Category: Science

TL:DR – There is no evidence that Blackcaps that overwinter in the UK are “demigrating”, they all tend to leave by mid-April. That said, much of their migratory behaviour remains a mystery.

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I’ve written about a warbler species we see here in the summer known as the Blackcap, Sylvia atricapilla, several times on Sciencebase. Commonly, for many, many years, thousands in fact, Blackcaps that migrate to the UK in the summer have spent the northern winter in Iberia or North Africa. They migrate north to south and back again, year in, year out.

There is, however, a number of Blackcaps that tend to spend the northern summer in mainland continental Europe, Southern Germany and Austria, for instance. Those in the western part of that region then head somewhat west for the winter to spend the colder months on the Iberian peninsula or in the northern parts of Africa. Those in the eastern part head South East presumably to Serbia, Croatia, Greece etc.

In the last 20-30 years or so there has been a shift in behaviour among these German Blackcaps, ast least the ones in the west. Some of them seem to have lost their internal compass calibration and have been turning up in the UK for the winter instead of the slightly balmier climes of Spain and Portugal. Their internal compass is genetically programmed into these short-lived migratory birds though and so this “mistake” has proven beneficial as it has proliferated in a sub-population of the birds that find well-stocked garden feeders in British gardens. They have thus taken to over-wintering here and then find their way back to the continent for the summer come late March or early April.

Over the last few years, we usually have a male in our Cambridgeshire garden, and one year recently we had both a male and a female. They seem to join in the stripping of mistletoe berries from the trees as well as partaking of the fat balls we hang in the garden. Previously, they have departed in the spring not to be seen again until the winter. We never see Blackcaps in our gardens in the summer, but we do see them in local woodland and these would’ve been birds that overwintered in Iberia and North Africa.

There is now some intriguing evidence that the UK overwintering Blackcaps are not mixing so well with their Iberian counterparts and that we might begin to see speciation taking place. In this, they might ultimately stop interacting and mating and begin to diverge genetically into two sub-species. If the evolutionary pressure is sustained they might one day split into two distinct species.

Now, the male that has been in our garden through the whole of the winter of 2022-2023 has recently been singing rather vigourously from our rowan tree. It was as if he was attempting to establish breeding territory although given his shyness at the feeders compared to the sparrows and even the tits, maybe he was just singing with frustration. Of course, the males do start to sing, or at least partially sing, in March under normal circumstances, perhaps rehearsing for their leading role once they get back to their summer breeding grounds.

I asked Greg Conway of the British Trust for Ornithology (BTO), who has undertaken a lot of research on Blackcap migration about the misinformation that seems to be circulating that claims some overwintering Blackcaps are becoming resident. You can read about work on Blackcap migration in a paper from van Doren et al. on which he is co-author here: https://doi.org/doi/10.1111/gcb.15597

“Virtually all the wintering birds do migrate in the spring and head across the Channel,” he told me. “Currently, there are just a handful of British-bred birds that have remained here to winter, amongst the more than 1000 of ringing recoveries.” He adds that “The winter tracking work did not reveal any resident individuals.”

The bottom line then is that demigration is not happening in Blackcaps. Conway expanded on what I suspected about the behaviour of my own overwintering Blackcap:

“Typically, the wintering birds depart between mid-March and mid-April.,” he explains. “And, just before they go, they gorge themselves at the garden feeders to build fat stores for their long journey. So you might see a bit more feeding activity this week.” He added that I should expect my Blackcap to depart in the next spell of fine weather. Indeed, by the 9th April our overwintering Blackcap had departed.

We know a lot about bird migration and movement but much remains a mystery. I’ve previously discussed the cases of Little, Great White, and Cattle Egrets, Spoonbill, and Glossy Ibis. All of those species have extended their range into the UK over the last few decades. Of course, many migratory patterns have been altered by the ice ages, shifts and changes in environment and food supply, and in the modern age climate change.

