What's happened to this bird I have no idea. It seems to have lost the feathers from its head.
No interactions present.
Head feathers worn/damaged through feeding young.
Perhaps the nesthole is a little tight and over the course of coming backwards and forwards to feed the young has taken its toll on its head feathers. Agree with previous comment also. Adults focus their attention on feeding their young. Lots of adult birds look very tatty at this time of year.
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I doubt this is caused by pushing through a hole into the nest, as that would affect the body feathers as much or more than the head. It is more likely to be caused by feather mite or some nutritional deficiency or something similar. It will be fine and almost certainly regrow the feathers at the next moult. I once ringed a Blackbird with a bald head like this in my garden, and that was perfectly normal after the post-breeding moult (in fact, it is still in the garden, often, some 5 years after ringing).
This bird looks like a bit of an old bruiser, with missing toes! Shame the ring number cannot be read, as then you could have found out more about it.
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Wear would be likely to be on the widest part of the bird. I was thinking disease or parasite.
Oh, to be able to read the rings on birds in photographs! The small metal ones are designed in such a way that you can get a few characters at best (see added photo), but given their size I suppose its inevitable.
In some cases a combination of colours are used, but I imagine it's only any use with small populations that return to regular haunts. An example is the nightingales at Little Paxton. I've photographed two that have been easily identifiable. One was this year, and it was ringed in 2008 as an adult male; so it's at least five years old. It was in fine condition still.
Unfortunately, you would need several different angles in order to see the whole ring number, which consists of a letter and six digits.
I was adding a comment to go with the picture along the same lines. It's such a shame that we can take photographs with such clarity that we can read the characters, but we can't see them all. I've groaned with frustration lots of times.
(I suppose people used to complain because they couldn't read the characters at all, not all that many years ago...)
The number of people going out to read rings specifically is increasing. Even with the smallest rings, some people have managed to get the necessary shots. Really, though, colour ringing provides so many more returns, especially of the bigger species. Local to me, the Lower Derwent Valley ringers do a lot of colour ringing of ducks and other species and are getting some great returns. Their blog is always worth a look...
Am I right in thinking that, with a lot of these small metal rings, you'd need several shots from different angles to get the whole number, because it runs around the ring?
If that's the way they're marked, then it's all in the lap of the gods rather (or the whim of the bird); unless the birds are netted of course, but we're talking about photography.
As you say, coloured rings are great, but they don't provide as many combinations as numeric ones, unless you use very subtle colours; you'd need ten shades to match number combinations, and then you might run into lighting and white balance issues, such as one colour being taken for another.
Not such a problem with big birds, where you can use colour AND numbers combined.
Oh for an easy life. But then I'd get bored! ;-)
(Just looked at the blog; got to admit I didn't take into account the ringing of BOTH legs. That considerably increases the combinations.)
Lat/Lng: 52.1159966, -0.257848
OS grid ref: TL193478