Tawny owls fighting

[Hedgehoggy]

Last night I heard the most blood curdling screams in my garden. The roosting magpies were going ballistic and it didn't sound like the foxes mating screams, it sounded far worse. It was two Tawny owls, presumably fighting over territory. I thought one of them had been seriously injured, or killed, judging by the screams but I saw the 'intruder' fly off. I hope he wasn't hurt.

The remaining 'resident' stayed in the tree making those 'disgruntled yaps' which I'd heard last year when I videoed a magpie harassing a very disgruntled Tawny in my garden (possibly same Tawny?). I hear the Tawnys' calls a lot in my garden, the calls seem to differ depending on the time of the year.

I also had a Tawny (possibly same one again?) roosting in a tree in my garden a couple of summers ago - the magpies and blackbirds blew its cover early morning and harassed it all day, but it remained unperturbed in the same fork in the tree up until dusk!

Does anybody know how large the Tawnys' territory ranges are approximately? Although I have a small woodland in part of my garden, there aren't any natural nesting holes in any of the trees (not that I can see anyway), so after last night's melee I'm now curious as to how near, or how far, their chosen nesting site might be.

[mbaldw]

Hi Hedgehoggy,

It was two Tawny owls, presumably fighting over territory.

Awesome, I've never heard a fight between these critters before, although I know that they are tough cookies. They are monogamous and nocturnal all year and have been documented to drive larger owls (even long-eared owls), cats, foxes etc. from their territories. Indeed, you probably saw the footage on SpringWatch a year-or-so ago, where a Tawny swooped to attack a fox feeding in someone's back garden. There was a fascinating account in a 1964 issue of British Birds describing an apparently unprovoked attack on a Red fox drinking from a pond in the owl's territory during January (even though this is too early to be protecting a nest!). There are also reports of Tawny owls eating other owl and raptor species, including Barn owls, Little owls and sparrowhawks.

Given that the males start singing in defence of their territory during late winter, and that females typically lay eggs during March, it may be that there was a nest nearby.

Does anybody know how large the Tawnys' territory ranges are approximately?

In his fascinating Owls of Europe, Heimo Mikkola writes:

"At Wytham Wood, Oxford, the average territory was 12 ha in a closed woodland and 20 ha in mixed woodland and open land (Southern 1970). Territories were sometimes much smaller, but according to Southern and Lowe (1968), territories of less than about 8 ha were too small to be viable for breeding. Tawny Owls studied in Belgium had territories of about 65-75 ha when living in beechwood with little lesser vegetation (Delmee et al 1978). And in Norway, where the prey density is far less than in England or Belgium, the mean territory size was about 102 ha per pair (Hagen, 1948)."

Dr Mikkola goes on to mention that Tawny owl territories studied in Britain have almost always been in woodland, although subsequent authors have pointed out that they also inhabit parks, large gardens and occasionally conifer plantations. Additionally, in their 1997 paper to Ibis Bridget Appleby and Stephen Redpath note that the average territory size of Tawny owls in Wytham Woods during their study was 22 ha, while in their second study population in Monks Wood (Cambridge), it was 15 ha and in nearby Fens it was 126 ha. Unsurprisingly, food availability does seem to play an important role in territory size and movement dynamics. A study of juvenile Tawny owls in Northumberland’s Kielder Forest during the mid-1990s found that some 25% of variation in time spent in different locations around the territory could be explained by variations in Field vole abundance.

I hear Tawny owls in our (communal) garden in the autumn; even though the garden is fairly small, it does have a line of trees running along the railway embankment at the back of the building. I get the impression that the owl(s?) hunts along the railway line, but I doubt that they roost/nest in the associated treeline. Each time I have seen them, they're flying off in the direction of the Common (a large expanse of mixed grass and woodland in the city centre).

I hope this is of some help.

Cheers,
Marc.

[Hedgehoggy]

Good grief Marc, I was shocked to find my post had been resurrected, but more than pleased that my curiosity has now been satisfied. BIG BIG thankyou I had googled till I was blue in the face, to no avail!

Originally Posted by mbaldw

I hope this is of some help.

Cheers,
Marc.

More than helpful! I didn’t know Tawnies also predated other owls and sparrowhawks, but I did recently read that they predated squirrels too and I seem to have quite a large colony of these critters frequenting my garden. They built a drey in one of my trees last year and I witnessed a magpie raiding the drey and making off with a tiny baby, which looked like a hairless mouse! The mother squirrel put up a good fight, but was no match for a marauding maggie.

Their differing territory ranges are very interesting to note. I think my garden may be part of their extensive territory then, as they are here most nights. More recently, I’ve heard, what I’m almost certain to be, the parents communicating with their young. As I leave food out for the hedgehogs and foxes every night, and food for the birds during the day, I maybe inadvertently attracting something the owls are very interested in!