“There is still so much we don’t know about Blackcaps and their migratory behaviour,” adds Conway. Licensed ringing and citizen science observations and reporting from birders, of course, are helping build up a picture of what happens. Hopefully, one day we can expect to get a clear understanding of this little bird’s comings and goings. Conway adds that there is also an initiative for reporting ringed Blackcap sightings. If you see any ringed individuals you can send date and place details and whether male (black cap) or female (brown cap, also juveniles have a brown cap) to [email protected]

UPDATE: 24th March 2024 – Having not seen any Blackcaps in our garden this winter, sudden sighting of a male feeding on our mistletoe. So, maybe he’s been around but just not visible. The Merlin app has certainly heard him a couple of times in the last few days when I stick my phone next to our open bedroom window some mornings at 7am-ish. (Our overwintering male Blackcap had departed our garden by the end of the first week of April 2023).

TL:DR – Advice on how to improve your chances of attracting birds to your garden.

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Sciencebase reader Michelle messaged to ask how she might attract more birds to her garden. There are plenty of things she can try to see more of our feathered friends on her patch. Some things will have an almost immediate effect others might take a little longer. The rewards are always worth the effort to see the variety and numbers of birds that can appear. I have an article about the birds you might see in an English country garden. Here’s my garden tick list.

What to do

The most obvious thing to do is to ask what the birds need and then try to fulfill those needs: water, food, shelter/cover, somewhere to nest.

So, you could put out a couple of shallow, but wide, bowls and fill them with water. One could be at ground level another on a stand. Of course, there are plenty of birdbaths of different kinds to be bought at garden centres or online. A more ambitious approach might be to dig a wildlife pond, stock it with a few native pond plants and let it thrive (avoid using a pump, don’t add fish). Birds need water to drink and to bathe and it’s always a treat to see them doing so. It’s a good idea to clean birdbaths and bowls and to refresh the water often. Birds are not particularly hygienic and will often leave poop in the water, which can spread disease to other birds.

Bird food is the next easy way to attract birds. There is a massive variety of feeders and food available for garden birds. I’ve found that it can take a week or more for birds to find a new feeder, but other people tell me birds are drawn immediately as soon as they hang one out even if they have never done so before.

Feeders designed for particular foods are usually needed. Suet and seed balls (great for Blackbirds, Blue Tits, Great Tits, Cole Tits, Robins, Starlings, overwintering Blackcaps). Nyjer seeds and a feeder that mimics a plant seed head will be a draw for Goldfinch and perhaps even Redpoll.

Black sunflower seeds (husk still on) need a strong bill and will tend to be eaten by Greenfinch and Goldfinch but sunflower hearts (dehusked) can be eaten by a wide range of additional species. Peanuts also need a special feeder that forces Blue Tits and others to break off tiny pieces of nut rather than grabbing a whole nut that might choke them or a chick. It’s important to use good quality peanuts sold for birds to avoid feeding them peanuts with toxic and cancer-causing black moulds.

Dried mealworms are tasty and attract different species. You can put them in a hanging feeder or on a ground feeder (use a refuge if there are cats that visit your garden or you have a dog). Products such as “Flutter Butter” come with special wall- or post-mounted dispensers and encourage birds to stay at the feeding station as they cannot easily grab a morsel and fly off to a tree to eat it.

Longer-term plans might involve that pond or if you haven’t already got native bushes and trees in your garden think about planting some. I’d be keen to plant bushes that produce lots of berries to provide a winter food supply for Blackbirds, Mistle Thrushes and visiting Fieldfares and Redwings. Also, let any ivy go to flower (attracts lots of insects) and produce berries (again, feeds those birds in the winter). The bushes and trees in your garden provide shelter and potential nest sites for a wide variety of species as well as hosting insects.

