Early birds:

the Cambridgeshire Tawny Owls January-March 2007


Another breeding attempt in this box is featured on the BTO site. See link further down this page

MUCH CAN BE LEARNED about the lives of tawnies with a nestbox camera. For example, last year (2006) this female laid four eggs between February 1st and 9th -- a large and early clutch. On 2 March, late into the brooding period, a fight with an intruding squirrel resulted in 3 of the 4 eggs being broken. Three weeks later (20 and 22 March) she laid a replacement clutch of two eggs, though these failed to hatch. What is significant about this is that's it's still unknown whether Tawny Owls will lay twice in a season to replace a lost clutch (as opposed to having a second brood after a successful first brood; see Zuberogoitia et al (2004) "Possible first record of double brooding in the Tawny Owl Strix aluco" on my References page 4, last entry. The paper's actually about a second successful brood in the same year, but on p.437 the authors write that "failed early clutches have not been reported to lead to second breeding attempts" by tawnies). Well, here it's happened and been recorded by a nestbox camera!

But what really strikes me about the events of 2007 described on these pages is the evidence for delayed incubation -- the way the female uses this method to ensure that three eggs laid over seven days hatch within 24 hours of each other. This has the obvious advantage that the young fledge at about the same time, with less risk that last-born chicks will be lost because they're not ready to follow the mother from the nest when the older ones fledge. This goes against a commonly held view that egg-laying is spaced to reduce the hunting load on the father.

The owls visit the nestbox regularly out of season, and the male brings food to the female long before she lays. Especially interesting is that the present pair start so early -- in 2005 the first egg was laid on Feb 22nd and in 2006 on Feb 1st. This year the female has excelled herself by starting on 20 January. Average first laying date in the UK is more like 20 March.

Infertility may be a problem for this pair. In the last two years there have been 9 eggs, four of which have proved infertile. Of the others, one hatched, three were broken during the squirrel fight, and one was abandoned after the fight. The infertility rate is nearly 45%. Only one owlet has been raised in two years. We must wish the mother better luck this year.

In 2005 the single owlet that hatched (the two other eggs were infertile) appeared at the entrance of the nestbox on its 28th day. The owlet continued to climb up over the next four days, but had to be enticed to leave by the mother on day 32 by her witholding food until it came out. This supports other evidence given on my website that tawny chicks often don't leave a suitably protected site before about day 32, and that chicks found on the ground earlier than that -- i.e. before they can fly -- may be there as the result of a mishap.

Nestbox details: There's a bit more about the nestbox at this anchor on my Tawny Owl Nestbox pages.


February 2010 — Another nesting attempt

The Cambridgeshire owls are being followed on a page on the BTOs website here. There are short video clips from this years breeding attempt mixed, a little confusingly, with footage from previous years. It is not known whether this is the same pair as in previous years — an owl was found in a poor state on the ground in the garden early in the winter, but we are not told what happened to him or her. The female in the current clips looks like the 2007 female, and she laid early in 2010, like the female of previous years.

There are four eggs, the first laid on 28 Feb, then 3 March, 7/8 March and 8/9 March, a spread of 8 or 9 days. It is stated that The female begins incubating as soon as the first egg is laid thus staggering the date at which they hatch and the age of each chick. This is surprising as the evidence from this females previous successful attempt, described on these pages, is quite the contrary. There is now quite a lot of evidence from other monitored tawny nesting attempts that delayed incubation, with the aim of synchronising hatching dates, is the norm. (My article on delayed incubation presents the evidence for Tawny and other owls. The Estonian tawny brood, webcast in 2009, followed exactly this pattern.)

But there is real drama at this box. Sometime in mid-March the male disappeared, possibly (my guess) the victim of a vehicle collision in this semi-urban area. This means that the female is having to feed herself, something that’s taking her out of the nestbox for long periods, and, if the eggs hatch, she will have to find food for up to four chicks.

