Crataerina - a problem?

[Carol Collins]

Whenever I read about swifts having Crataerina (which I presume they all do) the accepted view seems to be that the parasites do them little harm - that the swifts are not much bothered by them.  My experience seems to be different.  

I have a pair of swifts nesting under the eaves (not in a box) and I have a camera on them.  The first year they were on camera (2024) they produced three eggs and initially all appeared to be going well but the infestation of Crataerina gradually built up (several on each bird and others obviously moving around the nest area) and in due course all three of the chicks died.  Admittedly the spring was cold and far from ideal, so I was unsure whether the failure was due to the parasites, the weather, or perhaps a mixture of the two.  

The following year, 2025, the birds returned, used the same nest, and successfully reared three chicks - the weather was much better and they had few Crataerina - so I don't know which factor was the more important.  

This year they are back, in the same nest but with only two eggs. For the first week or so all was well but now they are obviously being bothered by Crataerina - they are continually restless, preening, apparently trying to get rid of the parasites.  Watching them is no longer a pleasure because they seem so uncomfortable and I suspect that the same will happen as happened two years ago.

If most folk are watching swifts in boxes, perhaps cleaned out each year, it may be that the population of Crataerina never builds up?  Does anyone else see swifts obviously bothered by them?

Carol

Hathersage, Peak District.

[Tony Croft]

Funnily enough I've always wondered about these parasites and I was hoping now we've had no returning swifts for 4 years we might be rid of them?

Best Regards
Tony

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[Dick Newell]

For questions like this I often turn to AI, Most times I get what seems like A sensible answer.

I asked Google "Do Crataerina cause harm to swifts?" and again, the answer seems reasonable to me.

Dick

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[Louise at Bolton and Bury Swifts]

Hi Carol,

Very upsetting to watch the chicks perish. To lose all 3 seems highly unusual. Even in wet cold summers here the eldest 2 swiftlets fledge with the 3^rd perishing due to lack of food. That makes me wonder if something else was going on that year besides the cold and the crat? 

I'm sure they make the Swifts uncomfortable if there are several. I am lucky in that I have easy access to boxes to clean them out.

I once rescued a grounded adult Swift that was underweight and carrying 37 crat!!! A horror show as they all jumped onto me or hid in the carry case! However, it is known that parasites can thrive with a sickly host. This particular Swift was relieved of them all, fed well and released. 

I wonder if numbers of crat build in some nest sites causing Swifts to abandon the nest? Or whether the majority of Swifts cope well with them for most of the time? 

Do let us know what happens in your nest this year, it will be very interesting to know. I suspect they will be okay (like last year) 🤞🏼

Best Wishes,

Louise

 

      

 

Bolton & Bury Swifts

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House Martin Conservation

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From: swiftsloc...@googlegroups.com <swiftsloc...@googlegroups.com> on behalf of Carol Collins <carolwc...@gmail.com>
Sent: Thursday, June 4, 2026 9:53:02 pm
To: swiftsloc...@googlegroups.com <swiftsloc...@googlegroups.com>
Subject: [SLN] Crataerina - a problem?

[Rowena Baxter]

Hi Carol

This is just my opinion and not based on observations.  It's no advantage to a parasite to kill its host, so I would think under favourable breeding conditions for the swifts, Crataerina are not a problem, but when you factor in poor weather conditions and a parasitic overload, that is a problem.

Horrid to watch under those conditions, I sympathise.

Rowena Baxter

--

[hythescream]

Ive seen many films of this, and it's not nice to watch.

I'm just using common sense and mathematics here;

When you scale up the blood loss, it's equivalent to half a cup of your blood per day! All animals that have blood work on the same basis. If you lose blood you become weaker. You might not die (parasitic theory), but your performance will suffer. Try running a marathon after making a blood donation, for instance.

So, in my opinion, there is no doubt that this problem will weaken ANY swift, especially one that is feeding it's young. You don't need much to cause the loss of a chick. The weather or a slight lack of local food could just tip the balance. 

If a swift is on top form, it will feed it's chicks well (might even get three out if it's lucky) and they will go onto breeding themselves, thus increasing the population even more.

The reduction of swifts by 60% over a few decades, that we are told about, isn't actually that much mathematically, because these effects compound over generations. 

