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Hoi Gerard,

Definitely a Tachinid.
Difficult to determine based on the picture.
Given the circumstances: early spring on tree stems an educated guess would be:
Campylochaeta praecox.

This species is easily recognized by the ocellar bristles (the strongest bristles between the ocelli) being reclinate (= curved backwards). In nearly all Tachinidae they are proclinate.

So, let me know.

Theo Zeegers