My feeling is that this will be an impossible task. Even if she somehow brings the eggs to hatch, the tiny, barely feathered chicks will not survive if she is not able to sit on them continuously in their first days — normally until about their tenth day. I fear that one way or another we are going to see an abandoned nesting attempt this year.

Whatever happens, we will have useful information on whether a female can raise a brood if her mate perishes during the nesting period, something I have always wondered.


20 January 2007

The female has laid an egg! It's very unusual for a brood to be started as early as this.

20 January: The female laid an egg last night some time between 4.40 and 6am. Left: Here she is sitting soon after 3 pm the same day. This is astonishingly early to start, and she must be confident that there'll be enough food when (if) the brood starts hatching about 17th Feb. Average British laying date is around March 20th. Right: Same day, 7.40 pm. I was lucky to catch the female perched in the doorway, looking around for a couple of minutes before she flew off . . . allowing a nice view of what's at the bottom of the box (below).

That's the egg at the centre. The smaller egg-like object is in fact a walnut shell brought in by squirrels. A squirrel pair raised a family in the box in summer 2006 and were not turned out by the owls although they dropped by once or twice to check on things. Just northwest of the egg is what looks like a bird skull.


29 January, 2.22 am

The female has left to stretch her wings, and there are three beautiful eggs on display. The first egg was laid on 20 Jan and the third by the 27th. My impression from not very systematic checks is that she's been brooding pretty seriously -- this is the first time I've found her off the eggs recently. (See previous page for comments on other objects on the floor.)


7 February, 8.18 pm

She's brooding so determinedly that I never find her off the eggs despite checking quite frequently. Here she is with her eyes reflecting the infrared of the lights. Note the rungs of the "ladder" up the wall in front of her. This is a good feature and should be present in all these deep nestboxes. Videos show the owls making use of the slats when they climb out of the box.

Only ten days to go until the first egg hatches on February 17th . . .


10 February

She's still brooding away, and in three days (nights!) of checking since the last entry I still haven't seen her out of the box. Seven days to go.


Nesting pairs can be very vocal. As these owls can't be heard over the external video link, here are three short recordings of my own to illustrate what might be heard near this nestbox. The first two are exchanges that happened when a male came to a nestbox (first clip) and to a nest (second clip -- these are different pairs recorded in spring 2006). In the second clip the female is clearly given food as she is unable to call properly with her beak full. Clip 1: Nestbox pair (549.81 K). Clip 2: Pine nest pair (589 K). In the third clip the pine nest female leaves the eggs and the pair have a good chase around in nearby trees. This "wild" behaviour doesn't appear to be unusual among tawnies! Clip 3: Owl caper (685.73 K).


17 Feb: First egg due to hatch today.

Once again, in the last week of checking I've not found her off the eggs. So, aside from striking lucky, all one will be able to do is to look for signs that she's attending to a newly hatched chick beneath her -- for example, preening it to help dry out its cover of fluff.

7 pm: Yes, she seems to be doing this. The photo at right shows her arranging her wings to form a cosy space for something she's taking care of! She's only doing this occasionally and confirmation will need sight of a chick.

12.55 am (18th): Just caught the dad about to leave after a visit. Good to know meals are being served. The female is being very active, often changing her position and sometimes looking beneath her. It does seem a chick may have hatched.


18 February

Anticlimax -- no chick hatched yet

9 pm: Checked in to find the female out of the box for the first time since she laid on 20 January. She will have been off the eggs from time to time of course -- it's just that I've not logged in while she was taking a break.

But there it is. As the pic on the right plainly shows, there are still three eggs. None has hatched yet. We're now well into Day 29 after the first egg was laid, and some time early tomorrow Day 30 will begin. So we can only hold our breath and hope she hasn't laid another infertile clutch. 28-29 days is the time usually given for tawny eggs to hatch.

Possibly all the interest she's been showing in the eggs lately (it has continued this evening) is because she can hear noises from inside. Let's hope so. It would be too disappoinitng if she has another failure.

The lowermost pic shows her just after she returned at 9.05 pm after slithering down the box. The clock on the camera's running about five minutes fast.