If two swift chicks survive because they and their parents are not infected or weakened themselves, and bring them to fully fledged condition, this is what could happen:  They might start breeding at the age of 3, and breed two chicks per year for 7 additional years, repeating each generation in the same way,.......after 20 years you could have 900 swift descendants from just two surviving swifts. Just think about that for a minute.

There's no doubt that they have survived these creatures for Millenia, but we are now aware of the problem, and more importantly, we can do something about it. Our mission is to increase the swift population, right?, so if we can do something to help just two swifts, you could be helping 1000 more in the next 25 years.... 

In the world of sport, this is called "Marginal Gains" and can be the difference between a gold medal and coming last.

The simple cure for this, if you can do it, is to take down your box out of season and place it in a bag and put it into a freezer for 48 hours. This will kill off anything that's waiting in the cracks for next year. Of course swifts are likely to catch these again, but there doesn't seem to be any evidence that they survive a trip to Africa. Why would the Crataerina pupate in the nest site if it wasn't planning on the future? Having a 9 month hot ride isn't the best approach to it's own survival, but setting up for the return to the nest is a good plan. If your swifts come back to a clean nest that gives them a good start. It's quite possible that a monogamous couple may not catch this parasite anyway once you've got on top of this. The more of us that do this simple procedure, the better, so if you can, schedule it into your autumn plans.

Just imagine how they would feel, coming back home and not having that to deal with. Their host has kindly intervened to help them on their way. And you don't need to watch them suffering either. It's clearly distressing for them. Try a couple of days in Scotland during midge season if you want to know what it feels like!

If you are planning on fixing up new boxes where this cleaning procedure is feasible, try to use a fitting that can be unclipped easily for this annual procedure to take place.

Every little helps, as they say.

Ron

[Rowena Baxter]

Sensible deductions and good advice Ron

Rowena Baxter

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[Dick Newell]

Hello Ron,

I am sorry that I disagree with you. As multiple studies have not found any negative effects of crataerina on Swifts, why should we worry about it?

I agree, the studies have not shown anything positive either but just because many of us are born with aracnophobia, an irrational condition, is no excuse for taking such a negative stance to flat flies (or spiders).

Isn't the truth of the matter that crataerina cause no harm to Swifts, but a sick swift may attract crataerina. Cause and effect could be the other way round.

These things have been evolving together for a lot longer than millenia - many millions of years in fact.

Who knows, there may be something symbiotic going on.

A note on your numbers, the average Swift lives for 7 years; as you say, they may not breed until they are 3, so they would have only 4 or 5 breeding seasons. By 2025 Swifts declined by over 70% in the UK since 1994

I would prefer people to not waste their efforts clearing out crataerina, but more on things that would make a difference:  preserve nest sites, more nest boxes and a nest form in every nest box

Dick

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[hythescream]

Thanks for your reply Dick. No need to be sorry, we are simply trying to improve swift survival, so all thoughts are welcome.

There are studies into this, but studies on swifts are difficult to say the least. There's a very high first year mortality rate for instance. Does anyone know that that's because a weakened swift becomes prey, struggles to make it to Africa, doesn't have enough energy to catch enough food etc. How do we know what they get up to, and to what effect this blood loss has on them? They will exit the nest for their first time and have to immediately get their own food. Probably carrying a large voracious fly for at least a while, just when they need the maximum amount of energy and fuel. The fly has done it's biological duty, then it pointlessly sucks swift blood until it dies.

All I know is that every animal based on a blood system will suffer with a shortage of it. Look at how anaemia affects people. Swifts will suffer from irritation too. These things all sap a certain amount of energy and in a poor food, bad weather year, that could make all the difference to a number of birds, and that effect will certainly be above zero whatever some scientist has come up with. And it's preventable at a local level.  At a species level it makes no difference. Swifts shouldn't be red listed really as there are plenty of them. They are possibly the most successful and adaptable bird ever.

What I'm talking about is the UK dwindling population. It's on a knife edge. Every bird counts. Even those rescued. Watching people's cameras it's obvious that they are suffering. As humans, we can now help with that suffering and give them the very best chance to reproduce. It's just one tool in the box. We can't wipe out the flat fly population.

I challenge the idea that weak swifts could be targeted. If that's true, then they will end up weaker still. We would find dead swifts in boxes probably. These flies have found a way to exist, just like malaria and ebola have. It doesn't make it right to ignore it. It empowers us to act upon it. We do this for our own species to remove suffering, and prolong life. Why not do it for swifts too if we are able to in such a simple way?

