In the US, urban development is causing monogamous birds to divorce

Alone again. Image: Getty.

Breeds of birds which generally tend towards long-term monogamous relationships feel the strain of the city just as much as anyone else. A recently-published study showed that urban development in North America is encouraging more monogamous duos to “divorce” one another, as they’re forced to flee encroaching urban environments.

The conclusions were based on research undertaken by scientists at Washington University, Seattle, alongside others. It was published in the journal PLOS ONE under the title “Breeding Dispersal by Birds in a Dynamic Urban Ecosystem”.

The scientists differentiated between “sensitive forest” species and those “tolerant of suburban lands”. They termed the first group, which consisted of the Pacific wren and Swainson’s thrush, “avoiders”; the latter, which consisted of four other species (song sparrow, spotted towhee, dark-eyed junco and Bewick’s wren), the labelled “adapters”.

All six species studied were considered “socially monogamous”. But new urban developments appear to require some species of birds to abandon their old homes and find new mates, when they otherwise would not have done.

“What we saw with those two avoiders, in the areas that were being developed, where trees were literally being cut down and roads put in during our study, we had birds that were forced to move and divorce by that activity. And those birds suffered a reproductive cost,” explains John Marzluff, a professor of wildlife science at Washington University.

This was particularly problematic for “avoiders” who “divorced” at a steady rate, the study found.

“You see an actual bump in success in the adapter group in the sparrows and the flexible species. But you don’t see that in the avoider species,” one coauthor, David Oleyar, a senior scientist at HawkWatch International, told Smithsonian Magazine.


Between 2002 and 2010, the scientists mapped 6,363 territories of a set size, assessing the locations of individual adults and adult birds’ locations over two consecutive years. They studied hundreds of birds to form their conclusions.

“By far the hardest piece of bird biology to get a hand on is movement. How do they move between successive breeding attempts,” Marzluff explains. “The reason we were able to do it was that we marked thousands of individual birds at the sites we studied and then monitored them over a 12 year period.”

Defining “divorce” among birds has itself had a difficult history. “I remember publishing a study when I was a graduate student back in the 1980s,” he said. “Boy, we got a lot of static for trying to use that term in that paper. So we took it out”

But it’s since been used on a regular basis, he adds. “It’s simply when an individual gets a new mate at the same time as former mate is still alive and in the area.” Mazluff theorised that, while the sample size wasn’t quite big enough to examine gender differences, if could be that males try to attract females on territory which the female deems unsuitable. “It’s the males that stay longer,” he said.

Urbanisation isn’t always bad news for birds, however, and can even lead to an increase in the diversity of bird species. “On the fringes, where we mix things up a lot, have a lot of different land cover – diversity is high there,” he said, adding there had been an increase in bird species in inner London over the last 100 years.

While the study revealed a previously unnoticed impact of urbanisation on certain species of birds in North America, the impact could be different among different species and in different cities. There have been no such studies conducted in the UK to date, but there is significant potential for the research to be expanded.

Professor Richard Gregory, from the RSPB Centre for Conservation Science, says that the study “casts new light” on the potential impact of urbanisation “and why there might be winners and losers” among birds.

“The costs of breeding dispersal, with lower reproductive success and survival, mean that some more sensitive bird species will decline in the face of urbanization. Yet less sensitive birds might thrive,” he says.

“We’d very much encourage similar studies in the UK because our understanding of the urban environment is surprisingly poor.”

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Was the decline in Liverpool’s historic population really that unusual?

A view of Liverpool from Birkenhead. Image: Getty.

It is often reported that Liverpool’s population halved after the 1930s. But is this true? Or is it a myth?

Often, it’s simply assumed that it’s true. The end. Indeed, proud Londoner Lord Adonis – a leading proponent of the Liverpool-bypassing High Speed 2 railway, current chair of the National Infrastructure Commission, and generally a very influential person – stood on the stairs in Liverpool Town Hall in 2011 and said:

“The population of Liverpool has nearly halved in the last 50 years.”

This raises two questions. Firstly, did the population of the City of Liverpool really nearly halve in the 50 year period to 2011? That’s easy to check using this University of Portsmouth website – so I did just that (even though I knew he was wrong anyway). In 2011, the population of the City of Liverpool was 466,415. Fifty years earlier, in 1961, it was 737,637, which equates to a 37 per cent drop. Oops!

In fact, the City of Liverpool’s peak population was recorded in the 1931 Census as 846,302. Its lowest subsequent figure was recorded in the 2001 Census as 439,428 – which represents a 48 per cent decline from the peak population, over a 70 year period.

