Sunday 26 November 2023

A flock of local Lesser Redpolls

The coldest morning of the year so far, I take a walk around my patch. Lesser Redpolls are a rare winter visitor in the city, and it's hard to predict where to find them but my local patch has been quite reliable every year and cold weather pushes them south. Today I encountered a small flock quietly feeding on catkins in some small birches in the cemetery. There were at least five, one of them, a male. They allowed me to approach and watch them hanging from the thinnest branches, holding a birch catkin against the branch with a foot while feeding from it so that they can extract the seeds more efficiently.

Adult male Lesser Redpoll.

Here you can see how they hold catkins while they feed.

A female/immature feeding.
A view of the flock. From a distance, very hard to see!

They kept feeding, until a sudden Blackbird alarm flushed them all but one. The lone individual called, and left flying towards the flock.

Monday 9 October 2023

Rooks hoarding acorns

It is a mast year, with a bumper crop of acorns across the city. October is peak acorn season and last week I saw the first Rooks carrying acorns. Acorns are packed with energy, and several bird species take advantage of them. Woodpigeons, presumably swallow them whole from the trees, but Rooks and Jays collect the harvest and store it for use during the winter. Both species are scatter hoarders, and cache individual acorns on the ground, and use their extraordinary spatial memory to recover them later in the year, when other food resources are scarce. Although the Jays are best known for this behaviour, Rooks are also amazing acorn hoarders. Rooks can transport acorns - and other food items - in a pouch under their tongue, which obviously bulges as they fly over with their pouch full. The number of acorns they can carry depends on the acorn size, and varies from 2 to 7. They prefer to cache the acorns on grass, and can fly up to 4 km from oaks to suitable grassland. Once they find a good spot, they drop all the acorns they are carrying and bury each one by one, by first making a hole in the ground with their bills, and then hammering the acorn in and covering it with grass, leaves or soil. Later on, they will visit the caching sites in the winter, find their stored acorns and crack them open to feed. 

A vocal Rook on an oak canopy, surrounded by plenty of acorns.

Rooks, unlike Jays, are very social and engage in communal acorn collecting, becoming very vocal when landing on the oaks canopy. They prefer to gather acorns with other Rooks, and individuals appear to join other individuals gathering acorns by flying in the opposite direction of individuals with full bills. When it comes to caching though, Rooks prefer to be alone, to avoid cleptoparasism, when other individuals try and steal their stored acorns.

Rooks displaying, the individual on the right, with distended sublingual pouch, passed an acorn to one on the left, presumably they are a mated pair.

More information

Waite, D. R. K. Food caching and recovery by farmland corvids. Bird Study 32, 45–49 (1985)

Källander, H., 2007. Food hoarding and use of stored food by rooks Corvus frugilegus. Bird Study, 54(2), pp.192-198.

Friday 24 March 2023

Siskins Galore


There is something about birdwatching that I find so rewarding: learning a simple thing, like a species contact call, opens a new world. Although they are regular wintering birds, I don't think Siskins are becoming more common around Hull. It is just that once I learned the beautiful soft, sad 'pew! call, then I am seeing them everywhere, and this year I have really enjoyed watching Siskins. 

Siskin feeding on Italian Alder this afternoon.

In the depths of the winter, when days are gloomy and cold, Siskin flocks rove around the city looking for Alders. They feed on both Common and Italian Alders, clinging from the cones sometimes upside down, deftly extracting seeds with their pointy bills. In quiet areas they might come down and feed on seeds on the ground.

In the last few weeks, the chattering singing chorus of Siskins have alerted me to flocks feeding in Lombardy Poplars in three different locations around the city, possibly as the seeds in Alders are becoming depleted. I was intrigued as I thought these catkins were pollen catkins. Lombardy Poplars are male clones, which produce red pollen catkins. Being wind pollinated I didn't think they had nectar (which would be odd for a finch to eat anyway), but I've never heard of a bird feeding on pollen. What are they feeding on then? Insects, possibly aphids, is a possibility, but it appears unlikely so early in the year.

Male Siskin feeding on Lombardy poplar catkins.

So, a bit like the Goldfinches feeding on lichen covered branches, I don't have an answer to what Siskins are feeding on, but Lombardy Poplars are plentiful around the city parks and playing fields, where they are planted as wind breakers, so hopefully they will stay around a little longer.

Please do let me know in the comments if you know the answer to the mystery!