You can't use AI to solve these problems, because it can't think outside of the box... yet.

For those that feel empathy for their swifts, cleaning the boxes will reassure you. It will give your swifts a comfortable life and the best chance of survival during difficult times.

Maybe someone here with known infestations of a regular pair and a camera system would like to follow this up by disinfecting their box this autumn and reporting back next year. That's the way that we should be pushing the boundaries of our own endeavours to helping these birds. Any offers out there?

You mention more nest boxes and nest cups. Thats another gripe that I have! Why are boxes so expensive? A swift doesn't care how much it costs. The sad thing is that out of a £40 box, plus cup, plus postage, 20% of that cost goes straight into the treasury. It's a luxury item! There's also income tax too and lots of carbon dioxide. Many groups obtain grant money to be spent on these boxes. It all feeds back into the government coffers. They profit out of swifts! They can't even get builders to install boxes, or deal with the crimes in other threads here.

 When I decided to make boxes myself, I was shocked at how much it actually costs. Using the SLN discount, using simple hand tools only, and casting my own cups, the total cost of a upvc box, glued, screwed, and cupped, comes to under £5 per box (including VAT!). Thats ten boxes for the price of one. The effect of this on the swift population compared to flat flies is enormous! What are we doing about that?......

Thank you for listening.

Ron

[Swifts (SOS)]

Morning all

The mention of £40 nest boxes made me want to reiterate the sustainable business model we have here in Suffolk for Swift conservation. SOSSwifts is a joint Suffolk Bird Group/Suffolk Wildlife Trust project, we are both registered charities. Claiming GiftAid back from the government is a very important part of it. For this to be achievable voluntary donations have to be request after any of our nest boxes have been given out so no evidence of trading visible.

If local Swift groups are not registered a registered charity then having a county bird group or wildlife trust handle their finances can be very beneficial financially. 

We are fortunate here to have a volunteer donating his time to make nest boxes. We supply the marine ply and uPVC plus glue, wood preservative and fixings for him to use and occasionally fund replacement tools to help too. He makes both flat lidded nest boxes for fitting tight under soffits or sloping roofed model 30 style boxes for walls. On this basis our box costs are around £7 each, similar to you Ron. Our volunteer can just about keep up with demand and has now made over 1,000 boxes in 2 years

We have sourced nest boxes from Men’s Sheds for around twice that price but found quality variable with odd baffles added, cheaper ply used and glue missing. The ace John Stimpson is a much more reliable source of quality nest boxes when we need a top up during excess demand though his lead-in times have become very extended.

SOSSwifts provide free surveys and guidance and suggest donations of £25 per box AFTER delivery, GiftAided if possible. The funds generated allows us to offer free Swift Community Sets for prominent elevations of public buildings, these consist of multiple nest boxes and a 240V call system. The surplus also allows us to donate nest boxes to locations where or adjacent/opposite to Swifts are nesting to boost local populations over time.

Some folk prefer the PeakBoxes gable boxes or Woodcrete alternatives rather than our, and they purchase directly.. We don’t receive any funding on this basis but it is still new locations around the county installing nesting space and playing calls, many with success now.

We don’t advocate the cleaning out of nest boxes here unless the occasional wasp or hornets’ nest have filled them up. 

Suffolk Bird Group use the same business model for nest boxes targeting our rarer breeding species that will readily take to them; Redstart, Spotted Flycatcher, Nuthatch and Barn/Little/Tawny Owls for instance. We fund the materials and even have someone volunteering to weave Long-eared Owl and Hobby nest baskets.

The 2026 season is still young but it is great to hear of Swifts using boxes for the first time, we also have received many reports of locations with increased uptake over 2025 box numbers. 

Very best wishes

Eddie Bathgate

@SoSSwifts

SuffolkBirdGroup.org/sos-swifts

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[hythescream]

Thanks for explaining how you run your superb outfit Eddie.

Clearly, you need to have a large scale concern to run it as a charity and work around the deferred funding hack. It's great to hear that you are making your boxes "in-house" via your productive volunteer, and this is definitely the way forward to maximise swifts per pound! How do you deal with the installations (what type of people do this for you and what does that cost?) and sound systems within your system? Who pays for those elements? Do you recycle the players? With 500+ installations per year, you must have a slick system in place.