It's curious that there seems to be quite a bit more debris in the nest than in the egg pic on the previous page. I don't know what it is, apart from the walnut shell(s) left by the squirrels.


Delayed incubation -- do owls practise brood planning?

Checking with this owl's previous record and with information in Mikkola's Owls of Europe has thrown interesting light on the situation.

First, in the past two years, with three broods totalling 9 eggs, this owl has had one success -- a single tawny owlet raised in 2005 (see top of page for what happened to the other eggs). I thought I'd check with the site owner's notes to see how long the chick took to hatch, with surprising results. Because if one assumed that it was the first of the three eggs laid to hatch, the incubation period would have been 33 days! Only the last egg laid gave the correct incubation period -- 28 days. The second egg, laid two days after the first, would give 30 days. Later tests showed that the two unhatched eggs were infertile. So it seems that in this case only the last-laid egg was fertile, with a lower probability that it was the second.

More interesting information comes from Mikkola. While he accepts the 28-29 day incubatiion period as normal, he refers to work by H.N. Southern on Tawny Owls in Wytham Woods and says: "It is always stated that incubation starts with the first egg, but Southern (1970) observed no incubation before the second egg was laid, and he calculated the average incubation time to be 29.7 days for a single egg. The first egg laid was not always the one to hatch first, proving also that incubation may not take place before the second egg is laid." (Mikkola op cit pp. 151-2). What's meant by this is that the female lays a first egg but then leaves the nest. The egg is left to go cold and incubation only starts with the second egg.

So does the Cambridgeshire female practise delayed incubation? The answer is that in 2005 and 2006 she did, most notably with her first clutch in 2006. On that occasion she laid four eggs between Feb 1st and 9th. The owner notes that over the two days (2nd-3rd) after the first egg was laid she left the nestbox six times, mostly for an hour but once for 7.5 hours. After the fourth egg was laid she started serious brooding, with absences not exceeding 10 minutes. Oddly, in the previous year she spent most time away between the second and third eggs of the three-egg clutch, again spending a total of just over 7 hours away over two days. She was only away for a couple of hours in total between the first and second eggs. After the last egg she limited her absences to 30 minutes a day. There are no records of how she behaved with her second clutch of 2006.

This year my impression from limited viewing is that she was not absent for long periods while she was laying. One of our own females in Kent was away from her eggs for much of one night in 2006, and delayed incubation is clearly recorded for the Barred Owl in the OwlCam video.

All this goes to show is that one has to be careful in making predictions from laying dates about when the first chick is likely to appear, that this female's eggs do seem to have a 28-day incubation period, and that there's hope yet. The bottom line is that the last egg laid has to hatch by about 24th February. We just have to be patient.


19 February -- Still no chick hatched

9.22 pm: She's been out for a break (don't know when she left). Just three eggs still. Here's a composite pic of her returning to the box, when she perched on a branch in front of the camera for about a minute before going back inside.

It's very unlikely that she goes out to hunt -- her mate deals with all her food requirements. The reason for these brief excursions is to relieve herself and exercise her wings. She's been brooding almost non-stop now for more than four weeks. With luck, if the eggs hatch she'll do about another 10 days. If they don't hatch? . . Last time this happened she spent another three weeks on the eggs before abandoning the attempt.


20 February -- No chick hatched

7.45 pm: She's been sitting each time I've checked this evening (first check late afternoon while still light). No sign that she has a moving chick under her. This is becoming quite a drama and I have to admit to being quite on tenterhooks! With luck she'll leave for her break around 9 pm, allowing one to see if anything's changed. (She didn't)

9.48 pm: She popped up to the doorway and had a look round for a couple of minutes but then ducked straight back in. As I'd switched to the outside camera I missed seeing whether there've been any developments with the eggs! From her movements in the box this evening I'd guess not. In the upper pic she's attending to the eggs shortly before appearing in the door.

00.25 am: Despite regular checking I didn't find her out and gone for a break until almost half past midnight. Not encouraging news: all three eggs unhatched. The first is now "overdue" by nearly four days. I don't know when the second was laid, but by now that probably should have hatched too. This time she was away from the nestbox for at least 20 minutes.

