Wednesday, 9 January 2013

Variation in Adult Jays

Me and Moxey had a busy day on Saturday starting at Brook Vale in the morning, moving on to Scarisbrick in the afternoon for a roost session. Neither session was particularly productive in terms of the number of birds ringed, however there was quite a nice variation in species. I'm not going to give you a narrative of our session on Saturday, but instead focus on two birds that we caught at Brook Vale in the morning. Jays.

We don't catch many Jays each year, we only ringed four in 2012 and eight in 2011, so to catch two together is a significant proportion of the birds we are likely to ring this year. When we came to ring the birds, we took a little bit of time to compare the two birds with each other. We had aged both of the birds as adults based on the criteria that we were familiar with, namely looking at the barring on the outermost greater covert, consistency of barring on the primary coverts and the tail shape.

Jay (Garrulus glandarius)

After comparing both of the birds, we noticed that there was still a difference between the two birds, one of the birds demonstrated a ruffous-brown tip on the inner web of the outermost GC. The other bird didn't. So we decided to consult the one of the guides that we, as ringers, use to help us confirm the age of the birds that we catch by plumage characteristics - Identification Guide to European Passerines by Lars Svennson.

According to Svennson, birds with nine or less black bars on the outermost GC are likely to be first year birds whilst birds with ten or more are likely to be adult birds. As you can see from the following photo's of the two birds, one bird has ten (and a skinny eleventh) and the other has eleven bars.

Adult Jay with eleven bars on outermost GC

Adult Jay with ten clear bars on outermost GC

We also double checked the tail feather criteria by taking a measurement of the the width of the 5th tail feather at 40mm from the tip, both birds had 5th tail feathers in excess of 40mm once again indicating the birds were adults.

The very broad tail feathers of an adult Jay

Whilst this isn't a groundbreaking observation, it was interesting to see the variation between different adults of the same species and also demonstrates the importance of looking at a range of criteria when determining the age of a bird. I, for one, will be taking a much closer look at any subsequent Jays that we catch.

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