Tuesday, 10 March 2020

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! 

Sunday, 1 March 2020

Black-headed gulls paddling

I've written about foot-paddling for worms on wet grass in Herring Gulls and Common Gulls in this blog before. My local park has flooded in the last few weeks and Black-headed Gulls have moved in, enjoying the flushes, which bring up a lot of worms. Black-headed Gulls foot-paddle in water, but apparently they don't do it on wet grass like other gulls (but see the clips by Ralph Hancock in the comments). In an article on foot paddling in gulls Niko Tinbergen speculated that the reason could be that Black-headed gulls are not heavy enough to produce the vibrations on the ground that stimulate the migration of worms to the surface.
I had never seen Black-headed Gulls paddling on water before. This morning there were at least 30 Black-headed gulls and I noticed a one briefly paddling on shallow water, and checking in front of its feet. A young gull came along and when at the same spot it started paddling too. I managed a short clip of it.

After a while one of the young ones got a worm, unclear if due to the paddling. Walking around the pools on areas where the gulls don't go by the road it was apparent that there were a lot of dead (drowned?) worms. I think that's what the gulls are after in the flood. The Herring gulls were also energetically pulling grass and leaves and soil from the shallow puddles to expose food.

More information
Tinbergen, N. Foot-paddling in gulls. Br. Birds 55, 117 (1962)

Friday, 28 February 2020

Wing-spreading in Cormorants

The spread-wing posture of cormorants, like the individual above in a local park earlier this month, is one of their most distinctive behaviours. Multiple reasons have been offered to explain this wing-spreading behaviour, is it to dry their plumage? or is it thermoregulation (basking)? Even aiding their balance, signalling successful captures, or help swallowing fish have been put forward as possible explanations for this behaviour. The evidence in the Cormorant overwhelmingly supports the plumage drying hypothesis.
A study by Robin Sellers on wintering cormorants in the river Severn provided strong evidence for wing-drying as the function of this behaviour. Cormorants engage in the open-wing posture almost exclusively after having been diving, when their plumage is wet. After a bout of diving, the cormorant will do some bathing movements in the water, flap its wings and shake its plumage before flying to a perch, where the wing-spreading takes place. Not only the wings, the tail is also spread at the same time. The longer they've been immersed, the longer they will stay open-winged.
Wing-spreading was almost exclusively observed in individuals that had been in water the previous 30 min, and most of the cormorants that had been in the water did engage in wing-spreading shortly afterwards.
Wing-spreading is often followed or preceded by preening the plumage. The duration of the behaviour is also inversely correlated to temperature and wind-speed, supporting the drying hypothesis. Rain tends to inhibit the behaviour.
Wing-spreading in Cormorants is not associated to a successful fishing event, rejecting the 'digestion-aiding' hypothesis.
 Sunny conditions did not trigger wing-spreading, but at low wind speeds cormorants tended to orientate away from the sun.

So, why don't other diving birds, such as grebes, auks or goosanders engage in this behaviour? The mechanistic, proximate reason is that cormorant feathers are not very water-proof, unlike other diving birds, in fact, they are very become waterlogged after a few minutes in the water. This is not due to a deficiency of their oil glands, but to a different microstructure of their feathers, which is thought to help reduce their buoyancy, and therefore energy requirements, during diving.

Different birds different functions?
Evidence also supports wing drying as the primary reason behind wing-spreading in other studied cormorant species (Shag, Double-crested Cormorant, Galapagos Cormorant, Bank Cormorant and Little Cormorant). However, in the tropical Anhingas, a group related to the cormorant family, wing-spreading takes a much larger proportion of their time than in cormorants, and the behaviour tends to happen for longer in sunny, cool weather, with their back oriented to the sun, and doesn't require wet plumage. Anhingas have a very low metabolic rate, which might favour efficient basking behaviour, and in captive, controlled conditions, oxygen consumption decreased as they engaged in wing-spreading. Therefore, the primary function of the behaviour in anhingas appears to be thermoregulatory ('sunning') rather than drying (although this may also have a secondary role). It is a cautionary tale that apparently identical behaviour in related species can have different functions.

More information
Sellers, R. M. Wing-spreading behaviour of the cormorant Phalacrocorax carbo. Ardea-Wageningen, 83, 27–27 (1995).

Hennemann, W.W. Energetics and Spread-Winged Behavior in Anhingas and Double-Crested Cormorants: The Risks of Generalization. Am. Zool. 28, 845–851 (1988).