Compare this to the population figures for the similarly sized City of Manchester. Its peak population also recorded in the 1931 Census as 748,729, and its lowest subsequent figure was also recorded in the 2001 Census, as 392,830. This also represents a 48 per cent decline from the peak population, over the same 70 year period.

So, as can be seen here, Liverpool is not a special case at all. Which makes me wonder why it is often singled out or portrayed as exceptional in this regard, in the media and, indeed, by some badly briefed politicians. Even London has a similar story to tell, and it is told rather well in this recent article by a Londoner, for the Museum of London. (Editor’s note: It’s one of mine.)

This leads me onto the second question: where have all those people gone: London? The Moon? Mars?

Well, it turns out that the answer is bit boring and obvious actually: after World War 2, lots of people moved to the suburbs. You know: cars, commuter trains, slum clearance, the Blitz, all that stuff. In other words, Liverpool is just like many other places: after the war, this country experienced a depopulation bonanza.


So what form did this movement to the suburbs take, as far as Liverpool was concerned? Well, people moved and were moved to the suburbs of Greater Liverpool, in what are now the outer boroughs of the city region: Halton, Knowsley, St Helens, Sefton, Wirral. Others moved further, to Cheshire West & Chester, West Lancashire, Warrington, even nearby North Wales, as previously discussed here.

In common with many cities, indeed, Liverpool City Council actually built and owned large several ‘New Town’ council estates, to which they moved tens of thousands of people to from Liverpool’s inner districts: Winsford in Cheshire West (where comedian John Bishop grew up), Runcorn in Halton (where comedian John Bishop also grew up), Skelmersdale in West Lancashire, Kirkby in Knowsley. There is nothing unique or sinister here about Liverpool (apart from comedian John Bishop). This was common practice across the country – Indeed, it was central government policy – and resulted in about 160,000 people being ‘removed’ from the Liverpool local authority area.

Many other people also moved to the nearby suburbs of Greater Liverpool to private housing – another trend reflected across the country. It’s worth acknowledging, however, that cities across the world are subject to a level of ‘churn’ in population, whereby many people move out and many people move in, over time, too.

So how did those prominent images of derelict streets in the inner-city part of the City of Liverpool local authority area come about? For that, you have to blame the last Labour government’s over-zealous ‘Housing Market Renewal Initiative’ (HMRI) disaster – and the over enthusiastic participation of the then-Lib Dem controlled city council. On the promise of ‘free’ money from central government, the latter removed hundreds of people from their homes with a view to demolishing the Victorian terraces, and building new replacements. Many of these houses, in truth, were already fully modernised, owner-occupied houses within viable and longstanding communities, as can be seen here in Voelas Street, one of the famous Welsh Streets of Liverpool:

Voelas Street before HMRI implementation. Image: WelshStreets.co.uk.

The same picture after HMRI implementation Image: WelshStreets.co.uk. 

Nonetheless: the council bought the houses and ‘tinned them up’ ready for demolition. Then the coalition Conservative/Lib Dem government, elected in 2010, pulled the plug on the scheme. 

Fast forward to 2017 and many of the condemned houses have been renovated, in a process which is still ongoing. These are over-subscribed when they come to market, suggesting that the idea was never appropriate for Liverpool on that scale. 

At any rate, it turns out that the Liverpool metropolitan population is pretty much the same as it was at its peak in 1931 (depending where the local borough boundaries are arbitrarily drawn). It just begs the question: why are well educated and supposedly clever people misrepresenting the Liverpool metropolis, in particular, in this way so often? Surely they aren’t stupid are they?


And why are some people so determined to always isolate the City of Liverpool from its hinterland, while London is always described in terms of its whole urban area? It just confuses and undermines what would otherwise often be worthwhile comparisons and discussions. Or, to put it another way: “never, ever, compare apples with larger urban zones”.

In a recent Channel 4 documentary, for example, the well-known and respected journalist Michael Burke directly compared the forecast population growths, by 2039, of the City of Liverpool single local authority area against that of the combined 33 local authority areas of Greater London: 42,722 versus 2.187,708. I mean, what bizarre point is such an inappropriate comparison even trying to make? It is like comparing the projected growth of a normal sized-person’s head with the projected growth of the whole of an obese person, over a protracted period.

Having said all that, there is an important sensible conversation to be had as to why the populations of the Greater Liverpool metropolis and others haven’t grown as fast as maybe should have been the case, whilst, in recent times, the Greater London population has been burgeoning. But constantly pitching it as some sort of rare local apocalypse helps no one.

Dave Mail has declared himself CityMetric’s Liverpool City Region correspondent. He will be updating us on the brave new world of Liverpool City Region, mostly monthly, in ‘E-mail from Liverpool City Region’ and he is on twitter @davemail2017.