Sunday 19 February 2023

Waxwing gift passing display

 After a long hiatus, I came across some Waxwings today. Six individuals were perched high up on a Lime tree, taking the sun in. They were quite active, changing branches, pecking the shiny, red tree buds. Two in particular called my attention, as they approached each other touching bills, then coming apart. I took plenty of photos, some unfocused by branches getting in the way, but, looking at them in more detail back at home, I noticed that the two individuals 'billing' were doing more than that, they were exchanging an object, probably one of the tree buds, in a ritual display called 'gift passing' (top shot). The behaviour is seen both in the Bohemian Waxwing, which we get in irruption years in the UK, and in the American Cedar Waxwing. 

The behaviour consist on passing an object, then the individual with the object jumps away, then joins its mate, which 'receives' the object, then the behaviour is repeated and the object passed backwards and forwards repeatedly.

The individual on the left has the gift now.
The individual on the right has it now. Note the raised crests.
Here a Waxwing shows interest in the bright red, shiny buds of the lime tree.
And here, another individual holds some buds in its bill.

I found a description of the behaviour, based on captive individuals and illustrated with photos, which shows that the display is also accompanied from fluffed rump and belly feathers, raised crest and lowered tail, the latest something not very obvious in my photos. The male appears to initiate the behaviour, obtaining the object, and sidling to the female, presenting it. If successful she will accept the object and reciprocate, after jumping away, then close. It is difficult to sex Waxwings, males apparently have longer crests and a more clearly delimited black chin patch.

This is a clip of the behaviour in the Cedar Waxwing, there seems to be a lack of recordings for the Bohemian Waxwing, although the description and my observations fits both species sharing this wonderful display.

More Information
Meaden, F. M. & Harrison, C. J. O. Courtship display in the Waxwing. British Birds 58, 206–208 (1965)

Sunday 4 September 2022

Goldfinches feeding on Horse Chestnut leaf miners

Goldfinches' diet is mainly composed of small seeds that they can deftly extract from seed heads using their long, thin bill. I have blogged before on their agility and skill manipulating seeds and stems with their feet.  Loose flocks will move about in search of food. In spring they feed on dandelions, later in the summer on teasels and creeping thistles, whilst in the winter they feed on tree seeds including alder, birch and ash. They are an adaptable species, and have taken to feeding on garden feeders, especially on niger seeds and sunflower seeds. They have also learned to feed on the dangling fruits of plane trees, planted in city avenues. They are also seed predators of Rowan trees, feeding on the seeds and discarding the pulp of the rowan berries. Finally, in the winter they spend a lot of time pecking tree branches, and it is still a mystery to me what they are doing in there!

A Blue Tit feeding on Horse Chestnut miners. Several tit species are able to use this plentiful resource.

In a visit to my local cemetery I wasn't totally surprised when I found a group of Goldfinches feeding on leaf miners amongst Blue Tits. Using their feet to hold onto a leaf, they were extracting the miners, presumably pupae given the time of year (note the brown blotches on the leaves on the photos). The horse chestnut leaf miner, Cameraria ohridella, is a micromoth that has expanded its range throughout Europe from its native Balkans and is now widespread in cities and towns across Europe, responsible from the early browning of Horse Chestnut leaves in summer.

This is the first time I see Goldfinches feeding on invertebrates, but they are already known to be one of the few predators of this newly arrived species of moth (see Ralph Hancock's blog here). In a previous study, only tits, including Blue Tits, Great Tits and Marsh Tits were fed on the late larval or pupal stage of the miner. The bird species known to predate the miner are all small, dextrous and agile birds, and able to use their feet to hold onto leaves. This food resource is plentiful for a few months, as there are several generations of miner per year, so the miner provides a great resource of food in the summer months. The adult moth is also a source for food for other insect-eating birds.

More information

Grabenweger, G., Kehrli, P., Schlick‐Steiner, B., Steiner, F., Stolz, M., & Bacher, S. (2005). Predator complex of the horse chestnut leafminer Cameraria ohridella: identification and impact assessment. Journal of Applied Entomology, 129(7), 353-362.

Saturday 4 December 2021

Mistle thrush pairs defending berry trees

Several bird species hold territories during the winter, like Robins or Wrens defending their patch and singing through the winter. It is rarer that a pair will defend a winter territory. I have covered examples like Stonechats and Pied Wagtails before. The former usually include a male and a female, but they are unlikely to be a mated pair.