Ive seen the Men Shed style boxes in several places. They are laughable to be honest.

I've barely started, but knowing how much to ask for as a "voluntary contribution" is a good way to move forwards. If I fail to get momentum, I will go into full time box manufacture instead and support a larger more successful group.

Thanks Eddie,

Ron

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[Swifts (SOS)]

Hi Ron 

We keep it very simple and have a ‘no ladders’ policy for the project. We don’t have to worry about insurance policies or risk assessments, these made a previous Barn Owl project very tricky/complicated administer and keep compliant. 

If people are not installing their own nest boxes we have a number of contacts we pass on who will install for a small charge when passing; an aerial chap, chimney sweep and painter-decorator for instance. We do struggle to cover the north or the county though. 

For playing the calls the Community Sets we donate have a 240V PeakBox call system. They have a warranty and super customer care. Domestically our advice is use a Bluetooth speaker if they know the technology or for them to purchase a PeakBox system if they don’t. My advice is simple, don’t play calls if you don’t want Swifts. 

We don’t donate the Community Sets unless recipients agree to install and play calls, this is very important. Schools have used DIY friendly PTA parents or maintenance teams. Some of the local groups have a volunteer who installs which helps considerably. One of the groups persuaded a local hire company to drive a cherrypicker around Framlingham to install gable boxes, simply superb. 

Churches - we are now in a fortunate position to be able to fund the materials and provide a 240V call system for viable projects. Recipients must have a volunteer to make/install who is prepared to get up into the belfry. We’ve also had an increase in the number of churches permitting external boxes fixed to timber soffits or exposed rafter feet around the eaves, again we donate but them to install. 

A nest box maker like yourself is extremely valuable Ron. Please do shout out where in the country you are, I’m sure there a groups in need of stock. 

Good luck!

Ed

SuffolkBirdGroup.org

On 6 Jun 2026, at 23:34, hythescream <hythe...@gmail.com> wrote:

Thanks for explaining how you run your superb outfit Eddie.

To view this discussion, visit https://groups.google.com/d/msgid/swiftslocalnetwork/1b9efd7f-9179-447d-a99b-aa19035da93an%40googlegroups.com.

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[Carol Collins]

Hi Tony,

Yes, it would be good to think you might be but I have no idea how long the pupae can survive - obviously over one winter but perhaps longer?  My experience of watching them does make me wonder whether a build up of Crataerina numbers might cause a colony to move?  There are dozens of old nests under my eaves (apprarently a mixture of house sparrows and swifts) which suggests that there used to be far more than a single pair - indeed there were two pairs when I moved in six years ago.  I know that house martins have a reputation for moving for no apparent reason from a site they've used for a while - could parasites be a factor?

Cheers,

Carol.

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[Tim Norriss]

Hi Carol

 

We have also watched Swifts on camera here in Hampshire being seriously troubled by Crataerina to the point of nest failure. Where we have access to Swift nest boxes and check them annually such as at the Hospital of St Cross and Winchester Cathedral we now squash all Crataerina pupae that we find during the autumn box check. And Adie Smith in his large colony in Bishops Waltham also does the same in the boxes when they ring the youngsters. Every little helps.

Not all Swifts have Crataerina. We do find some used boxes that are ‘clean’. We haven’t kept records but perhaps we should.

And I disagree with Dick here, Crataerina are straightforward parasites. There is no evidence of a symbiotic relationship.

Cheers

Tim

Hampshire Swifts

 

 

From: swiftsloc...@googlegroups.com <swiftsloc...@googlegroups.com> On Behalf Of Carol Collins
Sent: 04 June 2026 21:53
To: swiftsloc...@googlegroups.com
Subject: [SLN] Crataerina - a problem?

 

Whenever I read about swifts having Crataerina (which I presume they all do) the accepted view seems to be that the parasites do them little harm - that the swifts are not much bothered by them.  My experience seems to be different.  

--

[Dick Newell]

Hi Tim,

On the symbiosis point,

Presumably loss of blood is a negative for Swifts.

However, the Swifts do not appear to suffer a reduction in breeding success, survival or anything else.

Therefore it is reasonable to deduce that the loss of blood is compensated by something else, we know not what.

Maybe the flat flies are doing something useful for their hosts as payment for the blood that they take.

What this may be, one can only hazard a guess: something to do with the immune system? A grooming service as they nimbly move through the feathers? Reducing other parasite load - who knows?

