Black-tailed Godwits will look and smell different by the end of May
Before they return to Iceland in April or early May, wintering Black-tailed Godwits have got a lot of extra feeding to do, both to fuel their journeys north and to undertake a complete change of body feathers. The grey plumage that has protected them since the autumn is discarded, to be replaced by summer finery. Although more colourful, these feathers provide cryptic camouflage in the habitats in which the birds nest – and they have less odour too.
There is a lot of individual variation but the timing of the spring moult of Icelandic Black-tailed Godwits seems to differ depending on the wintering area used by individuals. Many Icelandic birds that enjoy the sun in Portugal, for instance, can start to change colour before the turn of the year, but there is little red to be seen in British flocks until February or even March. We have the impression that there might be enough variation across Britain & Ireland to potentially pick up differences at this scale but we have yet to establish if this is true.
Some godwit observers have been scoring the moult of spring flocks of Black-tailed Godwits for years, especially for ringed individuals. This has given us quite a lot of data from a few sites, but spread over a number of years and a range of spring conditions.
We thought that we may be able to look at the regional differences more effectively if we could collect images from as many flocks as possible over one weekend. We chose Easter weekend 2016 to try this out, as we thought that this should give us as much variation as possible, with birds at several different stages of moult. Birds were scored from 1 (full winter) to 7 (full summer), according to the table below and images at the bottom of the blog, but all we needed was pictures from birdwatchers that we could then score. We had not reckoned on Storm Katie, which seriously affected opportunities to take photographs!
This was very much a pilot project. We know that we can score most birds when they are seen moving around, so that each bird gives a view of chest side and back. Could we capture this information from a sample of birds in two-dimensional pictures? If this worked, potentially we could have run a season-long project.
There is a big difference between the winter and summer plumage of Black-tailed Godwits. Over the course of a few weeks, a grey individual turns into a thing of beauty. Camera shutters start to click in bird hides when a flock of birds sweeps back and forth, flashing white as the sun shines on bellies and underwings and then russet red as it catches their backs. It would be easy to think that these summer colours make Black-tailed Godwits more obvious, when they are nesting, but that’s not the case. In rusty-orange sedge-edged pools and when creeping their way through the red stems of dwarf willow on their way to a carefully hidden nest, they are very well camouflaged.
Most wading birds moult into a summer plumage in spring. For Black-tailed Godwits, this process involves body feathers, some of the small coverts on the wings, scapulars and a variable number of tertials. Their individuality is apparent from the richness of colour, the mixture of red, black and white feathers in the body and of grey and red feathers in the back. When the moult has been completed, male birds tend to be redder in the chest and neck and they have fewer grey feathers in the back than females, but there is an overlap. When sexing methods were checked against DNA samples it was shown that experienced observers could correctly assign sex to 82% of marked breeding birds in the field for which the sex was also determined by DNA. There’s an interesting paper about this in Bird Study.
Breeding waders don’t just look different – it has been suggested that they smell different too. Birds of a range of species, including Black-tailed Godwits, have been shown to produce a different preen gland oil with which to groom their summer plumage feathers, with monoesters replaced by diesters as winter feathers are discarded and pre-nuptial moult progresses. This was something that was first shown for Knot by Jeroen Reneerkens and colleagues and then for another eighteen species, breeding in both high Arctic and temperate conditions. In nesting birds, secretion of diester waxes continues through to egg-hatching, with the lowest occurrence of diester production in male Ruff and male Curlew Sandpipers. Given that these are the only two species in the sample under consideration in which males don’t incubate eggs, Jeroen et al suggest that diester preen waxes are harder for predators to detect using a sense of smell. You can read more here. When this theory was tested, using a sniffer dog, Jeroen showed that less volatile diesters were harder to detect. (paper)
Did it work?
Although we received fewer images than had been hoped (thanks Storm Katie!) we discovered that pictures are not enough. The clue may well have been in the protocol; we had asked for pictures of 25 to 50 birds so as to show the chest and back of each individual, and that was too tough an ask.
However, it’s still really helpful to receive plumage scores alongside colour-ring sightings so please keep those coming. Black-tailed Godwit colour-ring sightings can be sent to any of the scheme organisers. If you are not sure who your Black-tailed Godwit belongs to, Jenny Gill (email@example.com) is happy to act as a clearing house. There are some sample scores below.
Scoring Black-tailed Godwit moult
WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research. Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.