Colour-ringed Waxwings

I went to see the Hessle waxwings this morning, where a flock has been present since last Friday. Straight away, I saw two photographers looking at a tree. I soon joined them and the 18-strong flock was there, in a small street-lining cherry tree, facing the strong westerly wind. The light was beautiful compared to my previous Waxwing sightings, most of the time in the depths of winter. The birds were quite mobile, trilling and moving from tree to tree before descending to the unlikely source of berries this time of year: hawthorns.

No supermarket car pars, but car dealerships this time.

The 18 strong flock. 

Berries, what berries?
Probably most hawthorns in the country were stripped of berries long ago. Not these. The leaves of some of them sprouting amongst last year’s crop of berries. The reason for the late berries could lay in the location of this hedge: a very narrow strip of land between the A63 and a busy local road on an industrial estate. Every time the waxwings descended to feed, roaring trucks on one side and fast delivery vans on the other, I feared for their safety. Photos don’t really convey the feeling of being there, the noise of the road and the wind, the feel of the acrid stench of the traffic fumes at the back of your throat. This stressful food location has probably kept the local thrushes and Woodpigeons away.

On the hawthorn: berries and fresh shoots.

Colour rings
Two of the waxwings had plastic rings with different colour combinations on their legs. The yellow plastic over metal of their right legs indicate that both birds are part of the 2019 ringing scheme by the Grampian Ringing Group. In fact, information supplied by Raymond Duncan from the Grampian ringing group indicates that both birds were ringed on the 7th December 2019, in Aberdeen. One of the birds, a female with white, black and red rings stayed put until the 5th of January at Aberdeen, the other, a male with light green over dark green and dark green ring was found in Thirsk (N Yorkshire) on the 30th December and 3rd of January. Waxwings move towards the UK at the beginning of winter, in large numbers when the berry crop in Scandinavia is poor, what is called irruptions, and then they move south and east through the country. Is it coincidence that both birds are together in Hessle? Although social birds, Waxwings are not known for forming strong individual bonds when they move about during winter. However, there are reports of the same birds returning to favoured spots in different irruption years, so the same individuals may meet again in these Waxwing hotspots.

The Grampian Ringing Group has many years of experience colour ringing waxwings: they have colour-ringed thousands of waxwings for over thirty years. This winter, their dedication has paid off with 100 ringed waxwings and some resightings across the country. Their blog is a treasure trove from everything waxwing including how to sex and age them. Resightings of colour ringed birds, particularly when the colour rings and their position can be ascertained, are crucial to follow the movements of Waxwings without need to recapture them.

The male was ringed in Aberdeen in December.

Female also ringed in Aberdeen in December.

Males, females and ageing
Waxwings of different sexes and ages appear similar at first sight, but they are subtly different. Examination of their plumage pattern can actually be used to age and sex them. Adults have their primaries edged with white and bright yellow, forming a ‘hook’, while young individuals have only one side of the primaries lined with white or pale yellow. When the wing is closed this forms a single line. First winter immatures also have a shorter crest. The ‘bib’ is larger and sharper in males, while the bib edges are diffuse in females. The yellow terminal tail band is wider in males. Males have over 6 waxy red feather tips, and these are longer in adult males, but there is some overlap between sexes. Females have a small number of smaller waxy tips. Some of these features are visible in flight, so you may want to have a go at sexing and aging your waxwings from photos.

The diffuse edge of the bib indicates this is a female.

After watching the Waxwings for a while, I went to their old haunts in Priory Park at Hessle. Trees lining the road between Sainsburys and Aldi supermarkets have been removed, but their favourite berry trees, rowans, are still there, of course, there wasn't a single berry left! 

Mute swan colour polymorphism

 After weeks of waiting for the cygnets to hatch, we were rewarded today by the mute swan family approaching us in a local lake. I noticed the two cygnets were different, one had grey-brown down and black beak and legs.