Dick

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[Tanya Hoare]

The literature I have read suggests that if Crataerina were to seriously cause a decline in the swift population this would not be in their best interests. Crataerina pallidus is specific to swifts. No Crataerina, no swifts. I suspect the relationship has evolved over millions of years. Our experience of pupae is that the flies emerge in response to heat. I on one occasion left a previously occupied swift box on a carpet by a window in the house (I know!) In about April I noticed the box was surrounded by dead Crataerina. They had hatched and died for lack of food. We do however clean out nests to which we have access. However in the wild this is not possible, yet swifts survive. Dead swifts have been found with heavy infestations, but it has been suggested that these swifts were in some way sick and the parasites had simply hastened the end. However a dead swift is no use to a Crataerina. The jury is out but I favour Dick’s view that the relationship is in some way symbiotic. A fascinating subject which could do with further research.

 

Edmund Hoare

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[Tim Norriss]

Hi Dick

Yes loss of blood will be a negative.

There seem to be a few instances where Crataerina have had an effect on breeding success.

Anything else is pure supposition. I will stick with the view that they are parasites that give nothing but misery in return. Interesting parasites but still parasites.

Tim

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[Dick Newell]

Let's agree to disagree Tim.

When hard evidence emerges, rather than a few aracnophobic anecdotes, I may change my mind.

It is indeed surprising that no research has detected a negative effect. According to Google, a Swift has between 3.5 and 5 mL of blood

One flatfly consumes between 38 and 50mg of blood every 5 days. That is about 1% of a Swift's blood - perhaps a Swift can easily cope with that. However 10 flat flies would take 10%, which sounds a lot - so how come the Swift copes with that? But they do. I suspect there is something wrong with Google's numbers.

Dick

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[Audrey Jarvis]

In case you haven't seen this and it is of any interest.

From Edward Mayer a while ago.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3029137/

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[Hythe Scream]

Thanks for the research link Audrey. I read it!

There's no real evidence either way, but I would say that's because proper studies haven't or can't be carried out accurately enough to have a clear outcome.

Some consensus is the idea that there's a hidden symbiotic arrangement. That's possible, but there's no evidence at all to suggest this. There are two types of parasites, some like the "cleaner fish" do a good job for the host, but some just live off the host and survive in their own balance by not killing them. There's no rule about being symbiotic. Take flu as a virus example. There's nothing symbiotic about that! From what I have read, and understand about blood loss and irritation, there's no doubt that swifts do NOT benefit from this parasite at all, and it could easily cause them to move out of a perfectly good nest, or be weakened detrimentally to their output of fit offspring. I would advise anyone that has the capacity to disinfect their boxes to do so. It's down to "marginal gains", and the swift population needs every gain it can get. If you keep your swifts fit and strong it can only be a good thing, and until better research comes along it makes sense to follow that guidance if you can. I'll reiterate the importance of this; Just two swifts gained from any cause related to this parasite, could, in theory, create over 500 additional swifts in two decades. There will be other losses in these totals of course, but it shows what the long term impact could be. Every little helps, as Tony said earlier.  

If anyone in this group has a faithful, regular pair under a camera, where the flies can be clearly seen at the moment, and/or where the swifts show irritation -  would they please disinfect the nest this winter and check what happens next year and consequently? This citizen science is the way we can do some of our own research. 

Thanks

Ron

[Dick Newell]

When someone gives a pint of blood, that could be 12% of the blood they have (an armful as Tony Hancock used to say) and that is in one go. It seems we vertebrates can tolerate a substantial blood loss without major deleterious effect.

We recover plasma and platelets pretty quickly, though red blood cells takes much longer.

Dick

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[Edmund Hoare]

 I agree it is sensible to remove pupae from nests to which you have access. However if is hard to imagine the general public would do it. As to disinfectant-which and would it affect pupae? Edmund

Sent from my iPhone

On 8 Jun 2026, at 01:07, Dick Newell <dick....@gmail.com> wrote:



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[Tony Croft]

Good thinking....

We really don't know what's best...

Just wonder if we "disinfect" the nest box would thus put off  a returning pair...?

We could try various practices in different colonies if that were possible?  Perhaps interest someone in doing / orgaising a controlled trial across a few sites for a pHD or similar?

Just a thought....

To clean and sterilise or not to?

