The secrets of silent flight

Tawny owls are rarely seen in daytime but are supremely adapted to night-time hunting.  Although they have good night vision they hunt mostly by sound.  They can hear the footsteps of a mouse in the dark and have evolved silent flight so the prey doesn't hear the hunter's approach and so they can still hear it during the attack.

Like most owls, tawnies have large wings which allow slow flight and slow wingbeats but much of the secret of the silent flight is in the feather design.  Last summer I found two moulted tawny owl primary flight feathers in the garden and I recently took a closer look at them.  

They have three special features which are different from other bird feathers.

The leading edge of the feather shows serrations which are tiny comb-like structures that modify the airflow over the wings to reduce noise although the exact mechanism is not known.  The serrations are apparently most well developed on the 10th primary (P10) which directly meets the airstream but are also present on P7-9 (I don't know which feather this is).  Interestingly serrations are much more developed in nocturnal owls (such as the tawny owl) than in diurnal owls.

For comparison here is the leading edge of the owl feather (L) compared with a primary flight feather from a wood pigeon (R).

The upper surface of the owl's feather has a velvet-like covering which is thought to deaden the sound of the feathers moving over each other as well as stabilising airflow to allow slower flight.  The velvet was a bit patchy on this feather but bear in mind it was a discarded moulted feather that had been in use for a year or so.

Here is a comparison of the owl feather and the pigeon feather.

The trailing edge pf the owl's feather is fringed.  The fringes prevent separation of the airflow between adjacent feathers, again reducing turbulence and reducing noise.

Here again is a comparison with a wood pigeon feather.

The amount of fringing varies along the length of the feather although I don't know if that is a function of wear.  This is at the feather tip.

The feather photos were taken with a Canon 100mm macro lens plus stacked Raynox DCR-150 and DCR-250 conversion lenses, giving +12.8 dioptres in addition to the macro lens.

You can read a lot more about the science of owl feathers in a Royal Society publication here.  You can watch a fascinating short video of a pigeon, a peregrine falcon and a barn owl flying in an acoustic laboratory here.

The Herald

I wasn't expecting to write a post about a moth in mid winter so this was a real treat.  This beautiful moth is The Herald (Scoliopteryx libatrix).  It overwinters as an adult, often roosting communally in caves, barns and outbuildings, as here.

These pairs are close together, presumably by chance rather than for warmth.

This one is tucked up with a peacock butterfly.

Herald moths feed up on ivy flowers and ripe blackberries before hibernating.  The caterpillars feed on willow, aspen and poplar leaves in the summer.

The origin of its common name is unclear.  Perhaps because it emerges from hibernation at the end of winter to herald the start of spring or, probably more likely, because its wonderful patterns and colours resemble a herald's tabard.  This is Thomas Hawley, Clarence King of Arms, in 1556.

Look who turned up last night

The foxes come and go so quickly that most nights I don't see who is here.  Last night I was lucky and saw the fox approaching and took a couple of photos to confirm it was the "long-tailed" cub from last year, now full-grown.

Before it had eaten all the meaty bits the fox trotted away but it returned immediately, or so it seemed.  Except that this was the short-tailed cub, which I haven't seen for weeks and assumed had gone.

It is very interesting that both of last year's cubs are still here so late in the season.  I haven't seen either of last year's adults for a while but I guess they are probably still around.  It will be fascinating to see which pair of foxes own this territory this year.  The resident female, whoever she is, is likely to be pregnant by now and her new cubs will be born around the end of March.  With luck they will be in the garden by July.

Last night in the garden

We had a little bit of snow this week, the first of the winter.  I glanced out of the back door window last night just as this fox was approaching so I was able to take a few photos.  I am not completely sure which fox this is - probably one of last year's cubs.  The dog with his torn ear, the vixen with her pale mask and her stocky body shape, and the other cub with half a tail are all very distinctive whereas this fox just looks unremarkable.  I have seen it a few times in the last week or so and it is very wary but hungry enough to approach the house.  Fortunately it can't see me or hear the camera through the door.

Sparrowhawk follow-up

In the rush to add that morning's new photos to Saturday's post I forgot to mention his tail feathers.  Because he was spreading his tail we can see all 12 feathers (rectrices) and it is clear that the second and fifth rectrix on each side is brown whereas the others are grey.  This means this is a second winter bird who has replaced most of his feathers in the first moult last year but has retained a few juvenile feathers.  These will be replaced in the second moult this year.