On an aside, can I tap into that brain of yours again? (Pretty please! ) There was a recent post where two posters were hearing Tawnies calling during the daytime, one of which hears the Tawnies all throughout the year during the daytime. I’ve personally never heard a whisper from a Tawny during daylight, so was curious (again!) as to your views on this diurnal calling phenomenon throughout the year, not just the breeding season?

[mbaldw]

Hi Hedgehoggy,

I was shocked to find my post had been resurrected, but more than pleased that my curiosity has now been satisfied.

Heh! I don't often browse the bird forum but I was looking for another post and came across your question about Tawny owls. It's interesting that despite the plethora of generally-published (i.e. in books, rather than journals) material on Barn owls, there are so few on Tawny's (the excellent studies by Lindhard Hansen in 1952 and Manfred Melde in 1989 in notwithstanding).

I’ve heard, what I’m almost certain to be, the parents communicating with their young.

Really? Awesome. What sort of sounds are you hearing? Very young Tawny nestlings make a sound described as "bi-bi-bi", which changes to "psji-ii" (a kind of hissing sound) then to a more squeaky "siii-siii-siii"; recently-fledged young produce a "ti-sweep" or "ti-swerpt". In his book Owls Chris Mead points out:

"One of the least enduring habits of the Tawny Owl is the persistent squeaking of the newly fledged youngsters for food. This has variously been described as a 'rhythmical and monotonous series of squeaks' and 'an infernal racket'."

This is all to do with the parent's feeding strategy whereby they can return at any time with food, so the youngsters yell almost constantly to attract their parent's attention.

There was a recent post where two posters were hearing Tawnies calling during the daytime, one of which hears the Tawnies all throughout the year during the daytime. I’ve personally never heard a whisper from a Tawny during daylight, so was curious (again!) as to your views on this diurnal calling phenomenon throughout the year, not just the breeding season?

This is very interesting. I’ve never heard a Tawny calling during the middle of the day, but I’ve read accounts from several people who have. There was a discussion about this on a birding forum on which I lurk a couple of years ago and the general consensus was that the OP was either hearing a Jay ( Garrulus glandarius) -- which can apparently produce a superb mimic of a Tawny’s call -- or were in a high density Tawny area (high density = space at a premium = more territorial calling). Whatever the reason, it doesn’t seem an uncommon occurrence and even the RSPB, in their brief bio on the Tawny, mention that it may call during the daytime, although they don’t pursue the subject further.

The BTO conducted a study between October 2005 and April 2006, which collected data on Tawny owl calling habits. Unfortunately, I’ve only seen Steve Freeman and Mike Toms’ summary (published in their Bird Table magazine) and it doesn't mention daytime calling. Nonetheless, I'd wager the full dataset makes interesting reading.

Arguably, from the scientific data I've seen, Tawny owls are probably better off keeping stumm. Between 1998 and 2002, a collaboration between scientists at the University of Copenhagen and the University of Lund studied the habits of Tawny owls in the Gribskov Forest, Denmark. The biologists found that of the 15 owls in their study site that died of natural causes, 11 were nailed by raptors (namely goshawks, Accipiter gentiles). It seems that prior to meeting a goshawk, juvenile owls perched out and roosted in open spaces; after encountering a goshawk, however, the birds roosted in less exposed places. The authors concluded that:

“ depredation [an attack by a predator] by diurnal raptors is the main factor shaping the diurnal behaviour of tawny owls.”

If we consider this Danish site to be representative, one might expect owls to keep quiet during the daytime where potential predators (or mobbers, which as well as being a pain can also advertise your presence to prey and predators) might hear them. With this in mind, it doesn't seem unreasonable to think that, in a relatively enemy-free space, they could perhaps afford to be more vocal during the day.

Anyway, that was kinda a long-winded way of saying that I don't really know why they sing during the day!

Cheers,
Marc.

[Hedgehoggy]

Oh Marc, thank you ever so for going to all that trouble sharing your researched data and your own views. I think your penultimate summarised para is by far the best and probable explanation There is still so much we don’t know about these critters (and many others besides). Perhaps this is your next calling and you should offer your services to RSPB/BTO/other to further research the missing gaps? If your animal tracking escapades are anything to go by, it would certainly make for very interesting reading and entertainment while you’re researching those missing gaps to add to your brilliantly informative website! - Wildlifeonline - Welcome to Wildlife Online

Originally Posted by mbaldw

What sort of sounds are you hearing?

Getting back on track! The sounds I’ve been hearing have been a squeaky tsiii tsiii (as you’ve described for a newly fledged youngster), coupled with a harsh low-key screeching from what I'm certain is more than one owl; almost sounds like they’re squabbling! Imho, the sounds are not from an adult’s repertoire of calls, of which I’m now fairly familiar with. So perhaps I can conclude that the parents are dumping their youngsters in my garden while they go off hunting! What I find so amazing is that their flight is sooooo eerily silent. Only once (didn’t want to frighten them into not returning), have I walked around the woodland area in my garden to see if I could spot the noisy youngsters, but they silenced the moment they spotted me and I just caught a brief glimpse of a silhouette fly off oh-so-silently. Magical.