Best Regards
Tony

On Sun, 7 Jun 2026, 22:56 Hythe Scream, <hythe...@gmail.com> wrote:

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[Dick Newell]

For those of you who still think it is desirable to clean out a nest box at the end of the season, I can say that if you remove the nest, that is a terrible idea. The Oxford museum did this at one time, with negative effects on breeding success. They stopped doing it. 

We proved conclusively that Swifts occupy nest boxes with a nest form much more quickly than nest boxes without. They are more likely to breed, as they don't have to first build a nest.

Preserving their nests intact is hugely more important than removing a parasite that has been proven to have no negative effects.

Besides, climbing a ladder to take down a nest box to put it in the freezer, then climbing the ladder again to put it back up, is twice as much ladder-climbing as putting up a box in the first place. Why not put up 2 more nest boxes for the same amount of ladder-climbing?

And as for chucking chemicals into the box, what are we thinking of?

What am I missing?

Dick

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[Jean Stafford-Baker]

Thanks for the reassurance Dick. We've seen Crataerina for the first time this year. It's good to know they won't negativity impact the Swifts.

We have no intention of touching the nest. We waited 8 years for our Swifts to choose our box & wouldn't interfere in that way.

Jean (Norwich)

Yahoo Mail: Search, organise, conquer

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[Tony Croft]

Nest provision more important than the effect of the parasite I think based on what Dick is saying...still don't like seeing them there though!

Best Regards
Tony

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[T Davies]

One of the things I like most about wildlife, and swifts especially, is that they aren't pets, they are free, and being free and wild includes a lot of dangers and chaos, and that is part of it, although that doesn't mean I can’t help them...

.. but I'd be somewhat disappointed if for example every swift nest in the future was monitored and cleaned and all swifts were ringed, and followed using GPS.

They are up there in the sky doing their own thing and they are amazing.

Let's give them spaces they can use, we are all agreed on that.

But the main point seems to be everything Dick has said, and also everyone needs to know how much vast and real experience Dick has with swifts, for much longer than any of us have been doing it........ most importantly don't remove the nest, and a 'nest' starts off as a vague collection of 'rubbish' that you would easily destroy by mistake thinking it needed to be cleaned. I have heard Crataerina pupae look like minstrels (chocolates) - even picking through the 'rubbish' and removing the 'chocolates' does seem an unnecessary risk and a lot of effort. 

Free the swifts! Less interference! Nests are not neat! Don't squash the little creatures just because they freak you out!

 

Best wishes 

Talya Davies 

Brent and Westminster Swifts 

On Monday, 8 June 2026, Tony Croft <the.cro...@gmail.com> wrote:

Nest provision more important than the effect of the parasite I think based on what Dick is saying...still don't like seeing them there though!

Best Regards
Tony

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[Hythe Scream]

Ok folks, let's deal with this sensibly. Some of you are getting confused.

I'll start again with the problem, the evidence, the cure and the benefits.

The problem.

From camera footage, it's clear that the birds visually suffer from this infection. That clearly upsets them, and also upsets me to watch any animal being uncomfortable. There is some suggestion that they will leave their nests even. But onto the physical effects, it's got to affect their production. Maybe not every time, but if the weather is bad, food supply a little low, and you have the straw that breaks the camel's back. No one knows how many young swifts perish from this in their first flight to Africa. The motality rate is high, and somewhere around 50%. They might struggle to get there for all we know because of this problem. Until they can shrug off the fly, they will be weakened and struggle to get enough food.

The evidence.

Firstly, all known animals that lose blood will fail to perform at their best. This can be demonstrated by taking half a cup of blood off you each day (the swift equivalent scaled up) and then sending you down to Tesco's to get me some food 20 times a day. I am guessing that it will soon have an effect on my food quantity! You can observe those with anaemia to see how they suffer. In the real but strange world of cycling, riders used to ride in the mountains to improve their blood, then take a few pints to pop into the freezer. On race day, they would replace their blood with the 5* frozen blood variety to win their races. Blood quality and quantity is important. Now onto blood sucking parasites. Research has suggested that there are ZERO known cases in ANY creature on this earth where blood sucking insects give any benefits back. A parasite and symbiosis are two completely unrelated things. Most blood suckers will cause blood loss and even spread disease. Human equivalents of that are Malaria, Dengue fever, Lymes disease, Zika, Trench fever, the Plague and many more. They all, without fail, disable people. The massive loss of swift blood is bad enough, but they may even have a disease that's not yet been discovered. It could even be part of their recent demise. A disease can be newer than 50 million years old. Take HIV as a recent example. Lots of diseases spread across species these days, and for all we know, a "relatively harmless" fly could now be spreading blood borne diseases too. They might kill the birds after 6 months! Think about that for a minute!! Reading the swift article, the evidence quality around this fly is very sketchy to say the least.

