Hybrid falcons Falco spp.
There's nothing that quite matches watching an impressive bird of prey in the wild, and nowadays with raptor viewing points in many areas and even guarded nest watch sites, it's perfectly possible for anyone to do so. Still, most members of the public seem to prefer to visit a falconer's display and watch miserable tethered captive birds of prey which spend their days and nights cooped up in a tiny cage before being forced to fly around some screaming children for a few minutes now and then. The sooner the public realise how much more rewarding watching wild birds can be and the sooner the practice of falconry is stopped once and for all, the better.
As well as keeping birds of prey in unnatural conditions, many falconers are also fond of mixing falcons in unnatural ways, and a very large proportion of birds kept by falconers are hybrids. Some are even hybrids with hybrids, having three (or more?) species in their ancestry.
As many falconers aren't even very good at keeping falcons, many falcons and other birds of prey escape from them and may be encountered in the wild. The presence of escaped hybrids makes identification of large falcons extremely difficult and any claim of Saker, Lanner or dark/intermediate-phase Gyrfalcon tends to be tainted with uncertainty.
The following individual was present in North Norfolk for a number of months and changed in appearance during its stay. When it was first seen it appeared much darker than shown below and was identified as a Saker. By the time I saw it the underparts especially had become much paler and the bird was almost more likely to be mistaken for a Gyrfalcon than a Saker. The identification is uncertain, though Gyr x Saker is probably a good bet.
Escaped birds of prey are often (but not always) found to be wearing jesses, the leather straps put on their legs by falconers. Some, like this one, also carry bells, though these bells obviously didn't ring loudly enough for its owner to find it.
escaped Large Falcon, possibly Saker x Gyrfalcon, Choseley (Norfolk, UK), 29th July 2005