Successful nest box results on reserves in Brecknock

Checking nest boxes at Coed Dyrysiog. Photo: Andrew King

Checking nest boxes at Coed Dyrysiog. Photo: Andrew King

Inside a Blue Tit nest. Photo: Andrew King

Inside a Blue Tit nest. Photo: Andrew King

In grateful recognition of a legacy donated to Brecknock Wildlife Trust (before it merged with WTSWW), Trust staff and volunteers wished to strengthen the provision of bird nest-boxes on two woodland reserves near Brecon.

These are Coed Dyrysiog (Soar), a predominantly oak woodland on a sloping bank of the River Bran, and Drostre Wood (Llanwern) an even-aged broad-leaved wood in a pastural landscape.

Seventeen new boxes were placed across these reserves in early-April. Even though some early resident bird species had already started to nest, the uptake of these new boxes was very impressive, particularly at Coed Dyrysiog. The boxes were up well before summer migrants arrived.

All boxes were of a wooden construction, with a forward-facing entrance hole, and a hinged side-panel to allow inspection and cleaning-out at the end of the breeding season. This is an important requirement as too often commercially obtained boxes are sealed, and rather limited in their design and practicality.

Easy access to open these boxes was important as a Trust volunteer, a licensed bird-ringer through the BTO (British Trust for Ornithology who regulate and oversee bird-ringing in the UK) wished to embark on a new project to investigate productivity of common hole-nesting species in these woods.

The boxes were made with a range of entrance hole diameters:

·        24mm, ideal for Blue, Coal and possibly Marsh Tits.

·        28mm, ideal for Great Tits and Pied Flycatchers.

·        32mm, ideal for Redstarts and Nuthatch.

All of these species were known to inhabit the reserves. Eighteen boxes were placed in the two woods in early-April 2018, just in time for the 2018 breeding season. Another two boxes of the same design (24mm hole diameter) will be erected in readiness for the 2019 breeding season.

There appeared to be a good supply of natural nest-holes, especially in Coed Dyrysiog where mature oak-trees form a greater component of the wood, and therefore the degree of uptake of new boxes would be interesting.

Table 1: a summary of the use of these 18 boxes in 2018, in relation to entry-hole diameters

Hole diameter No. of boxes fixed No. occupied in 2018 No. by Blue Tit No. by Pied Flycatcher
24mm 6 4 4 0
28mm 5 5 3 2
32mm 3 3 1 2
‘Lost’ 24mm 4 1 ? ?


Some bird-boxes, of varying age and utility, had already been erected by BWT in these two woods, but relatively few were accessible to nest-monitoring volunteers.  Uptake was reduced in these older boxes, but two were occupied successfully by Great Tits and one by Nuthatch, all at Drostre Wood. No Redstart were seen in either wood. At least one bat box at Drostre Wood was also used by Great Tits, but the parents deserted at the egg-stage.

Those boxed marked as ‘lost’ in Table 1 have not been permanently lost or stolen!! Erection of the boxes in spring, spread throughout the reserves at a time when few leaves were on the trees, was such that by the time monitoring started on 15th May, some of these boxes were obscured from view and could not be re-located. During autumn, they were re-located to find that only one of these boxes had been used in 2018. All boxes have been numbered, and full GPS data to mark location was taken at the end of the season.

Table 2: provides minimum figures for average clutch size and %age chick fledging rate for each of the two occupant species


No. Boxes occupied Av. Clutch size % eggs hatched %  chicks fledged
Blue Tit 8 9.4 eggs 83% 77%
P Flycatcher 4 5.3 eggs 100% 100%

NB: % chicks fledged is calculated against chicks that hatched.

It was surprising that uptake of the new boxes was so high at 86%.

It was also interesting that across all boxes, old and new, 6 pairs of Pied Flycatchers used boxes in Coed Dyrysiog and 1 pair in Drostre Wood. The latter wood is less suitable for this species. No other territories of Pied Flycatcher using natural holes were noted in either wood. It is possible that the provision of boxes in spring 2018 pulled these birds in from other locations, thereby increasing the population, but that cannot be proven.

Comments on box placement and design

The new boxes supplied in late-winter 2018 are well constructed, and have obviously been attractive to birds. All have been placed at relatively low height close to the perimeter paths at both reserves, such that they can be inspected via a 3 or 4-step step-ladder. Some of the older boxes at greater height were removed in October 2018 (when all boxes were cleaned out), and brought lower to match the height of new boxes.

Also, some of the older boxes have been removed from the woods to carry out repairs. The range of hole-sizes made these new boxes more accessible for a wider range of species, even though only two species used them in 2018. The hinged side-entry of the new boxes generally worked well too. Care needs to be taken as broods get older (and close to the latest advisory age for ringing pulli) as they can ‘erupt’ onto the woodland floor perhaps 1-2 days before they are properly ready to fledge.

This did occur with one brood, but generally, judicious use of empty bird-bags to close off the entry hole and edges of the side-panel as it was lowered, prevented this.

Appendix 1: Totals of birds ringed

Site B Tit adult BT pulli Gt Tit pulli Pied Fly ad Pied Fly pulli
Coed Dyrysiog 3 50 0 1 24
Drostre Wood 2 9 11 1 7

NB – more pulli than this were handled/observed but were outside the critical growth stages for safe ringing. Seven visits were made for ringing birds – 4 at CD, and 3 at DWood.

Andrew King, Breconshire County Bird Recorder, and Licensed bird-ringer

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