The cure

We can't cure this disease, that's clear. But we are in control of their population increase in the UK, and we want the best for our swifts. When I used the word "disinfect", that doesn't mean hoovering out their nest or using bleach!! I explained that the best way is to put the box into a bag, and then into the freezer for 48 hours. Alternatively, you can hand pick the pupae. Obviously, many boxes are no longer accessible. But we have a plan. In my own case, I don't yet have any birds in my boxes, but I designed them to be taken off easily so I can do this when it's time. I suggest that when it's possible, you install boxes in such a way that they can be maintained. You might need to remove dead chicks or unhatched eggs as well. Many boxes are designed with removable fronts. But it's better to take them down if possible. (As a side issue, I'm working on a box design that can be removed and put back up from the ground. This would revolutionise this issue)

The nest itself should not be damaged by sweeping it all out. As mentioned, this is important. I pre-feather my new nest cups before installation anyway.

The benefits

Firstly, the swifts will live comfortably, and be able to get on with the job at hand. We will know that they are not suffering, even if we can't watch this on camera. Any marginal gains can have a massive impact on their population's plight, although we cant measure this.

The science

If there is anyone with cameras and known infections, would they please raise their hand if they are able to help with some citizen science. The chances of a PHD being conducted in this day and age are near to zero, so it's down to people like us. We are the knowledgeable custodians. We need boxes with infections and cameras. We need someone to hand pick, someone to freeze, and someone to monitor a control box. The more participants the better. I'm happy to design an experiment and collate the data.

As the original OP stated, watching them suffer is horrible. No one wants to see this. It's possible to stop it for any birds in your custody. That is a positive enough reason to do something about it.

Without real scientific studies, it's easy to dismiss this problem, but using logic and experience of other animals including humans, there's a good hypothesis to proceed with some remedial action. Every little helps as you say.

Thanks for listening to me repeating myself ;-)

Ron

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[Dick Newell]

Well Ron,

If you want to study this issue, there are plenty of Swift webcams here: https://gierzwaluw.website/webcams.html

Let us know when you find one with a big infestation.

Dick

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[Hythe Scream]

Thanks for the links Dick. I'll take a look, but I can't get experimental data out of these without cooperation from the owners. 

Thats why we have the best opportunity within our group, where the issue was originally mentioned and clearly upsets many of us.

I am happy to facilitate everything and report on the outcomes, but I need data from those with problem boxes, and the ability for the conditions to be changed and monitored for a couple of years. If no one comes forward, then we lose our chance to help these lovely birds. If anyone reads this message, please read my previous one too, and message me privately if you want to take part. You can use a different email address to the one you use here if you wish (send me a message to hythe...@gmail.com  Put "Crataerina survey" in the subject line.) I will not divulge your name or location, I will just speak with you individually and collate the results anonymously. If you know anyone else outside of this group that would like to take part, then talk to them and show them my previous email if you are able to.

To take part in this important research, you need to have boxes where they are monitored with a camera, currently experience flat fly infestation, and where you are able to remove the box for treatment during the off-season.

I know that many of you will be worried about the swifts under your control and the affects that this parasite has on their general wellbeing. It's horrible to watch, so let's help them to lead happier, and more comfortable lives, and to be more productive towards our goals to slow their demise in the UK.

Thank you

Ron

[an...@tali.me.uk]

Much of the rationale for demonising Crataerina seems to be based on its less than appealing appearance and unfortunate dependence upon Swift blood to survive.

 

But what are the facts? Review of the literature suggests that there is no evidence that infestation with Crataerina has any impact on the breeding success of Swifts. This seems at odds with common sense or “logic” which would surely dictate that any parasite which removes blood must be having a negative impact on survival?

 

However, we know Swifts and Crataerina must have evolved together over millions of years and so, on a population level, given that Swifts are still with us as a species, they can clearly cope with some blood loss. On the other hand, at the nest level, on occasion, perhaps an overwhelming Crataerina infestation can either be the consequence, or the cause,  of chick weakness and death?

