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.

Spotting the Sparrowhawk

The pigeon flock in the park took off all of a sudden, like a loud clap. In the silence that followed, I looked up: The male Sparrowhawk flew into the tree tops, I managed to see where it landed, high up in a sycamore. The leaves made it tricky to have a clear view, but I managed a few shots. When I find a predator by interpreting the behaviour of their prey I get a satisfying 'Dr. Dolittle moment'. The rattling call of the crow that gives this blog its name, or the alarm call of the blue tit have pointed me to many a Sparrowhawk. The Feral Pigeons are probably safer in flight than sitting on the cafe roof when a Sparrowhawk is out hunting, so they take off as soon as they see the raptor in flight.

Redwing and berries

This lone Redwing feeding on cotoneaster berries in the park did not lose track of the sky above. When feeding alone, birds have to keep watch more often than when in flocks.

Treecreepers and mixed flocks

As winter sets in, small, resident insectivorous birds including Long-tailed tits, Great Tits and Blue Tits, Coal Tits, Goldcrests and Treecreepers join in loose, vocal mixed-species flocks that travel and forage together. Why do they eschew from following the saying 'birds of a feather flock together'? Well, first, their small bodies lose heat easily and the days are short, so they need to obtain as much food as possible. On the other hand, the leafless trees makes them more exposed to predation, so, as they feed, they need to be vigilant, keeping an eye on predators and an eye on food, so to speak. Does flocking together help with this challenge: increasing foraging efficiency and reducing predation risk?
 Birds of different species can learn from each other while foraging together: where to find food and how to obtain it. Many of the birds in the mixed species flocks are young of the year, with much to learn. Also, birds spend less time watching for predators while in flocks than when solitary, which allows them to increase the time they can spend looking for food. Despite being 'in a crowd', Individuals in a flock made up of different species may be able to compete less amongst themselves than if they were in a single species flock, as different species often search for food in different microhabitats in trees and bushes. In these mixed flocks, there is a pecking order, with some species dominant, and others subordinate. In Wytham Wood, Oxford, the pecking order was from the more dominant, larger Great Tit, Blue Tit, Marsh Tit, Coal Tit, Long-tailed Tit, and finally the Goldcrest.
  Importantly, the more individuals in a flock, the more eyes and ears available to detect predators. There are similarities in the alarm calls of different species which can allow them to respond appropriately to each others calls.
 Treecreepers are specialists, exploiting the invertebrates found in the little nooks and crannies of tree trunks and thick branches, which they probe with their thin, curved bill while creeping up the trunk aided by their phenomenally curved nails and stiff tail, woodpecker style. This niche overlaps little with other members of the species flocks, and so, they are not expected to benefit from learning from other species or finding new food sources. What do they have to gain from joining mixed species flocks? Arevalo and Gosler investigated the behaviour of Treecreepers joining species flocks in Wytham Wood during two winters. When they found a treecreeper, they noted if if was part of a flock, and if it was, the size of the flock. They also recorded the number of hops and pecks to calculate the percentage of time the bird was looking for food.
 Treecrepers joined mixed flocks more often when the temperatures were lower, so all Treecreepers were found in mixed flocks in the coldest winter months, when flocks were also larger. When part of a flock, Treecreepers were most often found in the main trunk, where competition with other species was minimal, while when solitary, they used internal branches more, so they reduced their niche when in the flock.
Their key result is that the pecking rate was positively correlated with flock size, - indicating that treecreepers could devote more time to searching for food the larger the flock it was in. The pecking rate was not affected by temperature or by the species of tree. This suggests that treecreeper in mixed flocks reduced vigilance, concentrating their effort in foraging.
Overall, this research showed that Treecreepers make a decision to join a mixed flock based on the benefits and costs they are going to obtain. The colder the temperature, the more effort they need to make searching for food, and joining a flock allows them to use a larger proportion of their time feeding, as opposed to being vigilant. At higher temperatures the treecreeper could afford to spend more time keeping an eye on predators, and use a wider niche. The treecreeper, being very cryptic compared to the other species in the flock, might benefit directly from joining the noisy, more visible tits, if they are also less likely to be noticed by a potential predator finding the flock. Arevalo and Gosler also noted:

Treecreepers in flocks were seen to respond to the alarm calls of Long-tailed Tit and Coal Tit during Sparrowhawk attacks by 'freezing' and only resuming foraging after the other species had done so. Play-back experiments with Treecreepers have also shown that they can use information from other species within the flocks in which they usually participate.

