Reflections on Complexity | Part 1: Generalists v Specialists

Matthew Hay, Natural Capital Manager, Nattergal

The willow tit (Poecile montanus) – our fastest declining bird

The rarest bird you’ve never heard of

You’ve probably never seen a willow tit. I certainly haven’t. Sadly, our odds of stumbling upon this endearing species are rapidly diminishing – no other British bird has suffered a comparable decline in recent times, with numbers down 96% since the 1960s.[1] In many counties, willow tits are already extinct. Where they cling on, ironically, are post-industrial brownfield sites – places that are overlooked and forgotten, where outside influence is minimal and nutrient levels are low, allowing natural processes to creep back in.

Why does the willow tits’ decline matter? Well, for several reasons. Firstly, if we are trying to conserve biodiversity in any holistic sense then every species matters, they are all pieces of the ecosystem puzzle. British willow tits are also considered to be a separate subspecies from those found on the continent, specially adapted to our (formerly extensive) wet woodland habitats, hence their name. Most of all, willow tits matter because many of the factors leading to their decline are generic, affecting a huge variety of wildlife right across the planet. This is especially true for those animals that can be considered specialists: species which have evolved to take advantage of a highly specific niche.

It is vital, therefore, that all of us working to deliver Nature recovery try and understand what is driving the decline of specialist species like willow tits, to see if and how our efforts can benefit this broad category of imperilled wildlife. Put simply, unless we can stop the rot with specialists, we won’t be able to halt the decline of biodiversity in any meaningful way. For us at Nattergal, the loss of willow tits is also personal, with East Anglia’s last population recently going extinct just across the water from our West Norfolk site, High Fen Wildland.

Not all heroes wear capes

Willow tits may not look remarkable but these tiny birds have a super power, which has historically given them a competitive edge: they can excavate nest holes. This may not sound like much but other tits cannot do this and instead rely on natural cavities forming in older trees, restricting them to breeding in mature woodland. In contrast, willow tits can colonise young woodland; the dense, scrubby stands of birch and sallow that soon appear when land succeeds towards tree cover.

This ability to excavate nest holes historically allowed willow tits to avoid competition, carving out a distinct niche in which they flourished. But all that has changed and now their survival is under threat because human pressures on our landscapes have stacked the odds against them.

First up is the loss of habitat. With the UK substantially deforested and the average willow tit pair needing 7 hectares of habitat to breed successfully, the species’ range has contracted significantly since historical times, especially in Scotland where willow tits were once widespread.[2]

Far more damaging in recent decades, however, has been the proliferation of bird feeders in people’s gardens. While lovely for us to get good views of wildlife, this constant supply of food has massively benefited some species, especially generalists like blue and great tits, which compete with willow tits.

Historically, blue and great tit populations would have been kept in check by winter food shortages. Now, with access to year-round food from garden feeders and climate change reducing the frequency of harsh winter weather, their numbers are booming. The blue tit population has grown 25% since the 1960s, with an estimated 3.4 million pairs now living in the UK as of 2022, while great tit numbers have risen by an astonishing 88% over the same period, to 2.4 million pairs. In contrast, less than 3,000 British willow tits remain.[3]

Why does this matter? Well, because by removing the natural checks and balances on blue and great tit populations we (humans) have caused their populations to expand, forcing individuals to disperse in search of new territories. As a result, these species are now colonising areas of young woodland, occupying willow tit nests in the process by physically turfing them out of the holes they have excavated and preventing them from breeding.[4]

Bird feeders are also subsidising another generalist species, the great-spotted woodpecker (GSW), whose population has increased by an incredible 388% since the 1960s. As the willow tit’s main predator, this huge increase in GSWs is undoubtedly impacting their breeding success, with many pairs’ chicks being eaten by woodpeckers before they can fledge.[5]

But ecology is always more complicated than one simple, causal relationship and part of the explanation for GSW’s success may be due to the recent decline in breeding starlings in the UK (down 54% since 1995).[6] Starlings do to woodpeckers what blue tits do to willow tits, usurping their nest cavities and preventing them from breeding.[7] So, in addition to bird feeders, the loss of starlings has released GSWs from a major constraint on their population size, with knock on effects for GSWs’ prey, which includes willow tits and other red-listed species like lesser-spotted woodpeckers.

What’s caused the decline in starlings? Well, the exact causes are unclear but the primary hypothesis is intensification of agricultural practices, reducing food availability for young birds in their first winter. Regardless of the precise drivers, the starlings’ decline and the knock-on effects for GSWs highlights the complex chains of causality that interact in ecology to shape an individual species’ fortunes.[8]

But by just looking at willow tits, we can clearly see that three human-driven factors – climate change, subsidisation of generalist species, and agricultural intensification – are all conspiring, directly and indirectly, to form something of an extinction vortex for the species. Sadly this is true for many other specialists too.