*Will now stage a vigilant lookout on your website for a Tawny Owl section!*


P.S. Love your updated homepage for this season

[mbaldw]

Hi Hedgehoggy,

thanks for your kind words regarding my website – most appreciated.

So perhaps I can conclude that the parents are dumping their youngsters in my garden while they go off hunting!

Given that you appear to have fledglings in your garden, maybe the owls did roost there. As I understand the situation, the owls leave their nest and move out into nearby trees and bushes where they wait (often highly vociferously) for their parents to return with something good to eat.

What I find so amazing is that their flight is sooooo eerily silent.

That is very cool. I had always been told (and read) about owls being silent in flight, but I was still amazed the first time I saw a Tawny owl fly off without making a sound. The ‘secret’ to their typically silent motion lies in their skeletal anatomy and their feathers.

The skeleton is very light; many of the larger bones are hollow and air-filled. In conjunction, there are several aircells situated around the body (some of which integrate into the hollow wing bones) that provide buoyancy. Additionally, the majority of the flight muscles attach to the keel (breast bone), which takes their weight out of the wings. The skeletal adaptations, coupled with broad wings means that owls have what aeronautical engineers call a low “wing-loading”. In other words, owls have a large wing area for the weight of the bird – the larger the wing-loading, the more effort is required remain airborne. It also controls how easily birds (e.g. raptors like buzzards) can soar; the higher the wing-loading, the larger the turning circle and thermal pocket required to gain height. Anyway, with a low wing-loading, the bird can fly buoyantly and with ease through the sky. So, Tawny owls -- which have a wing-loading of 0.40 grams per sq-cm -- can fly rather effortlessly, with few flaps of their wings. Compare this to, say a grouse, which has a wing-loading of 1.34 g/sq-cm and needs to flap like crazy to say in the air*. Basically, fewer noisy flaps and more quiet gliding (it saves energy too).

It’s not just the anatomy of the skeleton and wing that explains the silent flight – the feathers are perhaps the most important component. Each wing has ten primary and ten secondary feathers numbered descendently – so, the tip of the wing has primary ten, running down to primary one at the bend of the wing; here secondary one is situated, running to secondary ten. Secondary ten is separated from the tail by three tertial feathers. It is the primary ten at the edge of the wing that we’re particularly interested in, because each barbule has a stiff, hair-like projection at the tip – together, these form a comb-like fringe (ca. 2.5mm wide) along the feather’s leading edge. In addition to this ‘comb’, the primary and secondary feathers on the trailing edge of the wing possess a soft, hair-like, fringe and the wing itself is covered with a soft, velvety down. These three features combine to moderate the passage of air over the wing, helping to reduce the turbulence and hence noise. This may all sound great (and it certainly helps the owls move about unheard, unless the feathers are worn, wet or being moulted), but as A. A. Wardhaugh points out in his Owls of Britain and Europe:

“Unfortunately for owls, these sound-deadening adaptations cause more drag or resistance to the air during flight.”

At any rate, knowing how it’s done, doesn’t make it any less spectacular to witness

Cheers,
Marc.

* The values I’ve given for wing-loading are those presented by Mikkola (1983), who also quotes Brull’s 1964 study, giving a wing-loading for the Barn owl of 0.29 g/sq-cm. However, in his Owls of Britain and Europe, A.A. Wardhaugh gives the wing-loading of the Barn owl as 3 kg/sq-m. By my reckoning, this equates to a wing-loading of 3 g/sq-cm by Wardhaugh’s reference – an order of magnitude greater than Mikkola/Brull! I haven’t checked up the morphometric details to work it out for myself yet!

[Hedgehoggy]

Ahem, I won't pretend that I understood all of the technical stuff on wing-loading, but I managed to grasp the basic concept, I think!

[Hedgehoggy]

EDIT: just re-read your post s l o w l y and I DO fully understand! DOH!

*Goes off to calculate total weight of an average Tawny's wing expansion in comparison to a grouse's* - only joking

[thunder]

Quote
There was a fascinating account in a 1964 issue of British Birds describing an apparently unprovoked attack on a Red Fox drinking from a pond in the owl's territory during January (even though this is too early to be protecting a nest!).
Endquote

Not necessarily so. A few years ago I had a reliable report of a Tawny Owl chick in mid January (in south Wales, as I recall).

henrya

[mbaldw]

EDIT: just re-read your post s l o w l y and I DO fully understand! DOH!

Heh! I'm glad my ramblings made some sort of sense!

Not necessarily so. A few years ago I had a reliable report of a Tawny Owl chick in mid January (in south Wales, as I recall).

Thanks for that henrya - very interesting. That does seem early for a Tawny chick (given that they typically start laying March-time). Do you happen to know whether it fledged successfully?

Cheers,
Marc.