23rd May -- Back in London

The diary's going on hold for a while as we've a journal to typeset and the deadline's approaching too fast for comfort!

Further efforts to find the fledged chicks will have to wait until we return in early June. They'll stick around with their parents until August at least and will probably only be chucked out of the territory in September or October. It will be interesting to find them as the parents seem to have moved their main hunting area from north of the wood to west of it, and finding the chicks will mean we know where the parents are.

So, that's it for the moment. Check back in early June for more news of the growing chicks.

HIGH TIME we had a Summary! I feel I've learnt a lot. Here it is.

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SUMMARY

The owls

We've been monitoring two pairs of breeding Tawny Owls in the High Weald in south Kent. One pair which was using a crow's nest in a pine tree behind the house abandoned the nest in early May for no known reason. No evidence that chicks hatched in this nest was heard, so the eggs may have been infertile. The other pair, which used a nestbox in woodland 600 yards from the house, has successfully raised two fledglings.

We've been following the breeding attempts of the nestbox owls since 2003. We believe the pine nest pair to have met up only recently -- the female has been around the house since 2004 but the male was not heard before 2005. If so, this year's unsuccessful breeding attempt would have been their first. (Postscript: neither pair bred in spring 2007.)

The nestbox

The nestbox was put up in autumn 2004 in an attempt to stop the woodland pair losing their broods. The entrance to the box is low enough to allow nesting activities to be observed but high enough (about 5 inches from the floor) to act as a barrier to prevent the chicks from falling out accidentally. This can happen when they walk backwards to relieve themselves and appears to be the main reason why an open nest is so risky for Tawny Owl chicks. The woodland pair lost all their chicks from open nests in the three previous seasons. The nestbox design has worked, although modifications need to be made to deal with sewage problems.

This pair did not use the nestbox the season after it was put up. It is possible they only used it this year because of the deteriorating state of the crows' nests they were using before. This year's brood may be the first they have succeeded in bringing to the fledging stage. We do not know how old the parents are or when they began breeding.

Egg-laying

By calculating back from when the first chick fledged, the nestbox female must have laid her first egg on about 17th March, with the second laid a few days later. This is earlier than we thought. The pine nest pair are thought to have started around the 23rd March, though this date is very uncertain.

In other years the nestbox female has laid on 19th March (estimated date for Owly, who was a second chick, in 2003), 12th and 18th April (estimated dates for Tubby and Tiny tot in 2004), and 3rd April (estimated date for Sophie, another second chick, in 2005). This is a wide range of dates, with a spread from before and during to after the range of laying dates given by the BTO for Tawny Owls (approx 17 March to 10 April, with an average of 24 March, in 2002). The 2004 chicks were laid notably late. This year's clutch was begun a few days before the average laying date for UK tawnies.

Incubation

The incubation period is about 28 days. During this time the nestbox female was on the eggs almost constantly day and night, although one night (12/13 April) she was away for several hours, spending only two hours in the nestbox between 1 and 3 am. This may have been because she was disturbed by the pitching of the tent the previous evening. It may also be because she needed to supplement the food the male was bringing her.

During incubation the male visits the nest four or five times a night. The evidence from one visit of the pine nest male (3/4 May) is that he brings the female prey he has caught. Some visits are characterised by noisy reunions during which the female may leave the nest and fly around excitedly. The male sometimes stays for a few minutes but more often he makes off almost immediately. The function of the warm welcome from the female is presumably to encourage him to continue hunting. Some of these visits may be merely social as it is unlikely that an inactive female would need food more than once or twice in 24 hours (as I have found with Sophie), depending on the size of the meal.

The pine nest female abandoned her incubating effort some time between 3/4 May and 11/12 May, presumably because the eggs hadn't hatched. So she would have spent about 45 days on the eggs, or about 17 days more than the standard incubation period.

After hatching

The nestbox chicks hatched between 14th and 18th April (dates estimated by working back from the fledging of the first chick). The first chick fledged on about 16 May, and the second by 20th May, when the mother took them away from the nesting area. We believe they left the wood and went to open fields several hundred yards to the west, where the family (including the dad) was seen together by local residents. As this area is where the male had been bringing food from at the nesting stage there is no reason to think that it was another family.

Hatching to fledging (the point when a chick is capable of flying from the nest) takes 30-32 days. In the days immediately after the eggs hatched the nestbox female stayed with the chicks and the male continued to bring food. On one wet night (30 Apr/1 May) the male appears to have hunted despite quite heavy and continuous rain -- at least he visited exactly as on a dry night. Some time after the chicks hatched (10-15 days) the female left the nest permanently and took up a watching position in a nearby tree. I don't know whether she did any hunting at this stage. The nights I was there she stayed near the nest and waited for the male to bring food. This he presented to her and she then took it to the chicks. Whether she occasionally helped herself I don't know either!

Oddly enough, however, there were at least two nights during egg-brooding when she left the nestbox for much of the night. It seems that at this time she did occasionally go hunting for herself. This was well after the initial egg-laying stage, when some females appear to practise delayed incubation. On one occasion she may have left because she was disturbed by my presence, but later when had become accustomed to my being there this cannot have been the reason.

This year I did not see the male sitting with the young chicks during the day, as in one previous year.

From the point of view of nestbox design, it's interesting that the chicks appeared to make little or no use of the outer ledge during the "branching" phase (at age 25 to 30 days). Possibly this was because they had a good view of the world from inside and so felt no need to emerge like owlets of the same age in tree holes or deeper boxes. I often saw them looking out, both during the night and when we visited during the day. All wing exercising was done in the safety of the box, which was designed to be roomy enough for this type of activity.

Disturbance caused by monitoring

Apart from the initial disturbance to the nestbox female caused by pitching the tent, neither pair of owls was unduly disturbed by the monitoring. With the pine nest owls I was sitting in the open near the nest tree. Both must have been well aware that I was around -- certainly the male had a good look at me one night! They have probably become accustomed to human activity because their territory includes the house and garden.

The case of the woodland owls is different. This pair nests in a much more secluded area and the male showed distinct signs of objecting to my presence at first, especially once when he spotted me through the open flap of the tent. The female on the other hand has seen a lot of us over three years and has clearly become accustomed to our presence. Both showed obvious signs of knowing when I was around, even during long periods of concealment in the tent, and would often look in its direction. (Interestingly, though, in one study (for which I'll find the ref!), the researchers found that they could watch tawnies hunting at night.)

So it was satisfying that once when I went to the nestbox area during the day soon before the first chick fledged both parent owls turned up and flew around in the tops of the oaks paying little attention to me.

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