 

It’s worth bearing in mind that wild birds host an astonishing array of parasites. I have no information for Swifts but swallows, for example, have been found to host “..at least 16 species of helminths, 8 species of mites, 3 species of louseflies, 7 species of fleas, 1 species of feather louse and 11 species of protozoan blood parasites” (this is a quote from Ian Newton’s book Bird Populations) so leaping to conclusions about the one parasite which is actually visible may be unwise.

 

Returning to the statement that Crataerina has not been shown to have any impact on swift survival, might it be worth someones time to review the publications which were used to come to that conclusion? Did they have control nests (i.e. crataerina-free?) against which breeding success in infested nests was compared, or did they show that the level of infestation was unrelated to nesting success? Either way, surely now we can look at this in more detail?

 

Every year thousands of Crataerina-free swift nests are installed across the country, many with cameras. According to the literature the Crataerina lifecycle takes place entirely at the level of the nest site. Swifts leave for migration free of Crataerina and arrive back free of Crataerina, so the only way a nest site can become exposed to Crataerina is if a returning swift happens to visit an infested nest en route. Although recent work suggests this is more likely than was originally thought to be the case, you’d think that new site sites would only gradually be infested with the parasite, allowing us the opportunity to compare breeding success in Crat-free nests with that in infested nests. This would need a fair number of nests to obtain a meaningful conclusion but having the equivalent of a placebo-controlled trial to assess the impact of Crataerina would make for an interesting piece of research.

 

Andy Broadhurst

 

 

Member of SLN Swifts & Planning Group

 

abroa...@derbyshireswiftconservation.org

07941 349002

 

Derbyshire Swift Conservation is a Charitable Incorporated Organisation.

Registered Charity Number 1206439

 

From: swiftsloc...@googlegroups.com <swiftsloc...@googlegroups.com> On Behalf Of Hythe Scream
Sent: 09 June 2026 01:49
To: swiftslocalnetwork <swiftsloc...@googlegroups.com>
Subject: Re: [SLN] Crataerina - a problem?

 

Thanks for the links Dick. I'll take a look, but I can't get experimental data out of these without cooperation from the owners. 

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[Dick Newell]

I must say Ron, after many years of watching swift webcams, I have never witnessed the kind of distress that you describe. I see Swifts fidgeting all night when there are no flat flies, and I have seen swifts with a lot of flat flies which they seem to ignore. They also fidget.

Do you not think that the immense fear that some people have of spider-like things is colouring their thinking?

Personally, I like spiders.

This year, so far, I have seen very few flat flies. This surprises me when we have had quite a few warm days this spring, which is supposed to be favourable for them. I spent some time scrolling through Jaap's webcams and didn't find any.

Good luck with your research.

Dick

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[Hythe Scream]

Some people don't like spiders, but I don't think this has got anything to do with that really because we don't usually handle them. Ive seen one run up a re-habber's arm very quickly though 😀

If it's a bad year for the Crats, then that's a bonus.

I'm only responding to the initial OP's comments really. It made sense on reading the info, watching youtube, and listening to responses that I've since had and read about from various other sources, that it's time to see what can be done, if we are able to. It's not about wiping out a parasite, which can't be done. It's about maximising your output to this worthwhile cause, and knowing that you are helping a sentient creature to live a comfortable life and maximise their chances of a long and productive life which goes towards the aim of saving the swift from further demise. 

I've offered to take this research on board, and will ask formally for volunteers in a seperate post. If anyone comes forward to take part, then I'll do the paperwork. If they don't then I'll leave a summary of advice that I've collected so people can do what they wish going forward.

Thank you to all those who have contributed to this conversation. I've read them all.

Cheers

Ron

[Tim Huckvale]

People may be interested in a video from one of my nest boxes: a swift that took up residence on 25th May this year is seen, 2 days later, bringing in a partner. Both are carrying crataerina but don't seem to be bothered by them. The first swift appears to have a couple of much smaller parasites as well.

The video is under 27 May at https://thesibfords.uk/about/swifts/swifts-2026

The pair have since laid two eggs. Unusually, I have recently seen several crataerina crawling over the eggs when both parents have been out, so perhaps this box has a higher than usual infestation.

Tim Huckvale

Sibford Swifts