Adding a couple of strategically placed bird boxes also gives the birds another reason to visit your garden. They need to be placed fairly high up on a post or tree in a relatively small garden and generally have a line of sight from a post or tree opposite and several metres away if possible. If you have large windows, consider adding some decals, perhaps bird-shaped stickers, to the windows to help flying birds see that there is an obstacle and so avoid collisions with the glass. I wrote about Wood Pigeons leaving behind a powder down imprint on our windows back in 2017. Look into installing swift boxes or swift bricks too and perhaps adding a squealer to attract the birds arriving in the spring.

What NOT to do

A couple of things not to do. Don’t hang or install shiny, moving ornaments that glint and shimmer in the sun, these might shock easily startled birds and discourage them from enjoying your garden.

Also, don’t trim and prune too hard, in fact, don’t tidy your garden too well. If you leave the bushes and trees to grow a little wild and allow wildflowers (weeds!) to show through, you will encourage insects and that means an additional food source for the birds in terms of the adults and their larvae and caterpillars.

I’d also avoid pesticides and herbicides, these are generally easily avoidable in a small garden. Also, don’t put down ant powder or ant traps in the garden, ants are the staple food of Green Woodpeckers (yaffles). If there are no ants in your garden, you’re unlikely to see that bird visit.

I don’t recommend scattering bread or other cooked products for birds in your garden and no bacon rind. These are generally too high in sugar and salt for birds to cope and can also attract rats.

If you notice birds looking ill or with lumps on their heads or bodies, it’s a good idea to remove all feeders, discard uneaten contents, wash thoroughly in soapy hot water and leave them to dry. Don’t put the feeders out again for a couple of weeks so that the diseased birds might disperse.

Don’t use Astroturf and don’t pave over or gravel too much of your patch.

More on ponds

Your garden wildlife pond can be as ambitious as you like, but you can also create a great micro ecosystem using a half barrel or a Belfast sink. You might need to line them, you can sink them into the ground or leave them free standing. Let them fill with rainwater and then get some native pond plants (it’s best not to acquire materials from other garden ponds to avoid sharing disease).

Put some pots or stones within strategically placed to allow any frogs that turn up to easily get in and out and to avoid trapping hedgehogs that wander in. Terracing and shelving is essential for a full-size pond in this regard too.

An additional thought about ponds is to create a spillover area behind your point that will provide a permanently muddy patch, this will provide habitat for plants and organisms that favour something in between dry land and deep water. Again, increased natural diversity for the sake of the wildlife and for your own interest is the aim.

 

TL:DR – I wrote about bird migration and so I thought I’d write a poem about it too.

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I have a post in which I discuss the intriguing and changing migratory behaviour of the Blackcap, Sylvia atricapilla. For several years, we have seen this species spend the winter in our garden. One year we had a male and a female.

The likelihood is that these birds flew from their summer breeding grounds in southern Germany and instead of reaching Iberia or North West Africa they got slightly lost and ended up in Old Blighty…England. It’s been happening like this for a couple of decades at least.

The male that overwintered with us 22/23 has departed and soon the Blackcaps that migrate from sub-Saharan Africa will arrive in our woodlands to breed in the summer.

Anyway, I wrote a poem for Sylvia.

Overwintering

She came in from the East. Her compass quite askew
Should’ve spent the winter in Iberia, but Old Blighty will have to do
Her chestnut cap is fluffed up. And her buff breast is scruffy too
Those chilly winds chill the feast, there’s nothing else that she can do

but peck at firethorn berries behind sparrows in the queue
And sulk among the mistletoe. Till Christmas takes its due
This land beyond the floodlands, with a date she’ll take in lieu
No nest, no mate. No direction yet, a Blackcap looking blue

When seasons change, they’ll send her back to the place where once she flew
And with the spring and high on life, she’ll bid us all adieu
Then find a mate, and build a nest for now she takes her cue
She’ll live her life and raise her brood with not a thought for you

But the world will turn and the winds will change as always they will do
Bring her back to Old Blighty. Yet, with no different a point of view
She’ll sulk and peck at mistletoe. The winter she’ll see through
And bravely hide from Easterlies, there’s nothing else that she can do

–David Bradley