You can see this short clip I took a couple of days ago of a Treecreeper showing this 'freezing' behaviour, when a Magpie was nearby. The treecreeper was not part of a flock.




More information

Morse, D. H. (1978). Structure and foraging patterns of flocks of tits and associated species in an English woodland during the winter. Ibis, 120(3), 298-312.

Morse, D. H. (1970). Ecological aspects of some mixed-species foraging flocks of birds. Ecological Monographs, 119-168.

J. E. Arévalo and A. G. Gosler (1994). The behaviour of Treecreepers Certhia familiaris in mixed-species flocks in winter. Bird Study, 41 (1), 1-6 : 10.1080/00063659409477190

Blackbird alarm calls

For a long time, I have been fascinated with the rich vocal repertoire of Blackbirds. I have recorded their alarm calls and what do they appear to react to, and did some research to find out what was known about them. Here I have compiled what I have found so far in my attempt to understand 'blackbirdese'. I have excluded male song and chick and fledgling begging calls from the post. Given how difficult is to transcribe bird sounds, I have illustrated each call with embedded sonograms and clips from the website Xeno-canto, which contains a fantastic collection of bird calls from around the world, published under Creative Commons licences.

Chink, chink, chink, chink, chink!'
A mobbing call given to owls, kestrel, magpie, carrion crow (and ornithologists checking nests!) which may be followed by attacks. The blackbird is exposed, agitated, flicks tail and wings - which are kept dangling -, from an obvious post (top shot and above) facing the potential predator, very nervous sounding and calling repeatedly and monotonously for a long time. It indicates a moderately aggressive tendency with a low escape tendency, although somewhat inhibited from attacking. Mobbing might be a way of cultural transmission of enemy recognition. Inexperience birds exposed to conspecifics mobbing a predator will join in the mobbing and mob the predator themselves in the future.

Alarm call before roosting or 'chinking'
Territory holding males give a persistent 'chink-chink' calls in the evening, possibly to deter other Blackbirds from roosting in its territory. Other territory holders might join in 'chinking'. This is interesting as it is very similar if not identical to the mobbing call. Is this a dishonest signal 'don't roost here, there are predators around'? or it is an assertion of territorial ownership with a general meaning 'move on'?

Seeee!
The bird sits tense, still, with feathers flat on the body and head. In response to crow and sparrowhawk, whether they are flying or just nearby. Slow, well spaced and high-pitched with open beak (above). Many other birds have a similar thin, high pitched alarm call for threats from aerial predators since the features of this sound makes the calling bird harder to locate, therefore the bird calling does not endangering itself. It has also been called 'hawk alarm', but it is also uttered in response to crows flying overhead or when the nest is threatened. Nestlings react to this call by becoming quiet and still. In experiments using magpie dummies, the parents uttered this call when the dummy was 6-7 m from the nest, while they used the mobbing call when the dummy was very close to the nest. Interestingly, there appear to be differences between urban and rural blackbirds use of this call, as D.W. Snow reported that woodland birds use this call also to humans (the one in the photo above is a woodland bird, so maybe was reacting to our presence), and then it indicates that the nest is very close.

Alarm rattle
A loud, sudden and accelerating outburst, ending on a noisy scream, with the bird flying away. Alarm call when the bird is suddenly startled, also during fights. May starts when the bird is perched but finished in flight. If this call has the same effect on a predator as if an observer disturbs a blackbird at close quarters - it has made me jump more than once - it might give the calling bird a few moments advantage to flee from danger.


