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.

December songsters

It is not unusual to hear bird song in December. Robins and Goldfinches sing year round, Wrens will burst into song at any time. Do Starlings ever stop singing? I don't think they do.  Collared Doves and Stock Doves, after a short rest at the end of their breeding season, can now be heard calling again.
 In sunny winter days, Song Thrushes will start singing, maybe more towards January.

Wren in full song in October

Collared dove singing a couple of days ago.

Starling singing yesterday

 Today I heard two bird species singing, however, which I've never heard singing at this time of year. This morning I heard distant fluting phrases that sounded like a Mistle Thrush. I went to investigate and there it was, a Mistle Thrust atop a tree, singing contentedly.

 Blackbirds, normally will sub-sing in the winter. This is an eery, very quiet song, that usually subadult individuals carry out, as if 'practising' singing, well hidden in bushes. But as I was leaving work today, as it happened last week, I heard the surreal full song of a blackbird, its beautiful notes raising over the cacophony of the Carrion Crow roost at campus, and transporting me into spring. I had to check this out. There it was a full adult male Blackbird was singing from a wall. Given that it was quite dark at 16:15, the photo on top was the best I could manage without using the flash, but I recorded a short video of it. A very very unusual thing to happen in early December. I wonder if the unseasonally mild weather we are having is confusing this normally spring songster.

The female Sparrowhawk and the 'missile' thrushes

There is a breeding pair of Mistle Thrushes in the park. Last week, one was busy finding earthworms on the lawn and carrying them neatly folded in its bill. This means they must be feeding young.  Today I heard the rattling call of annoyed Mistle thrushes. They use this call when they guard their berry trees in the winter, against Blackbirds and even much smaller birds like Chaffinches. The pair of them called and called from a tree. I tried to spot them and saw that a female Sparrowhawk was sitting on a branch. The Mistle Thrushes rattled and swooped over the hawk, the hawk ducked every time they dive-bombed her, although they didn't actually touch her.
 The female Sparrowhawk is an impressive raptor compared to the almost cute, much smaller, male. She is as large as a Woodpigeon, with broad chest and piercing yellow eyes. She can actually bring down Woodpigeons. Although the thrushes appeared fearless, they always stayed over the hawk, not underneath. The hawk moved onto an ash tree nearby, and although higher up, there were fewer leaves in the way, and after some trying I found a clear line of view and managed to take some photos and a video of the 'missile' thrushes dive bombing her. This behaviour, known as 'mobbing' involves potential prey individuals harassing or sometimes actually attacking predators when they are encountered. Mobbing is at its most intense during the breeding season. It is a behaviour that is not completely understood. One explanation, the 'move-along' hypothesis, is that mobbing diverts the predator attention from places where there are nests or young. When the Sparrowhawk's young hatches, they will be fed almost exclusively on fledglings, so harassing the predators might encourage them to move on, and hunt somewhere else.

Mistle thrush with worms in bill (14/4/2015)

The 'missile thrush' was my photo management software autocorrection, but I think it is actually quite fitting! This is an initially very shaky clip (no tree nearby to hold on). Watch out at 11 seconds.

The Sparrowhawk silence

There are three ways I spot Sparrowhawks. First, during their spring soaring flights, high over their territory, when they are very apparent then, though a bit distant. The second is when Carrion Crows alert me to their presence, when they are mobbing them, chasing them mercilessly while uttering the call that gives the name to this blog: a call that I directly translate in my mind as 'sparrowhawk!'. The third one I have got to call 'the Sparrowhawk silence'. It is an eerie silence just after the synchronic clap of many pairs of wings - Feral pigeons, Gulls - that replaces alarm calls in these birds, and the alarmed calls of Blue Tits or Blackbirds: no gull squabbles, no cooing, no chitchat. If I look up during the 'silence', I am bound to see the brown silent shadow of the hawk, flying low across the trees in the park. I have witnessed this a couple of times this week. Today, when the silence happened I was photographing a Mistle Thrush. It was feeding on a small, but exposed grassy area, but it didn't fly off. It just froze, for maybe two eternal minutes, tense, its tail pressed against the ground, just watching. Afterwards, it relaxed and carried on hunting worms as normal.

