Dormice don’t ‘happen’ upon bird nests - they go looking
Nine breeding seasons of individual tracking show Edible Dormice repeatedly visit nest cavities at night and most encounters end in predation
Why this matters
Nest predation is the leading cause of breeding failure in small birds, yet we rarely know whether predators strike by chance or by design. This long-running Czech study tagged individual Edible Dormice Glis glis and monitored their night-time visits to nest boxes used by tits, flycatchers and Nuthatches – revealing that most encounters were deliberate, nocturnal raids rather than incidental clashes.
What the researchers did
Over nine breeding seasons (2006–2015) in a mixed deciduous forest near Dlouhá Loucka, researchers fitted a subset of nest boxes with circular antennae and readers that logged every PIT-tagged dormouse entering or leaving. They paired these time-stamped visits with detailed breeding records to identify exactly which dormouse visited which active nest and what happened next.
The headline results
They recorded 23 dormouse–bird encounters involving 18 tagged dormice. In 18 of those encounters – almost 80 per cent – the nest was predated, usually on nestlings (14 cases) but sometimes eggs (four cases). All predation happened at night, on average a little over three hours after sunset.
Who was hit hardest
Collared Flycatchers Ficedula albicollis suffered disproportionate losses – 12 predations from 13 encounters – compared with Great Tits Parus major, where several first encounters were repelled. The authors suggest better in-cavity defence by tits (including hissing displays and direct attacks) helps some nests survive initial intrusions.
Repeat visits – a sign of learning?
Five dormice made repeated visits to active nests within the same season. In two cases they predated both nests; in three, the first visit did not end in predation but the second did – a pattern consistent with learning or purposeful searching for bird broods. No individuals were recorded as predators across multiple years, likely reflecting seasonal emergence patterns and turnover.
It’s a boys’ club (and it’s nocturnal)
Males dominated predation events (16 of the 18 predations), probably because males emerge from hibernation earlier and range widely while seeking mates. Every recorded visit occurred at night – the dormice are strictly nocturnal in their nest-visiting behaviour.
What it means for nest-box managers and conservation
- Expect targeted nocturnal checks: once dormice are active (late May–June), occupied boxes are at greatest risk a few hours after dusk
- Box placement can matter: reviewing box height and spacing may help in some woods.
- Species respond differently: tit boxes may benefit from entrance restrictors or predator guards; flycatcher boxes might require different siting, densities or seasonal removal in high-risk areas. Monitor individuals, not just nests: tagging can identify repeat offenders and clarify whether control measures are working.
Caveats and next steps
Sample sizes for repeated-visit sequences were small, and boxes sit lower than many natural holes. The authors argue for radio-tracking and diet analyses to nail down mechanisms – are dormice actively searching for cues from begging nestlings, odour, or box layout – and to quantify how often they also raid open-cup nests in the wider home range.
Bottom line
For cavity-nesters sharing woodlands with Edible Dormice, predation is usually not a roll of the dice. It’s targeted, male-biased and happens after dark – and flycatchers, more than tits, pay the price.
October 2025
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