Carrion Crows feeding on Plane Tree fruits

This morning I took a walk near a local park. On a square lined with Plane Trees, some pollarded, a flock of Goldfinches fed on the bauble-like fruits, often hanging from them acrobatically, as they often do.

Then I noticed the crows. They flew to the tip of the branches, where the plane tree fruits clustered, once they gained their balance they moved to the tip of the branch, and picked on the fruits, pulling at their stem. Then they purposefully hanged down from their legs, then only one leg while holding the fruit on the other and attempting to break the stem. What complex thing to do! Some of the fruits had frayed stems, like they had been chewed a few times. If a crow breaks the stem but drops the fruit, the other crows will be quick to go and steal the price. I didn't see this, but I did see chases and close guarding of the fruit by the owner crow, often flying to a safer place holding onto it.

All three crows did this, often the three of them hanging from branches at the same time. At least twice I saw they were successful, and then they flew with the fruit on their bill, once transferring it to their feet before landing and starting breaking the fruit open for their seeds. I'm always amazed at crows, I have covered how they crack mussels and periwinkles at the beach.

Today's behaviour, however, really topped it up. Were they copying the Goldfinches? Crows are always to the alert, they must have noticed the Goldfinches feeding on these fruits and had a go at getting them!

This is a video made with three clips I took from them

And a few photos of the behaviour.

Hanging from one leg, holding onto the fruit with the other.

Crow with its prize.

The pink gape suggests this is a young crow, feeding on the plane fruit. I wonder if this is a family, with the young copying the adults in learning this skill.

I'll be very interested to head from anyone that have seen this behaviour in their local crows!

Herring Gull pair long call duetting

You have probably seen Gibbon pairs duetting in nature documentaries or videos. It is a territory display that shows how the pair is well bonded, and the turn-taking of the duetting is amazingly precise, the pair alternating calls to produce a single 'song' that advertises their presence and ownership of a territory. But you don't need to travel to distant tropical jungles to witness pairs duetting animals. Herring Gulls do it all the time. Because their long call is so familiar we tend not to pay much attention to it, but I was made to stop a couple of days ago when a pair landed on a chimney stack opposite my house (above). One of the pair seemed intent on long calling, and started to call, followed each time by its partner. Long calls become commonplace during the breeding season, when pairs defend territories or food, as it appears to have an aggressive function. Here is a sonogram of it. The gull starts calling with the neck pointing down, the calls well spaced, then the calling becomes faster in a staccato and the gull stretches its neck and points up and then the call dies in intensity. In 'The Herring Gull world, Tinbergen describes the call, saying that 'The voice of the herring gull is wonderfully melodious’‘There are few sounds as evocative, as stirring as the profound, plaintive beauty of their calls.' However, he missed the fact that it often occurs in pairs, there the pair message is not to each other, but to other gulls. The duetting has also been described in other gulls, like in the Western Gull.

A pair of Herring Gulls with their two young duet on a roof in Filey.

Often, one of the pair starts the call, and the other joins in and they end the long call together, necks stretched, bills pointing up.
Why do gulls long call? They seem to do it when they have asserted their ownership of something (food, for example) or finish a squabble, or sometimes the whole Herring Gull colony calls and pairs joins together. I'm surprised I've found so little about this amazing behaviour other than anecdotal references to it, but this is a wonderful photo of a pair duetting.

More information

An article on long calls in various gull species. Earbirding.

Pink-footed geese commute

One of the highlights of my bird year is to first hear and then see the wavy skeins of Pink-footed Geese flying over from their summer breeding quarters in Iceland and East Greenland.

This morning, I headed to my local park to carry out the Wetland Bird Survey. I counted the Greylag and Canada geese flock, the Common Gull flock, all recently arrived, and the resident Mallards and Moorhens. But al through, there was a beautiful background noise of calling Pink-footed geese. Flock after flock flew over, their arrival preceded by their echoing calls. Most were going north, 1500 of them.

You might be puzzled to see these geese flocks flying north or east early in the morning, how can it be that they are not going south? The reason is these birds are wintering or staging their migration in the Humber Estuary. What we are watching is their daily commute, in fact, their morning rush hour! Every night, the flocks gather to roost Read's Island and Whitton Island in the upper Humber estuary, where they are safe from disturbance and predation. At day rise, the birds fly to fields on Holderness or the wolds to feed, favouring fallow fields, or the spilt grain in those fields still to be planted, and also left-over potatoes! A similar movement happens towards Lincolnshire.

The Pink-footed geese population suffered strong declines in the 20th century, but it has been steadily increasing in the last couple of decades. The increase was followed by a renewed use of the Humber Estuary for wintering and also as a staging post on the way to Norfolk. UK coasts and estuaries now hold a sizeable amount of the total wintering population of this geese species, estimated to be about half a million birds.

With the constant daily movement, it is unavoidable that a few stragglers will separate from the flock. They call pitifully when trying to catch up. Many will eventually find the flock. Some others, which could mainly be young birds which haven't learned the route, may be disoriented and join a Greylag flock, as has happened several times in our local parks. 

More information

A Humber Goose spectacular. RSPB Blog by Pete Short.

Mapping the Pink-footed Geese.

WeBs survey results.

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! 

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)

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.

Super-wettability
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).

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!

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).