Nestcam footage has revealed how a pair of blue tit chicks have been unwittingly raised by a family of great tits.
The nest was first discovered in early June in woodlands at the Lodge Forest Visitor Centre in Aberfoyle, Stirlingshire.
Two of the eggs in the clutch quickly identified as belonging to a different species, but all hatched at the same time.
Despite being dwarfed by the much larger great tits, the blue tit chicks fought their way to the front at feeding time.
They managed to not only survive, but thrive, fledging at the same time as their adopted siblings.
Experts at the centre say the footage offers new insights into the private world of birds.
“Watching this little great tit nest with a mixed brood of chicks has just been incredible,” said Ami Kirkbright, wildlife information officer for RSPB Scotland.
“It’s something that no one here has ever seen before. It was really fascinating to research what was happening, and then follow their progress over the weeks.
“We get some really privileged views of wildlife on our cameras here at the Lodge, with birds like our ospreys and owls, as well as lots of other animals, including pine martens and red squirrels.
“But these little great tits and blue tits have been the star of the summer I think, not only for me but for many of the visitors coming into the centre.”
It is not unknown for blue tits and great tits to raise each other’s young, but it is rare to capture the behaviour on film.
Territorial fights between the species for good nesting sites are thought to result in great tits displacing their smaller rivals, sometimes after eggs have been laid.
However, blue tits have also been recorded sneaking back to their nest after a takeover and adding eggs of their own, perhaps as a last-ditch breeding strategy.
Blue tits may only have one breeding attempt in their short lives, so this furtive egg-laying strategy is thought to be an evolutionary tactic– allowing them to pass on their genes even after losing their nest.
Scientists studying the long-term effects of shared nests have discovered that fostered blue tits fledge believing they are great tits and start their adult lives using incorrect songs and calls, before eventually reverting back to natural blue tit behaviours.
Incidences where great tit chicks are raised by blue tits are even less common, but not unheard of. It’s thought such scenarios are accidental rather than a breeding strategy, as great tit chicks seem to continue thinking they’re blue tits for their entire lives and are never able to mate successfully.
The Lodge Forest Visitor Centre is managed by Forest Enterprise Scotland, which runs the wildlife viewing project with RSPB Scotland.