Two Familiar Tits, Two Different Timetables

Blue tits and great tits are the two birds most likely to move into a British garden nest box, and although they can look similar at a glance, their breeding routines differ in ways worth knowing. Both are hole-nesters, both sing from February onwards, and both depend on a brief and unreliable glut of caterpillars to raise their young. Yet a blue tit and a great tit sharing the same garden will rarely do the same thing at the same moment.

Separating them is straightforward once you have seen a few. The great tit is noticeably bulkier, with a black head, white cheeks and a bold black stripe running down the belly. The blue tit is smaller, neater, with a blue cap, yellow breast and a short, dumpy look. In voice, the great tit is the one repeating a ringing two-note phrase, often likened to a squeaky pump; the blue tit offers a thin, churring trill and a fussy scolding note near the box.

Choosing a Nest Box: Size Really Does Matter

Blue tits need a hole of roughly 25 to 28 millimetres. Great tits need something closer to 32 millimetres, and if the entrance is too tight they will simply look elsewhere. Buying or making a box with the correct hole for your target species is the single most effective thing you can do.

• Position: mount boxes 2 to 4 metres up, ideally facing between north and east so the box does not bake in afternoon sun.
• Shelter: angle the box slightly forward so rain runs off, and keep it clear of the prevailing wind.
• Protection: fit a metal plate around the entrance hole to stop squirrels and greater spotted woodpeckers enlarging it.
• Spacing: leave at least 5 metres between boxes, as tits defend a small territory around the nest.
• Avoid: perches below the hole, which help predators, and boxes with thin walls that overheat quickly.

Both species will also use natural cavities, wall crevices and old pipes, so a box is an addition rather than a necessity. Where sparrows and tits compete, more boxes in more positions gives everyone a chance.

Nest Building: A Fortnight of Moss and Wool

Nest building usually begins in earnest in late March or April. A blue tit's nest is a remarkable piece of work: a deep cup of moss, dried grass and strips of bark, packed with animal hair, wool, feathers and sometimes the odd piece of string. The female arranges it; the male often accompanies her and passes material across. Construction can take a fortnight, and some pairs start and abandon a site before settling.

Great tits build a similar but coarser nest, with a heavier base of dry grass and moss and a generous feather lining. If you leave out suitable material in spring, such as dog hair, short lengths of wool or a bundle of dry moss, you may see it disappear within hours. Do not put out long strands of thread or synthetic fibre, as these can tangle around small legs.

Eggs, Incubation and the Caterpillar Clock

A blue tit typically lays 7 to 13 eggs, one each morning, though clutches of up to 16 have been recorded. The female begins incubating before the final egg is laid, so the chicks hatch over two or three days rather than all at once. Incubation lasts about 13 to 15 days, and the male feeds the female while she is on the nest.

Great tits lay slightly smaller clutches, usually 7 to 12 eggs, with a similar incubation period. For both species, the whole timetable is tuned to a single event: the emergence of caterpillars as oak buds burst. Winter moth larvae and similar species are the perfect food, soft, plentiful and rich in protein. Get the timing right and the brood thrives; miss it by a week and the parents struggle to find enough.

Feeding the Brood: A Caterpillar Every Few Seconds

Once the eggs hatch, the pace becomes extraordinary. Both parents shuttle to and fro from first light, delivering a caterpillar every few seconds at peak demand. A growing brood of blue tits can consume hundreds of caterpillars a day, and the adults may make several hundred visits to the box between dawn and dusk.

Nestlings fledge at around 16 to 22 days old, often early in the morning. For the next two or three weeks the young follow their parents around the garden, begging noisily and learning where to find food. Blue tits occasionally attempt a second brood if the season has been kind; great tits rarely do.

Watching Without Disturbing

The best watching is done from a distance, with patience. A few practical rules will keep the birds safe and your records useful.

• Do not open the box between April and August. Tapping on the wood or shining a torch inside can cause the female to desert, or the chicks to fledge too early.
• Clean boxes out in September or October, removing old nesting material and giving the wood a scrub with hot water only.
• Avoid insecticides on trees, shrubs and lawns. A garden free of sprays is far more productive than one dotted with mealworm feeders.
• Keep water available year-round, and top up mealworms sparingly during wet, cold spells when caterpillars are scarce.
• Watch from a chair, ideally 5 metres or more away, and note dates of first nest-building, first egg and first fledgling. Repeated records over several years are genuinely valuable.

Once you know the routine, the breeding season becomes a story you can follow day by day: the first moss arriving, the change in the male's song, the sudden urgency at the hole, then the quiet morning when the box falls silent and the young are out in the leaves.