Why are House Sparrows in decline?
Updated: Feb 2, 2021
House Sparrows aren’t exactly the White-Tailed Eagle of our conservation world, so research on population trends wasn’t carried out until the 1970s. For those who haven’t read the previous blog, the figures found need to be a cause for concern. Urban House Sparrows have declined almost 71% since 1977 (60% of London’s House Sparrows were lost in just 10 years). But why’s this happening? The honest answer is, nobody knows for certain. The extensive research just isn’t there, and what’s available is unsure.
There are scores of hypotheses though, and below we’ve briefly gone over the commonest.
Threat 1: Invertebrate loss
Often stereotyped as exclusive ‘grain eaters’ House Sparrows need invertebrates. They’re protein-packed mouthfuls for the adults, and essential for the development of chicks. Without sufficient invertebrates, and the life-enhancing elements they contain, House Sparrow chicks won’t be able to survive.
Today, 40% of Britain’s invertebrate species are vanishing, and insect-rich wildflower meadows are constantly reduced to neatly trimmed turf- ecological desserts. When you think about it, it’s no wonder House Sparrows are struggling! Chicks are likely starving in their own nests (sadly the one place you’d think they’re safe!). Without as many invertebrates, House Sparrows have a much unhealthier diet, scavenging human rubbish instead, which in itself is a threat, inducing physical and mental stress on the birds.
Threat 2: ‘Efficient’ agriculture:
We’ve come a bafflingly long way from travelling everywhere via horses 100yrs ago, also the time when tentative research was first carried out on House Sparrows. It showed that from the 1920s, when we shifted from riding on horseback to comfy car-seats, the vast amounts of grain spilled by human (or horse) error whilst being transported disappeared. This robbed city-centre House Sparrows of a central food source-the first turn in a downward spiral.
Agricultural development in rural areas was also co-evolving, for example livestock were now living their entire lives inside. Flies, midges and other invertebrates attracted to the livestock, and their feed all disappeared, another important food source stolen. A rapid shift to pesticide, herbicide and fertilizer use further punched the curve of invertebrate food down, and from the Great Wars onwards, thousands of miles of hedgerows, a vital lifeline to House Sparrows and their prey, were ripped out so even more crops could be grown.
Threat 3: Plasmodium relictum:
Long surrounded by scepticism and suspicion, Plasmodium relictum is a contagious parasite, solely responsible for the previous extinction of several species of bird. Effectively, it’s a bird version of Malaria. It’s vectorised by infected Mosquitos, so Britain’s mild climate has let it be passed round like wildfire, especially in the South. Thankfully, fatalities are extremely rare, but it can inflict a lot of interior damage, like weakening vital organs, and attacking parts of a House Sparrows DNA, shrinking their lifespans.
Luckily, it’s most damaging after first infecting a population, and signs show infected populations are slowly but surely becoming immune to it. However, this is where the ever-present thorn in everyone’s side that is Climate Change comes in. With temperatures rising, the parasite is expanding into new territories, where the cold previously warded it off. Alarmingly, it’s recently been recorded in Alaska, for the first time ever.
Threat 4: Reduced nesting spaces:
You only have to wind the clocks back decades to see how drastically the buildings lining our streets have changed. Today, they’re sleek, sturdy bubbles of warmth, usually completely cut off from the outside world. And though we call it progress, House Sparrows probably have a different name for it, since it’s one of the drivers behind their diminishing urban population. Previously, houses (particularly the eaves) would’ve been poker-dotted with gashes and crevices in the brick/tilework, flawless nesting spaces for them. Now, with over ¼ of the population blocking current gaps, and so many new buildings being sleek constructions, made from glass and metal, they’re fast losing their specialist habitat.
We believe ivy could also have an unsuspecting role to play. Especially in dense, built-up areas, where space for plants can be luxury, climbing plants like ivy are invaluable, and often harbour small colonies of precarious House Sparrow nests, where gaps in buildings are scarce. But the increasing perception people have of ivy as ‘ugly’ and the common misconception that they kill
trees means most is killed off and cut down before it can be of use. Understandably, it’s also not always safe/practical to leave it growing there, as it can be destructive, but if you’ve got the room, it’s a brilliant plant for wildlife.
