MR, Hertfordshire
Identification of the species is key to solving this problem, and if you have not done so already, you should have the invaders professionally identified. Once you have done that, you can identify a solution.
Having said that, from your description they sound as if they are bird mites, which are primarily parasites of poultry. They live in the fabric of henhouses, feeding on the blood of chickens, on which they are dependent.
They can also attach themselves to wild birds but less successfully, because wild birds spend less time in their nests than chickens do in henhouses.
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It is only when wild birds are fledging their young that bird mites have a real opportunity to thrive. Then, when the nest has been abandoned, the mites lose their food source and migrate in search of alternative hosts, usually at night.
The timing and description of your problem matches this. Inspect your roof thoroughly for birds’ nests. Sparrows, for example, need only a small gap to find a way in. It is not uncommon for them to build a nest between the roofing felt and the tiles.
Also check pipes and chimneys. Where feasible, protect these from access by birds. If you have bird mites, then birds must be gaining access somewhere.
If it is any consolation, these mites are dependent on birds, and although they reportedly resist starvation for several weeks, they will ultimately die without their hosts. While they may bite mammals, our blood cannot sustain them and so, by the time this letter is published, your problem may have already diminished or even disappeared.
If it has not, then consider seeking another identification of the mites.
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The British Pest Control Association (www.bpca.org.uk) offers an identification service and access to bird specialists. For details, e-mail enquiry@bpca.org.uk.
The problem may well return next year. If you have not been able to locate any birds’ nests, spend some time early next spring watching to see whether they are gaining access and, if so, where.
Richard Strand is a chartered biologist with 30 years’ experience in pest control
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