Nest Box Selection
Blue Tit Nest Box Hole Size: Why 25 mm Matters and Where to Put It
A UK-focused checklist for entrance diameter, construction and siting, with BTO guidance and realistic expectations about occupancy.
By Pussies & Tits · 5 min read · Published
Sourcing and editorial standardsThe short answer
BTO recommends a 25 mm entrance hole for a blue tit nest box. Larger holes can admit larger species. Site the box with a clear approach, protection from harsh weather and a location cats cannot reach. A correct hole size improves suitability; it does not guarantee nesting.
See your next step →Key takeaways
- Measure the entrance diameter rather than relying on a ‘small bird’ label.
- Keep nest boxes away from busy feeding stations.
- Follow species-specific guidance and local wildlife protections.
Why a bigger entrance is not automatically better
BTO’s blue tit box plan specifies a 25 mm entrance. The purpose is species suitability, not simply making access easier. A larger opening can let larger birds enter and compete for the space. If you want to encourage several species, choose appropriate separate designs rather than modifying every opening to one generous size. Check the actual diameter with a ruler or caliper before installation.
Check construction before buying or building
A useful box needs suitable materials, drainage, a secure attachment and access for appropriate cleaning. Avoid choosing solely by painted decoration. BTO’s plan provides a practical construction reference; follow its full dimensions rather than extrapolating from one hole measurement. Do not add a perch merely because shop designs show one. Clear approaches and a suitable entrance matter more than ornamental features.
| Check | Why it matters | What to do |
|---|---|---|
| 25 mm entrance | Blue tit suitability | Measure, do not guess |
| Weather protection | Limits exposure and overheating | Avoid strong sun and prevailing weather |
| Secure mounting | Reduces movement and predator access | Follow fixing guidance |
| Cleaning access | Allows maintenance at an appropriate time | Check how the box opens |
Find a box with a documented 25 mm entrance
Compare entrance diameter, materials, cleaning access and mounting instructions before ordering.
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Choose a site with more than compass direction in mind
BTO advises a clear flight path, shelter from strong sunshine, wind and rain, and a predator-inaccessible location. Its guidance for small-hole boxes gives a 1–3 meter height range. Treat this as guidance to combine with the actual site, not permission to mount a box within a cat’s reach. Look for nearby branches, walls or fences that could act as a launch point. Keep the box away from feeding stations, where activity and competition can disturb nesting birds.
Install first, then watch without disturbing
Choose a stable position and check the mounting before birds begin using the box. Observe from a distance. Do not repeatedly open an occupied box for photographs or move it after nesting starts. Wildlife protections apply to active nests; check current local guidance before maintenance. A camera installation also needs careful planning in advance, not improvisation during an active nesting attempt.
If the box stays empty
An empty box does not prove a purchase failed. Habitat, local bird numbers, competition and natural cavities all influence use. Provide suitable garden habitat and avoid disturbing the box. Review siting at an appropriate time rather than making repeated changes during breeding. No seller or editorial guide can honestly guarantee occupancy. This article narrows a purchase decision but should not replace the full BTO plan or local conservation advice.
Frequently asked questions
Can I enlarge a blue tit box entrance?
Enlarging it changes which species can enter. If blue tits are your target, use the 25 mm design recommended by BTO and do not alter an occupied box.
Should a nest box sit beside a feeder?
BTO advises keeping nest boxes away from feeders. Busy feeding activity can be an unwelcome disturbance near a nest.
Sources and further reading
This guide summarizes the references below. It is not a veterinary consultation or an independently tested product review. Check current local wildlife advice before acting.
Find a box with a documented 25 mm entrance
Compare entrance diameter, materials, cleaning access and mounting instructions before ordering.
Compare on Amazon →Affiliate link: we may earn from qualifying purchases at no extra cost to you.
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