Thursday, 6 February 2020

Long-tailed tits hanging and feeding

Just a quick field note on foot use. I wasn't aware on Long-tailed tit foot use before, so I was very pleased to watch this behaviour. Sun flower hearts had been placed atop posts on a nature reserve and a flock of Long tailed tits was feeding on them. Individuals will pick a sunflower seed and jump to a branch to hang from it with one foot while holding the seed on the other food to feed on it. Other tits hold seeds between their feet while they feed, perching on a branch. Is it that Long-tailed tits are not able to balance on a branch while bending down on their feet, maybe because of their long tail? I saw the behaviour repeatedly, and I should have taken a video, but I just got this shot from behind that shows the foot holding onto the branch and the other with the seed on it. What call my attention was the fact that they had to jump to a branch to hang from in a premeditated way so that they could feed. The Two in the Bush blog also describes this behaviour, with much better photos. Hanging from one leg poses no issue for these tiny acrobatic birds!

Friday, 10 January 2020

Stonechat pairs in winter territories

Species like the Robin keep territories during the winter, with males and female individuals defending their own separate territories, while in the summer, territories are defended as a pair. There are some exceptions in which pairs defend winter territories, often in different areas that summer ones. One of these species is the Stonechat. In their wintering grounds on the coast and around the Humber estuary, it is frequent to encounter Stonechat pairs in the same area, feeding near each other and following each other, with no aggression (top shot, a pair at Alkborough, Lincolnshire, 11/12/2017). Of course, winter territory defence has nothing to do with reproduction, and often happens in areas that don't even hold breeding pairs in the summer. This unusual behaviour must offer some benefit to persist in the population, maybe allowing individuals to increase their chances of survival to predation or competition and to secure sufficient food resources.
A study of wintering Stonechats in the Negev desert (Israel), shed some light onto the behaviour. A total of 89 individuals were colour banded and followed through two wintering seasons. Their territorial behaviour was noted. The most surprising result of the study is that wintering Stonechat pairs are very fluid. Pairs often form after arrival to winter areas, and individuals rarely stay together all winter, instead pairs change frequently, with an average of just 4 weeks together per pair. In fact, none of the studied pairs stayed together all winter long and individuals arrived in the area and left at different times. This rules out that breeding pairs migrate together or settle on the same winter territory. Up to a third of individual are unpaired at any given time, but all associations are always between a male and a female. There were some aggressive interactions between members of a pair, which were commonly initiated by the male. Both males and females engaged in territory defence, with males more aggressive against intruders than females.
Male Stonechat at Kilnsea, 10/12/18.
Why pair in the winter?
If pairs are not stable, don't migrate together, or don't form in preparation for the winter season, what is the point in defending territories as a pair? Males and female stonechats feed on the same prey, therefore there may be a cost on sharing a territory during the winter, when food resources may be very scarce. A possible benefit outweighing the cost may be joint territory defence: two individuals are better spotting and chasing away conspecific intruders or detecting predators and defending the territory against individuals of other species with similar food resources. In this study, individuals spent a lot of time defending the territory against Mourning Wheatears, but there was no mention of differences when pairs or individual Stonechats were involved.
Another pair at Kilnsea (13/1/2020)

Why male/female pairs?
These results don't clarify why associations are not between two males or two females defending a territory. The authors speculate that as Stonechats are sexually dimorphic, even in the winter, there may be constraints that prevent two males from living peacefully in the same territory, and the increased male aggressiveness may mean two females may be unable to defend a territory against a male/female pair.

More information
Gwinner, E., Rödl, T. and Schwabl, H. Pair Territoriality of Wintering Stonechats: Behaviour, Function and Hormones. Behav. Ecol. Sociobiol. 34, 321–327 (1994).

Tuesday, 17 December 2019

Loafing gulls

 Gulls spend a good portion of their days doing what is known as 'loafing', in open areas of good visibility: playing fields, beaches, but also on water and on ice in frozen lakes, the same sites are chosen year on year. These loafing areas are different from night roosting sites. At loafing sites gulls appear relaxed: some sleep - head under wing - others sit or stand watching around, others preen. Immatures may play with objects such as sticks or leaves. Loafing areas are common areas, and tend to be quiet, with little or no squabbling or aggressive behaviour, often with several species mixing in the same area. Individuals come and go, commuting to their feeding areas, unless a disturbance provokes a sudden communal flight. On the coast, the number of individuals in a loafing area follows a tidal cycle - as low tides expose food resources the gulls move away the loafing area to feed- and also time of the year. Tidal influence in sleeping is highly prevalent in wading birds, which strongly depend on exposed shores for feeding. A study in Herring Gulls suggest that they have a dual sleeping pattern, with the proportion of sleeping gulls peaking at midnight and at midday. This may be a common pattern with urban gulls or away from the coast, where food is less related to tidal fluctuations.
A mixed flock of Black-headed gulls, Common Gulls and Herring gulls loafing on a rise on a local park.
Loafing gulls allow good opportunities to check individuals for plastic rings, as shown by this preening Common Gull.
A group of Common Gulls loafing on ice on a frozen pond of (16/1/2013)

More information
Shandelle M. Henson, James L. Hayward, Christina M. Burden, Clara J. Logan and Joseph G. Galusha. 2004. Predicting Dynamics of Aggregate Loafing Behavior in Glaucous-Winged Gulls (Larus glaucescens) at a Washington Colony. Auk 121, 380–390.

