Sunday, 13 July 2014

Some clues to ageing birds - July 2014

Normally by now we would have started ringing at Titchfield Haven, but unfortunately we are running very behind this year. In preparation for a start next week, we did cut in most of the net rides this morning, and hope to finish things off one evening during the week. With no ringing at the Haven my activity has been limited to occasional sessions in the garden and so for this post I thought I would go through some of the ageing features on the species that I have caught.

Over the last 3 or 4 years I have regularly caught Great-spotted Woodpeckers, particularly through June, July and August, when the young have recently fledged. Determining the age of a juvenile Great-spotted Woodpecker is pretty straight forward since they have red crown feathers (see below). Birds cannot be sexed until they have undergone their post-juvenile moult and the red nape patch has developed in males, or not in the case of females.

Juvenile Great-spotted Woodpecker

Juvenile birds undergo a partial post-juvenile moult that includes body, primary, upper wing coverts and tail feathers. Usually most, if not all, of the secondary, tertial and greater coverts and primary coverts are retained. Following completion of the post-juvenile moult it should be possible to see a contrast between retained juvenile feathers and new black adult type feathers. Juvenile birds also show white tips to the primaries, that is until they have undergone their post juvenile moult and replaced their primaries.

Active wing moult of Great-spotted Woodpecker - note the white tips to the outer primaries of the juvenile feathers which will not be present in a juvenile bird.

Blackcaps breed in the scrub behind my garden and so I often catch newly fledged birds when ringing in June and July. Juvenile birds have a dark brown cap, which to the unwary could be confused with the chestnut brown cap of a female bird. They cannot be sexed until they have undergone their post juvenile moult. Juvenile Blackcaps undergo a partial post juvenile moult (which does not include primary, secondary or tail feathers), whereas adults undergo a complete moult post breeding, replacing all their feathers. It may therefore be possible to see a contrast in retained greater coverts in juveniles to aid ageing.

Juvenile Blackcap - showing brown crown, note the deeper brown colour as opposed the the chestnut of a female.

The shape and extent of wear on feathers can be very useful when ageing birds in the autumn, particularly tips of primaries and tail feathers. Juvenile tail feathers are generally pointed and thinner, but also look at the ground colour of the feathers. Juvenile feathers tend to have a brownish wash unlike the greyish tone of adult feathers, although this can be more difficult to determine in female birds.

Blackcap - Juvenile tail; pointed tail feathers are often a good clue to the age of a bird

Blue Tits and Great Tits follow the same moult strategy as many passerine species, with adults undergoing a couple moult of all feathers post breeding where as juveniles undergo a partial moult. This post-juvenile moult does not include any wing or tail feathers and therefore any bird that is replacing it's primaries (as in the image below) must be an adult. In the image below it is possible to see a contrast between the new blue adult primary coverts and the old greenish juvenile ones, thereby making this bird as an age code 5.

Post Breeding Wing Moult in a Blue Tit

The image below illustrates the post-juvenile moult of a Great Tit. Although it cannot be seen in this image, there is no moult in the primaries or secondaries, but there is in the greater coverts. It is possible to see the old juvenile retained greater covert (and all primary coverts), contrasting with the new adult type feather, which is still growing and partially in pin.

Post Juvenile Wing Moult - note contrast between new adult type feather and retained juvenile one.

Another feature that can indicate the age of a bird is a fault bar or growth bar on the tail feathers. Such features are caused by structural differences that have occurred during the development of the feather. If, as in the image below, the pattern forms a band across the tail at the same location on the feather, this indicates that all the feathers have been grown at the same time, as when in the nest. With the exception of the few species that periodically moult their tail feathers, or where an individual has accidentally lost its tail, this can point to the bird being a juvenile. When an adult bird replaces its tail feathers, as with a post breeding moult, the bird would replace its feathers in pairs, and therefore any anomaly would not appear as a band across the tail.

Fault Bar on a Great Tit

It is important to remember when ageing birds to look at a number of features in combination before reaching your conclusion. Even if you have a bird of known age it helps keep you eye in for when you are faced with a difficult individual.

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