Mistle thrush collecting nesting material

The resident pair of mistle thrushes rattled while flying to the lawns in the park. One of them fed on worms for a while taking little notice of me, but then it found a feather and seemed to have a change of heart, as it started to collect nest lining material enthusiastically, and then it flew to a large horse chestnut rapidly growing its leaves.

Singing mistle thrush

The local Mistle Thrushes have started to sing in the last few days. This one surprised us on our way home, on a tree in a front garden, quietly warbling, in a tentative way reminiscent of the blackbird's subsong.

Thrushes and whitebeam berries

Whitebeams, a tree related to the rowan, is fruiting now. They have contrasting good and bad years in fruiting (masting trees), and the fruits do not last long, becoming brown and shriveled in about a month from ripening, but berries are large fruits with a lot of pulp compared to the seed content, so they are very rewarding food when available.
Woodpigeons and Crows like the fruit, and so do all thrushes, but they are too large for Robins, Blackcaps and Starlings. They are also eaten (pecked as opposed to eaten whole) or their seeds predated by Blue Tits, Chaffinches and Greenfinches.
 In my local park yesterday, there was a strong contrast between the colour of the fruits of a line of whitebeams. One of them had the brightest red berries, and Woodpigeons and Blackbirds were feeding on it. Note the bulging crop of the Blackbird in the photo above.
 Two trees at the other end of the line had still many unripe berries and as a flock of mixed finches and tits, including goldfinches, chaffinches and great and blue tits descended on it. Suddenly, it became apparent this tree had an owner. A Mistle Thrush started churring vigorously from its centre, while flicking its wings and tail. It seems to make little impact on the little birds, although it kept going for quite a while.

Woodpigeon feeding amongst the berries

Mistle thrush on guard.

And the very nervous one in this short clip.

More information
Snow, B. K., & Snow, D. (2011). Birds and berries. A&C Black. In Google Books.

Watching Mistle thrushes

A pair of Mistle Thrushes are resident in my local park. They are very used to people and can be quite approachable. I watched three of them, maybe a family group, feeding on the grass, 4-5 m away from me. One of them even came closer as it fed on worms.

Mistle thrushes sharing the turf

There is a resident pair of Mistle Thrushes in my local park. The resident male has been singing occasionally since the end of November from a high post on the tallest trees. Mistle Thrushes' song is reminiscent of the Blackbird's but with shorter, more repetitive, mournful phrases that carry quite far. Up to five birds gathered on the grass before Christmas, and when the snow arrived the locals birds had to share the few clear patches of turf under the trees with a flock of immigrant Redwings and a lone Fieldfare in search of earthworms and other invertebrates. Mistle Thrushes are large and bossy, and if the other thrushes get too close, they are chased away without trouble. Resident birds have a head start in the breeding season. They know their patch and as soon as conditions are right they can start nesting right away.
 The male was singing again in the last few days without snow, while the female fed on the grass.

Mistle Thrush and Fieldfare

Mistle Thrush, Redwing (back) and Fieldfare (foreground)

The pair feeding on the snow

Male singing yesterday

The Yew Thrush

Mistle Thrush on a Yew

A closer shot of a Mistle Thrush

The Yew tree is laden with bright red, soft berries at the end of a church garden. On top of it is a bird, with a ratchety voice, guarding the precious sweet load, a Mistle thrush. Individual Mistle Thrushes become territorial during the winter months and defend their tree of intruders, either conspecifics or other thrush species. They eat their berries economically, feeding on the ground when possible, and leaving the berries for frosty days. The tree, thus, retains its bright berries for most of the winter. Only in very cold winters, will the Mistle Thrush be unable to enforce his possessions and must relent to the flocks of other hungry thrushes that want to feed. They are seen to defend Ivy berries, Mistletoe and, preferentially Holly trees.

Yew Berries. The only non-toxic part of the tree is the red aril surrounding the seeds, which thrushes help disperse.

More information

Barbara Snow & David Snow (1982) Long-term defence of fruit in Mistle Thrushes, Turdus viscivorus.  Ibis, 126: 39–49. doi: 10.1111/j.1474-919X.1984.tb03662.x