Cooke, F. and Ross, R. K. 1972. Diurnal and Seasonal Activities of a Post-Breeding Population of Gulls in Southeastern Ontario. Wilson Bull. 84, 164–172.

Galusha, J. G., JR and Amlaner, C. J., JR. 2008. The effects of diurnal and tidal periodicities in the numbers and activities of Herring Gulls, Larus argentatus, in a colony. Ibis 120, 322–328.

Sunday, 15 December 2019

Wintering Chiffchaffs

In the last month, I've come across a couple of wintering Chiffchaffs. The last of them this Wednesday on my way to work, the calling bird revealed it's presence atop a maple, busily feeding on the top branches. Although most UK warblers are breeding migrants, four species can be found during winter. Two of these are resident species, the Dartford and Cetti's Warblers. The situation is more complex with the Blackcap, which I've covered before, and the Chiffchaff.

Sewage plants and wetlands
Chiffchaffs are insectivorous year round, so they can't rely on garden feeding like Blackcaps during winter. Overwintering in Chiffchaffs was documented in the early 19th century, but this seem to involve very few birds. In recent decades, however, there seem to have been an increase in numbers, possibly aided by global warming. Ringing and recaptures have allowed estimates of around 100 individuals gathering in suitable sites. Chiffchaff winter survival depends on a steady supply of invertebrates (mainly midges, gnats and aphids) and sheltered sites where to roost. They appear to survive several consecutive nights with frost, so the increased temperatures may have more important effects on insect abundance, not actual survival of the birds. Suitable habitats include sheltered coastal areas, but they also use urban and suburban sites, especially near water, sewage treatment plants (where warmer water means an abundance of insects) woodlands and hedgerows. The Chiffchaff on the top shot, fed low on the marginal vegetation around an urban lake, East Park, Hull on 2/12/2013.

It's complicated...
The Chiffchaff is part of a species complex and its taxonomy has changed quite a lot in recent years. What was considered a single species now includes the Canarian and Iberian Chiffchaff, previously races now elevated to species level. The Chiffchaff proper is now divided into various races: the nominal race, collybita, breeds in the UK, whereas tristis or Siberian Chiffchaff, and Scandinavian Chiffchaff abietinus. The three races are very similar, differing in the tone of the plumage and subtle morphological features, and more notably, song and calls. Given this and the occurrence of intermediate forms between some of the species makes them very difficult to identify in the field.

Residents, migrants and winter visitors
It appears that the bulk of the UK breeding population of Chiffchaffs, belonging to the subspecies collybita, migrate south in the winter. In addition, there is very strong passage through the UK, especially in the autumn, mainly of northern European birds moving to winter quarters in the Iberian Peninsula, and Northern and Western Africa and these include a sprinkling of abietinus and tristis individuals. What about the winter birds? Where do they come from?

Ringing and DNA analysis
Ringing recaptures in the winter don't include locally breeding Chiffchaffs, suggesting that the winter birds are also migrants. Most of them appear to belong to the collybita subspecies, which breeds in western Europe and Southern Scandinavia. Recoveries of birds ringed in winter in Southern England have been made subsequently in N England and Denmank during the breeding season. Recently, DNA analysis has come to the rescue. It is possible to obtain enough material for DNA analysis from tiny feathers, dislodged when ringing birds. Sending these feathers for DNA analysis allows to discriminate between subspecies, as they have distinct DNA. Individuals captured in the south of England include some Siberian chiffchaffs confirmed by DNA. Over 20% of ringed wintering Chiffchaffs in a mixed woodland by a lough in Ireland was confirmed to be tristis and two individuals were confirmed as abietinus. So it appears that wintering Chiffchaffs include individuals from all three European subspecies, that once finding suitable locations during migration they settle for a while to feed. Ringing recaptures have found the same individuals wintering repeated years in the same location. Another intriguing feature of wintering Chiffchaffs is that males, which are larger than females, were the bulk of wintering chiffchaffs in the past, although females are increasingly wintering. There is still so much to learn on Chiffchaff migration, taxonomy and evolution.

More information
Clement, P. 1995. The Chiffchaff. Hamlyn Species Guides. pp126.
Clement, P., Helbig, A. J. and Small, B. 1998. Taxonomy and identification of chiffchaffs in the Western Palearctic. Br. Birds 91, 361–375.
Birds in Cheshire and the Wirral. A breeding and wintering atlas. Wintering Chiffchaffs
O’Mahony, B., Farrer, D. and Collinson, M. 2008. Genetic identity of wintering Common Chiffchaffs Phylloscopus collybita trapped in County Kerry in 2015. Ir. Birds 
Ray Meads Ringing Group A review of our wintering chiffchaffs.
Simms, E. 1985. British Warblers. The New Naturalist Series. Collins. 432 pp.