The same could be seen when he was preening two days earlier although I hadn't really focussed on it then.  You can even see how the juvenile feathers are more worn or frayed, especially at the tips.

The first time I saw this bird as an adult was in August when his new tail feathers were part grown.

Sparrowhawk news

It is a good job this chap didn't turn up last weekend during my RSPB Big Garden Birdwatch, otherwise I might have seen only one bird.  This winter's sparrowhawk hasn't sat around much until the last few days although I think he comes to the garden quite often.  I suspect he is quite an efficient hunter and if he catches something straight away he probably takes it off somewhere to eat in peace.  Yesterday I saw two attacks and one fly-over.  Two days ago he was sat on the "sparrowhawk perch" for a couple of hours on and off, spending a lot of time preening and looking as though he had already eaten his breakfast.

One moment he is all fluffed up.

And the next he is sleeked down ready for action.

Here he is having a good scratch and giving his feathers a thorough sorting out - photos from two days ago.

This morning I saw one attack by the feeders outside the kitchen window, apparently unsuccessful, and a little while later he was back on the perch, sitting with his tail spread.  When I saw tail fanning last winter it was a signal of dominance from the adult bird to a juvenile who was watching from a nearby tree.  I haven't seen a juvenile in the garden this winter and I don't know if there was another sparrowhawk around.

If the sparrowhawk is going to make more use of the perch now that we have colder weather it will brighten up the lockdown.

Counting the birds

Like many of you, I expect, I took part in the RSPB* Big Garden Birdwatch (BGB) last weekend. It involved recording the largest number of each species seen at one time during one hour of birdwatching.  Although it has limitations the BGB gets a lot of publicity and encourages many people to take note of the birds in their gardens.  Despite its size, I have reservations about the way this survey analyses its results.  It prioritises abundance so it is biased in favour of birds that go around in groups and sit around on feeders long enough to be counted, such as finches, rather than birds that dash in, grab a seed, and take it off to eat in cover, such as tits.  In my garden I expect there are more coal tits than greenfinches, yet I rarely see more than two coal tits at one time whereas it is easy to see six greenfinches sitting on the feeder together.

I wrote previously about the differences between the RSPB BGB and other surveys such as the BTO* Garden BirdWatch, which is a weekly survey which has been running since 1995.  The main difference is that while the RSPB BGB prioritises abundance, the BTO GBW prioritises presence, so they reach different conclusions about which is the "commonest" garden bird.  For example, if in ten gardens a robin was seen in eight and six starlings were seen in each of two the RSPB BGB would decide that starlings are the commonest garden birds whereas the BTO GBW would choose robins.

In the RSPB BGB in 2020 nearly half a million people recorded almost eight million birds and the "top ten" were

1. House sparrow
2. Starling
3. Blue tit
4. Wood pigeon
5. Blackbird
6. Goldfinch
7. Great tit
8. Robin
9. Long-tailed tit
10. Magpie

This is despite the fact that house sparrows have declined by 53% since the survey began in 1979 and starling numbers have fallen by 80%.  Both are on the Red List of birds of conservation concern.

This chart shows the BTO GBW results for 2019 (green), the latest available, compared with 1995 (yellow), the first year of the survey.  It shows that the birds you are most likely to see in the garden are blue tit and wood pigeon, with house sparrow in 7th place and starling 13th.

This chart shows the winners and losers over 25 years.  House sparrows, starlings, greenfinches and song thrushes have fared badly for various reasons whereas goldfinches and wood pigeons have done well.

I suppose it doesn't matter all that much whether presence or abundance is given priority so long as it is clear what people are talking about.  However, the 2021 results in a few weeks' time will also get a lot of publicity and it will be a pity if it isn't made clear how much trouble these birds are in.

Last weekend I managed a few photos through the kitchen window when there was a brief break in the cold, wet, sleety, windy weather we are having.  These are biased in favour of birds that sit on my "woodpecker" feeder, a dead branch with suet in a hole.  If the weather improves I'll also take some of the birds on the seed feeders and post them later.

*For readers not from the UK RSPB = the Royal Society for the Protection of Birds and BTO = the British Trust for Ornithology.