The other, had creamy white down, with pinkish-grey beak and legs.

It was quite noticeable when they were together.

 Next up a close up of the male, I even used the macro setting, that close we were. He was very relaxed around us, but he reacted by opening his wings, raising his neck feathers and hissing loudly when a dog approached the family. Both parents were incredibly protective of their cygnets, chasing away ducks, moorhens and even fluffy ducklings!

 The swan family, on the right, the female, which has pale feet.

The white cygnet illustrates the fact that Mute Swans (Cygnus olor) have a colour form called 'immutabilis', or unchanging, and also known as Polish swan. The mutation is sex-linked and recessive, and occurs in a gene located in the Z sex chromosome. Unlike most mammals, female birds have two different sex chromosomes (ZW), while males have two of the same kind (ZZ). If the female has the mutation, she will be white as a cygnet and will moult into an adult white plumage directly. A male needs to have two mutated forms of the gene to be a Polish swan. 'Immutabilis' adults are hard to tell apart when they are in the water, as the only trait that is different is the colour of the feet, which is paler. The immutabilis form is rarer in populations than the normal form, and it is more common in female swans.

 Other than the 'cute factor' when cygnets, the immutabilis form carries advantages and disadvantages for swans. Swans are very territorial, and they react aggressively toward white.  As they start moulting into their white feathers while being in their parents territory, the adults may start behaving aggressively towards the immutabilis cygnets. This means that young immutabilis flegdlings could be expelled from the territory earlier and may suffer highest mortality, as they need to venture and find a territory of their own when too young. Swans do not usually breed until their 4th year of life, so moulting into a brown subadult plumage pattern advertises to other swans, including their parents, that they are not ready to breed. On the other hand, if population densities are low, it could be advantageous to moult straight into an adult plumage and breed earlier.

Conover and coworkers studied the feral swan population in the East of the USA for nine years to establish the survival of cygnets of both morphs and the age at which they first reproduced. They ringed and neck banded young swans in two populations and follow their fate by thousands of resightings. Their results were striking: immutabilis cygnets (which they called AP, or adult plumage) had significantly lower survival rates from hatching to fledgling (73%) than grey cygnets (which they called SAP, or subadult plumage) (87%). This effect was mostly due to the higher mortality of male immutabilis cygnets, and mortality seemed to occur between August and fledgling, when cygnets grow their flight feathers. They stated:

We watched the parents of eight broods, which contained both SAP and AP phenotypes (four in the Chesapeake Bay and four in Long Island Sound), attack and drive out of their territories their AP offspring during August or September, while continuing to care for their SAP cygnets for several more months. We were able to keep track of eight of these ostracized AP cygnets; four of them perished within a month.

But all is not lost for the immutabilis swans. Although males suffered a higher mortality, the survivors were able to breed earlier than SAP males: by age 3, all of the surviving immutabilis males had bred, while just over 30% of the SAP males had done so. In the studied populations, the frequency of the immutabilis form does not seem to be changing, so the persistence of the immutabilis form could be reflecting a balance between the costs of being expelled early from the parental territory and the benefits of early breeding.

More information

Conover MR, Reese JG, & Brown AD (2000). Costs and Benefits of Subadult Plumage in Mute Swans: Testing Hypotheses for the Evolution of Delayed Plumage Maturation. The American Naturalist, 156 (2), 193-200 PMID: 10856201

Munro, R. E.,, L. T. Smith,, & J. J. Kupa (1968). The genetic basis of color differences observed in the Mute Swan (Cygnus olor) Auk, 85, 504-505

UPDATE 23/0/2022. I'm reading 'The Swans' edited by Sir Peter Scott, and he discusses the possible force behind this colour polymorphism, which appears to incur in the cost for cygnets that they suffer more aggression from parents and are expelled from home territories at an early age: hunting pressure. Young swans were preferred as food as they were more tender, and therefore the immutabilis variety were hunbted less, as they were thought as adults.