels, so they are not as selective as Buzzards in their nesting sites). Doubtless lower pesticide levels also helped.

Discussion and Conclusions

Table 1 above assesses the associations between the increases of Buzzards, Carrion Crows and Magpies against the declines of six other farmland bird species. The table is not yet complete for all farmland birds. For those species that have been analysed, the method of splitting each index into the periods when predator numbers are fairly constant and when numbers begin to rise, show that the association with declining prey species is very strong. Combined with knowledge about other events occurring in the British countryside over the last 50 years, knowledge of the basic biology of each species and ecological theory of predator-prey interactions, the following hypothesis can be proposed:

That increases in populations of Buzzard, Carrion Crow and Magpie have caused declines in populations of many other farmland bird species.

The idea that Buzzards and Crows seriously affect the populations of many smaller birds will not come as a surprise to any gamekeeper in the country. Or, for that matter, any farmer or forester who daily observes nature and the way it changes with the seasons and the decades. These are the people who are out in the fresh air, every day of the year, and who watch and learn from the things that happen on the land that they husband.

It is true that there have been many changes since WW2 in the way the land has been farmed. It is true that the use of DDT on agricultural land was a disaster for wildlife. But at the time, no-one (least of all the farmer) knew what effects it would have – and would continue to have for a very long time after its use was banned.

It is also true that some changes have adversely affected populations of particular species. For example, there is not much doubt that the loss of permanent rough pasture in wet areas in the lowlands has contributed to the decline of the Redshank, Snipe and Curlew. The shift from making hay to two, three or even four cuts of silage from grassland has affected breeding success of Skylarks.

But for that critical time after the war, when British farmers were doing their utmost to feed a nation that was dependent upon imports for 60% of its food, and when they used DDT and other organochlorine pesticides in good faith, they have never been forgiven by the environmentalists. “Intensive agriculture” is now so permanently lodged in the environmental  lexicon that the establishment will continue to use it as an excuse for continued interference in agriculture – in the mistaken belief that they understand the land better than the farmers.

Having repeated this phrase over many years, the establishment has given licence to all those tele-media luvvies (spurred on by their publicists) to use “intensive agriculture” as a deeply opprobrious epithet. It is this central theme which has prompted so much of the negative media coverage of farming. The possibility, exposed by this article, that actually many of the changes we see in the countryside are in fact an entirely natural response to the more favourable environment that modern farming has produced, will be dismissed or ignored.

Meanwhile, the academic environment is crowded out with wannabe conservationists, post-doctoral students, university professors and their equivalents in the NGOs. For all of their expertise, for all of the taxpayers’ money that is ploughed into this body of learning and for all of the dismissal of “anecdotal” evidence coming from gamekeepers, farmers and the like, no-one has bothered to interrogate the data that they themselves have produced. The meme of “intensive agriculture” is just too easy and too comfortable to warrant further inquiry.

And finally, to answer the question posed in the title of this article: “Do Buzzards eat Partridge?” the answer is: “Yes. And when they do, they knock seven bells out of the Partridge population.”

 

References

The data used in the preceeding analysis has all been generated by the British Trust for Ornithology using surveys by thousands of volunteer birdwatchers over the last 50 years. The result is a matchless dataset of trends in bird populations in Britain and Ireland. It is to be found here.

[1] Cramp S and Simmons KEL (eds) (1979) The Birds of the Western Palearctic, Vol II, pp 177 – 190, Oxford University Press, Oxford.

[2] Cooke AS, Bell AA and Haas MB (1982) Predatory Birds, Pesticides and Pollution, NERC, Huntingdon.