After a fantastic spring and summer of bird ringing at Bedfont Lakes most of the warblers (aside from the resident cetti’s warbler) have left for their long journey to their wintering grounds. Before they leave they have a fair bit of fattening up to do so this is the time when ringers start noting the weight of these migratory birds and giving each individual a fat and a pectoral muscle score. This is a new learning experience for me and a few weeks back was a great opportunity for me as we caught over 102 birds at Bedfont Lakes South Side between 4 of us (and sadly I had to leave early that day!), including 36 blackcap and 25 whitethroat! This allowed me to compare individuals varying fat and pectoral muscle build up and get to grips with how they feel and look.
I thought this would be a nice time to look at some of the different species of warblers at Bedfont and some of the differences between the juvenile and adult birds. Starting with my favourite – the sedge warbler.
Juvenile sedge warbler
As far as I figure, generally juvenile sedges have more yellowish buff colouring especially on the underparts but from the photo above it is hard to tell if this is indeed a juvenile or an adult (I found it tricky!), you would need to look closer at the feathers; new, fresh wing and tail feathers and ‘fluffy’ under tail coverts can sometimes be a giveaway, although not always obvious to me. With this species a good clue is the spotty ‘necklace’ most juveniles have – not so much in the one pictured above but you can see here in the one below a line of small dark spots going across the breast.
Juvenile sedge warbler with ‘necklace’
Really these are beautiful birds, their creamy colouring and dark markings are just fantastic. Definitely have a soft spot for them!
Reed warblers seem to be the most common warbler ringed at the main sight (the Nature Reserve on the North side of the Park). Again new, fresh wing and tail feathers and ‘fluffy’ under tail coverts can indicate a juvenile bird but also in this species the juveniles are usually a more rufous or rusty colour, especially on the upper parts and rump as you can see in this next photo (juvenile on the left).
Juvenile (left) and adult (right) reed warbler comparison
Another indication of age with this species is eye colour – the iris is a grey charcoal colour in their first year then changes to a more olive brown as you can see in this adult bird.
Adult reed warbler
I haven’t managed to get a good photo showing the iris of a juvinile reed warbler but i did get this adorable one of a very newly fledged bird, how cute?! He hasn’t even grown his tail feathers yet!
Reed warbler fledgling
Only a little bit more developed is this rather fluffy juvenile cetti’s warbler…. This year was the first record of cetti’s breeding at this site!
Juvenile cetti’s warbler
Again the juveniles in this species have a darker grey iris as opposed to the brown iris of the adults.
Another common warbler species at bedfont are blackcaps. I mentioned in previous posts (this one and this one) that the juveniles have a duller brown cap, as opposed to the chestnut in the adult females and black in the adult males. The juveniles are are generally more brown and fluffy compared to the adults and have fluffy undertail coverts that are a light brown (or ‘mocha’ as I call it!) colour as opposed to grey in the adults.
Juvenile blackcap undertail coverts
Adult female blackcap
Upperparts of adult female blackcap
In the last 10 months I have been ringing at Bedfont I haven’t seen many willow warblers but I know the juveniles generally have more yellow underparts and generally brighter yellow colouring all round. This juvenile is a good example, and stunning in in the sunlight!
Juvenile willow warbler
Juvenile willow warbler
In my experience ringing at Bedfont other warblers that seem to be fairly common are chiffchaffs and whitethroats, the latter only at South Side. Along with willow warblers, other warblers not as common are garden warblers and lesser whitethroats. In fact I think I’ve only seen a handful of adults. In any case, it was a treat as they are both beautiful, smart looking birds – not at all ‘grey and dull’!
It’s been an amazing spring and summer and now I am looking forward to see what new learning experiences autumn and winter bring!