A pair of Mistle Thrushes near their cotoneaster tree. Northern Cemetery 30/11/21.

Earlier in the week I saw some Rowans heavy with berries along a street. A Mistle Thrush sat on a taller tree nearby. Then another individual flew from the berry tree, a pair! Later in my walk, I saw another pair of Mistle Thrushes near their defended cotoneaster (above). I then realised seen paired Mistle Thrushes in the winter before, near their defended trees. Are they a resident pair? Is a pair better than a single individual defending their berry larders?

Clockwise from top left, Yew, Ivy, Rowan and Holly ripe berries.

The berries

Mistle Thrushes are well known for their winter defence of berry resources. Holly is one of the most important defended trees in the UK, followed by Hawthorn, Mistletoe, Yew and, later in the winter, Ivy. At Hull, I've seen Mistle Thrushes defend Holly, Yew, Rowan, Whitebeam and Cotoneaster. Often the same trees are defended year after year.

Mistle Thrush on defended holly, one of an Avenues of Hollies at East Park defended by a pair, 23rd November 2021.
Mistle Thrush overlooking its defenced Rowan, 1st February 2021.
Mistle Thrush on General Cemetery on its defenced Whitebeam at Pearson Park. 17th October 2014.
Mistle Thrush on its defended berry-laden Yew.
A Mistle Thrush atop the church rooftop checks the yew opposite, on Cottingham Road Community Church garden. 14th November 2017.

Defence behaviour

Aggressive tree defence behaviour is spectacular: The Mistle Thrushes use their loud rattling or churring calls, repeatedly and chases other birds away from their tree. Most aggressive interactions involve repelling other Mistle thrushes, but also any other thrushes, like Blackbird, and various finches, Great Spotted Woodpeckers, Waxwings and even single Woodpigeons. Defence consists on chasing intruders, often silently and even if the birds were feeding on fallen berries under the tree. If there is a conflict then the individual might flick its tail and wings, their white underwings flashing, while churring persistently. Churring also occurs in flight during chases. Their defence is effective due to their large body size - they are the largest European thrush. Curiously Barbara and David Snow, who studied this behaviour in the UK note how the Mistle Thrush chooses who to attack carefully and it will chase only some species away so long as they are food competitors for the particular tree it is defending. For example Starlings are left alone in hawthorn and holly (whose berries they rarely eat) but chased from Ivy, which they eat. Greenfinches and Great Tits, are chased from Yew (they feed on the seeds) but not from Holly, which they don't eat. Eventually, after a few attacks, the local birds will learn to avoid the defended trees, and the Mistle Thrush just watches from a distance, and often this is how you'll find a Mistle Thrush, sitting on a vantage point near its precious tree.

Establishment of the guarded tree

Snow and Snow describe that the system of tree defence is established in October or early November, when song might be occasionally heard and other Mistle Thrushes are vigorously driven off until the territory is set. That way they can exclusively feed on these berries through the winter when they are needed. The defended trees become their winter larders, standing out on the landscape, packed full of berries when all other trees have been stripped out. Mistle Thrushes actually avoid feeding on the berries of their defended tree if they can, especially when the weather is mild and the ground is soft and they can rely on worms and other invertebrates. You need to pay attention to these trees full of berries later in the winter to recognise that the reason they are there is that a Mistle Thrush is just sitting atop a tree or aerial nearby, preening or resting. Unfortunately this effort will be worth nothing if a hard weather spell drives flocks of winter thrushes or Waxwings onto their area, then the Mistle Thrushes despite their best efforts may be unable to maintain their tree defence and lose their berry crop in a day or two. If the weather is mild, the tree can hold its berries until spring, and then, usually by the end of March, trees stop being defended. The Mistle Thrushes start nest building, often nearby. In the case of their defence of holly, the long-lasting berries, which can remain in the tree for up to nine months, are often used to feed their young. The hard work of defending the tree through the winter pays off not only to increase adult survival but as parental investment.

A pair of Mistle Thrushes near berry laden rowans at Auckland Avenue, 14th November 2017.