Threat 5: Pollution
I’m sure a lot of you could’ve guessed this one… While water and plastic pollution (microplastics worming their way into the food chain is an especially urgent problem) the main killer is air pollution. Toxic fumes from vehicles are the main killer, as one BTO survey found out. Out of a few thousand houses surveyed, they concluded that dying/dead House Sparrows were much more likely to be found in colonies near roads with dense traffic. So how is air pollution actually affecting them?
We’ll go more in-depth in this in another blog post, but it’s mainly down to something called ‘free radical pollution’. Free Radicals are atoms/molecules which lack an electron on their outer shell. To fill the gap, they ‘steal’ electrons from living cells (including ours!). Overtime, this weakens organs and tissues, e.g. the connective tissues you’ve got in your face-hence when you smoke regularly, the free radicals in cigarette smoke weaken the tissues and give you more wrinkles. They’ve also got the potential to cause cancer overtime, in wildlife and us.
Threat 6: Predators, predators everywhere
Humans are the ‘big bad’ when it comes to species damaging the environment, but we aren’t alone…
Cats. First kept to control mice on farms, are found on every road in Britain (averaged out). Cute they might be, but predators they also are, having been domesticated for a fraction of the time dogs have. In Australia, domestic (including released) cats kill over a staggering 1,000,000 birds daily. And in Britain, they kill 100,000,000 victims every breeding season. Globally, they’ve single-handedly caused 63 extinctions in recent times.
Specific data on their relations with House Sparrow decline in Britain is weak, but they’re suspected to have taken a bite out of London’s population.
Maybe a more unsuspecting curve-bender is the infamous Sparrowhawk. Less damaging than domestic cats, as evolution designed it and House Sparrows to live like they do, but nonetheless a reason for their diminishing numbers (exclusively urban ones though). It’s thought that House Sparrows were so quick to exploit the niche of living side-by-side with us, they had a time free of predators, bar the odd woodpecker. Now, other wildlife are slipping into similar niches. Take Sparrowhawks, which are regulars to large urban/suburban gardens. But, in their absence, House Sparrows have become sluggish and less wary, so before they re-gain their previous reactions to Sparrowhawks, a chunk of the population will be taken, before (hopefully) bouncing back.
However, this is a great example of a lack of evidence. While some research says Sparrowhawk predation is playing a part, ironically, in the BTOs extensive survey, House Sparrow numbers were higher in areas with Sparrowhawks than without. However, this could just be because Sparrowhawks only live in areas with cleaner air, so the numbers killed by Sparrowhawks are
encompassed by the masses killed by air pollution. Ideally, more research is needed.
And that’s it! Remember, these are only the commonest threats-later on (and on our social media’s) we’ll be writing more in-depth about these. If you fancy learning some of the things you can do to help lessen these threats, and encourage these bubbly characters into your area, you can read our blog on it here: Helping out House Sparrows (savethehousesparrow.com)
(Some useful links on House sparrow decline):
RSPB on House Sparrow decline: https://www.rspb.org.uk/our-work/conservation/projects/causes-of-population-decline-of-urban-house-sparrows/
BTO House Sparrow Survey 2002: https://www.bto.org/sites/default/files/u23/images/about_gbw/house-sparrow-survey/bird_table_63_housp_survey.pdf
General House Sparrow information by Wildlife Trusts: https://www.wildlifetrusts.org/wildlife-explorer/birds/larks-sparrows-wagtails-and-dunnock/house-sparrow
Effects of Pesticides on birds: http://www.wisconsinbirds.org/wp-content/uploads/2017/04/effectspesticides-1.pdf
2019 Blog post on House Sparrows + their decline: https://blog.nature.org/science/2019/03/25/where-have-all-the-house-sparrows-gone/
Wildlife Trusts-House Sparrow decline: https://www.wildlifetrusts.org/wildlife-explorer/birds/larks-sparrows-wagtails-and-dunnock/house-sparrow
Thank-you for reading!