Poor old girl. I'd guess she has a keen sense that it's time something was happening. At least three days to go though before one has to think about writing this clutch off completely. I really look forward to reporting better news!


A slightly out of date report from National Geographic News, 3 June 2003

For a bird, the bleak midwinter wouldn't seem an ideal time to go about the precarious business of starting a family. But recent winters haven't been particularly bleak in Britain, and last Christmas scores of fluffy owlets grew fat on mice, voles, and baby rats caught by parents that spotted a breeding opportunity too good to ignore.

Scientists at the British Trust for Ornithology (BTO) say 2002's mild temperatures and an explosion in rodent numbers sparked an unseasonal baby boom in the country's barn and tawny owl populations.

"We had five cases of tawny owls laying eggs from Christmas week right through to the end of January," said David Glue, a research biologist with the bird conservation charity. "This is exceptional - usually they don't start nesting until the second or third week in March."

Glue says the latest in a series of warm years - 2002 was the fourth hottest in Britain since records began in 1659, with five of the six warmest occurring since 1990 - contributed to a huge increase in the rodents that owls prey on.

"An early spring and a late autumn produced a heavy crop of beech mast, hazelnuts, conifer seeds, and other natural fruits, so we had large numbers of voles, mice, and young brown rats," said Glue.

Biologists estimate that two billion field voles were born in Britain last year. The usual figure is around 700 million.


Source page. The article runs to two pages but the rest is about the effect of climate on other birds in the UK.


21 February

7.25 pm: Several checks so far. Just now she's been upending herself (tail up, head down) to deal with something, but it could be just to rearrange eggs. As usual will have to find her out of the box for a definite conclusion.

2.25-3 am (22nd): Had to shop this evening so missed her going out. But the impression from several early morning checks is that she's still waiting patiently for something to happen. She's been sitting in very much the posture she was in in the first photo for 17 February, with her wings held slightly out and her head down, quite still. Then she'll turn with her tail in the air and head down to the floor, but there isn't enough room in the box to see what's under her. What I'm not seeing is an indication that she's being distracted (if that's the word) by the movements of a newly hatched chick.

Pics taken in the early morning of 22 February. It turns out that she was fussing a chick, hatched some time before 5.50 pm on 21st (see entry for 22nd below).


22 February -- An egg hatches at last

8.50 pm: Watching quite closely this evening as it really is make or break time with this clutch. Just at the moment she's fussing quite a lot with things underneath her, but until she goes out for a stretch it'll be impossible to tell what. One problem is that I'm not getting anything like the frame rate printed on these pics -- more like a new frame every 2 seconds -- so it's difficult to tell exactly what she is doing. But quite a few tail-up frames coming through.

Egg day-count situation as of today: It's now well over 33 days since the first egg was laid in the early morning of 20th January (31 days when all three eggs were seen on 20 Feb), so it looks as though egg no. 1 has to be written off. I have no information on when eggs 2 and 3 were laid, only knowing that all three were laid by 27th Jan. So if one asumes that the last egg was laid six days after the first, that egg must hatch by 24th Feb (its day 29). If the middle egg was laid about half way between, it has to have hatched by today as middle egg is now more than 30 days old. A recipe for a lot of nail-biting until she leaves the box!


News just in from nestbox owner:

"We have an egg hatched. Owlet was heard cheeping at 17.50 yesterday, 21/02/07. Visual confirmation made when she left the box at 01.09 today." WOW! Hope to have a pic soon. What a relief.

So, it looks as though it's egg no. 2 that's hatched, giving a sporting chance that there'll be a second chick. Time for a new page!!

Story continues on next page

powered by owls

27 January 2007: Mrs Owl, now sitting on three eggs, emerges at 8 am to repel an intruder. The squirrel is probably one of the pair that used the nestbox last summer for their own family, and the mother owl will take such approaches seriously as a fight with one of the squirrels in 2006 resulted in her losing a first clutch of eggs.