Pair defence

If Mistle thrushes stay to breed on the area near their defended tree, it would make sense that the pair defends the tree, rather than one individual. Snow and Snow describe how pair defence is more common and successful than single individual defence, although no data is given as to the proportion of pair defence. They say that "nearly all trees successfully defended throughout the winter were jointly defended by a pair of Mistle thrushes" and that  “pairs or individuals centred on a source of fruit which, if all goes well, will be defended through the coming winter.” In contrast, research on Mistle Thrushes defending Mistletoe clumps in woodland in Poland did not find evidence of pair defence. Could it be that pair defence occurs in some situations or for some berry sources and not for others? Anecdotal observations, however, suggest that members of a pair can defend different trees in the same area, and join forces if one of the pair loses their tree. On their article on Mistle Thrush defence Barbara and David Snow comment:

"When fruit territories are first established, in autumn, each defended tree may be guarded by a single Mistle Thrush or by a pair. It is probably common for members of a pair to establish themselves at neighbouring fruit-trees. Thus we had three cases where two Mistle Thrushes began by defending separate trees quite near to one another (60-180 m) and later, when one of the trees was stripped (as a result of the onslaught by Fieldfares and Redwings in the severe weather of December 1981), the two jointly defended the tree that still had fruit. In another case a pair of Mistle Thrushes jointly guarded the same holly tree from October or November in the two successive years, continuing throughout the winter in the first year but losing the fruit to invading Fieldfares and Redwings in the second year."
Snow and Snow indeed suggests that the same individuals, if they survive, may defend the same trees year after year, but their birds weren't ringed, so we can't know for sure. I'm surprised there is still so much to known about this widespread bird!

More information
Skórka, P. & Wójcik, J. D. Population Dynamics and Social Behavior of the Mistle Thrush Turdus viscivorus During Winter. Acta Ornithol. 40, 35–42 (2005).

Snow, B. K. & Snow, D. W. Long-term defence of fruit by Mistle Thrushes Turdus viscivorus. Ibis 126, 39–49 (2008).

Snow, B., & Snow, D. (2010). Birds and berries. A&C Black.

Wednesday 31 March 2021

Anting Carrion Crows

I've wanted to film crows anting for quite a while. Anting is a behaviour carried out by a range of birds (I dealt with Blackbirds anting in a previous post), but it is rare to observe it. Anting can be active - the birds picks an ant (or ants) and rubs it on its feathers - or passive - the bird allows the ants to climb onto its plumage. The behaviour involves distinctive movements such as sitting close to the ground or partially opening the wings and tail dragging. The bird movements on top of the ant's nest or direct disturbing of the nest will also encouraging the ants to come out of the nest and climb over their feathers, as anyone who has had a picnic on an anthill will attest.

A Carrion Crow anting at my local park. 27/07/2016.

 The first time I watched anting behaviour in crows I was driving, waiting on a red light. It is a very distinctive behaviour, but I had no chance for a video or a photo! I have seen it a few more times, but didn't get a good video of it. Today, as I got out of my car I noticed the local pair of crows on the verge opposite. I used the car as a hide and filmed them. First, one of them first seemed intent in getting some nest lining material, but the other went to the base of a tree trunk and pecked, and pulled some vegetation from around the tree. Then it pressed its body against the ground. The second crow joined the first and both enjoyed an 'anting' session.

Watch both clips and a clip of the ants nest:

Why anting?
It might be surprising, but is still unclear why birds carry out anting behaviour and there are a range of hypotheses. Some claim that it is some form of feather maintenance. Species of ants that eject formic acid (Formica, Lasius and Camponotus) as part of nest defence are preferred for anting. Formic acid has insecticidal and acaricidal properties. One of the first hypothesis is that the formic acid kills or repels bird ectoparasites such as feather lice or mites. This has had some support. In the summer of 1943, in Transbaikalia, Dubinin killed four Blyth's Pipits that had been anting, counted all the feather mites and checked the mites behaviour. Many mites were already dead and a total of a third of the mites died over the next 12 h, and the rest seemed very mobile. The birds feather smelled very strongly of formic acid even 12 h after the shooting. The control for this experiment were four pipits that hadn't been anting. Only 0.9% of the mites died after 24 h, and they stayed more or less immobile. However, other experiments tested this hypothesis were inconclusive.
Another hypothesis draws on the formic acid has anti-bacterial and anti-fungal properties, and could be beneficial to keep in check bacteria and fungi that feed on feathers. Although the experiment showed that pure formic acid inhibits feather bacteria and fungi's growth, the results were not conclusive when more natural doses were used that reflected ant's formic acid content. An ornithological enigma!

More information

Morozov, N. S. Why do birds practice anting? Biology Bulletin Reviews 5, 353–365 (2015).