Bird feeders subsidise generalist species, like blue and great tits, and contribute to the spread of disease

Generalists versus specialists

When it comes to biodiversity decline no trends are universal and not all species are faring badly. Some, like blue tits and great-spotted woodpeckers, which are adaptable and generalist in Nature, have benefitted from human influences on our landscapes and, in particular, from its homogenisation – which is to say the loss of complexity.

You’ll probably be familiar with these generalist species, as increasingly they are the only ones we see around. Looking at the population trends, it is clear that wood pigeons (up 152%), jackdaws (up 141%), magpies (up 102%), badgers (up >88%), buzzards (up 80%), ravens (up 39%), and many others are benefiting from human influences on the landscape.[9],[10] By reducing ecological complexity we are giving these species an advantage, whether through extirpating the apex predators that kept them in check, removing the vegetation their prey uses for cover, or increasing sources of human-produced food like cereal crops.[11]

The corollary of these generalists’ success is that the specialists have often declined. Examples include large marsh grasshoppers (widely extinct), fen raft spiders (widely extinct), corncrakes (extinct on mainland Britain), capercaillie (down 98%), wood warblers (down 81%), woodcock (down 75%), and many, many others.[12]

For some of these specialists, there is a migratory element to their decline, with pressures in their wintering grounds or on migration taking a toll. For others, such as the corncrake, intensification of agricultural practices was the clear cause of local extinctions. However, for many of the other species that are faring badly in Britain there is no one obvious problem or agreed on hypothesis to explain their demise. Instead, the drivers of decline are considered multi-factorial and best addressed by implementing the ‘Lawton principles’ for Nature conservation (i.e. more, bigger, better, and joined up).[13]

Specialists needs complexity

At Nattergal, we are fully committed to the Lawton principles on the land we manage. Size (bigger) and ecological connectivity (joined up) are excellent buffers against human pressures, which is why we have a minimum size threshold for our sites and why we work hard with local partners to enhance habitat corridors across the landscapes in which we are embedded.

The “better” principle is more open to interpretation and where subjectivity can creep in. But in our view complexity is an absolutely key ingredient if we want our habitats and ecosystems to be “better” for Nature.

Why do we think this? Well, because when we look at the diverse pressures affecting specialists like willow tits, almost all of them are reducing complexity. Landscape homogenisation, less frequent cold winter weather, the loss of seasonal food scarcity – all of these are leading to ecological simplification.

Aerial photograph of ‘rough and loose’ being created at Boothby Wildland, to increase topographical complexity and water storage in the landscape.

If we can agree that we won’t be able to bend the curve on biodiversity loss unless we find a way to reverse the population trends of our specialists, then it’s clear that the focus has to be on restoring habitats and ecosystems to their full complexity; on encouraging all the different ecological niches to exist, at scale, and in a way that allows for connectivity between populations.

So at Nattergal, we make no bones about the fact that we target complexity explicitly, on all our sites, as an outcome we are trying to deliver. Our view is that natural processes provide one of the best tools in the Nature recovery toolbox for doing this. In the next instalment of this blog, we will explain how we’re using them to increase complexity and drive Nature recovery on the land we manage.

[1] British Trust for Ornithology

[2] Broughton, Richard & Parry, Wayne & Maziarz, Marta. (2021). Wilding of a post-industrial site provides a habitat refuge for an endangered woodland songbird, the British Willow Tit Poecile montanus kleinschmidti. Bird Study. 67. 1-10. 10.1080/00063657.2020.1863333.

[3] British Trust for Ornithology

[4] Parry, W., & Broughton, R. K. (2018). Nesting behaviour and breeding success of Willow Tits Poecile montanus in north-west England. Ringing & Migration, 33(2), 75–85. https://doi.org/10.1080/03078698.2018.1631610

[5] Parry, W., & Broughton, R. K. (2018). Nesting behaviour and breeding success of Willow Tits Poecile montanus in north-west England. Ringing & Migration, 33(2), 75–85. https://doi.org/10.1080/03078698.2018.1631610

[6] British Trust for Ornithology

[7] Jackson, Jerome & Jackson, Bette. (2016). Dynamics of Woodpecker – Common Starling interactions: a comparison of Old World and New World species and populations. Ornis Hungarica. 24. 1-41. 10.1515/orhu-2016-0001.

[8] British Trust for Ornithology

[9] British Trust for Ornithology

[10] Judge, J., Wilson, G.J., Macarthur, R., Delahay, R.J. & McDonald, R.A. (2014). Density and abundance of badger social groups in England and Wales in 2011-2013. Scientific Reports, 4:3809.

[11] British Trust for Ornithology

[12] British Trust for Ornithology

[13] Making Space for Nature: A review of England’s Wildlife Sites and Ecological Network Chaired by Professor Sir John Lawton CBE FRS