Sriii
Also called 'trill' call. A flight or fight intention call. Perched and in flight. It can serve as an appeasement call by a subordinate bird indicating its intention to flee.



Soft call
Also known as 'pok' or 'pook' call, sounds like a soft bark, to me more like 'wow'. Usually from a tree, still or in flight. It is an alarm call to indicate the presence of ground predators, which in gardens usually means the presence of a cat, or a human approaching young or the nest. Fledglings respond to this call immediately by keeping silent and still and looking around, and especially below them. D.W. Snow used a playback of this call to a few day old nestlings reared by him and they acted in the same way.


Chooking
Low pitched, uttered with the beak closed. Anxiety call, mild alarm. Sometimes on its own, sometimes accelerating to the full-swing alarm call in flight. Flicks tail, horizontal body. To people, dogs, cats, etc.  Females use it when disturbed while looking for nest site or nest building. Also used when the bird is foraging in an unfamiliar situation where the bird feels insecure.



Chook, chook, chink, chink, chink
A variant in which the chook combines with chinking as the bird becomes more aggressive or excited.

More information

Snow, D. W. (1988). A study of blackbirds. British Museum, Natural History.

Kryštofková, M., Haas, M., & Exnerová, A. (2011). Nest defense in blackbirds Turdus merula: effect of predator distance and parental sex. Acta Ornithologica, 46(1), 55-63.

Alarmed wrens

A pair of Wrens in the wildlife garden called their rattling alarm calls. I am always intrigued by alarm calls. Many alarm calls are directed to the bird watcher, as we approach a nest or a bird more than it is comfortable with. In this case, the birds were directing their attention to a cat, which was going about its business in the undergrowth. The pair of wrens moved constantly around, chattering nervously. A Goldfinch sat on a branch paying a lot of attention, but keeping quiet. On the top is my best shot of one of the Wrens, and you can see a little video here.

The bird with the silvery eyes

In his most amusing book King Solomon's Ring, Konrad Lorenz described his experiences rearing a veritable menagerie of different animals including jewel cichlids, water shrews, a raven, cockatoos geese and ducks. His intention was to develop an intimate relationship with them and understand their behaviour and communication. He reared the birds himself then set them free. This allowed him to have very tame animals that could be observed from close quarters. The longest chapter is devoted to his colony of Jackdaws (Coloeus monedula), which he started by hand-rearing more than a dozen of them. He ringed his Jackdaws with colour and metallic rings and named them accordingly to their ring colours. This chapter is a vivid account of Jackdaw behaviour. Lorenz was of the opinion that you had to get under the animal's skin to understand their actions, and his fascinating account shows that he got very close to feeling like a Jackdaw and he was definitely treated by one with a male that adopted him as his partner and insisted in feeding him chewed worms.

Flying together
Jackdaws are very sociable corvids, they like to fly in tight flocks. The members flock feed together and breed together in loose colonies in natural holes in cliffs and old trees and in buildings commonly using chimney pots. He described the "kia" call as equivalent to "fly away with me" and a longer version he learned to distinguish "kiaw" as "fly back home with me". His first jackdaw, Jock found dark birds flying away from him irresistible, and would often pursue a Hooded Crow away in his walks with Lorenz. Adults use this instinct to entice young to fly with them and will keep a close eye "looking over their shoulders" as they do so, to make sure the young can follow. Lorenz noticed how adults adjusted their flight and made it slower when fledglings were flying with them, making it easier for the young to follow.

The rattling reaction
One of the most surprising discoveries made by Lorenz was the instinctive basis of the rattling reaction and how . He found by chance - by taking his black swimming trunks from his pocket - that a dark floppy object held by something was a very strong signal that started a furious attack. His tame jackdaw attacked his hand drawing blood. The pecking was accompanied by loud rattling calls. In other occasions - such as when he intended to ring the young jackdaws in his colony, he observed that holding the feathered young will draws the members of the flock together into a rattling reaction. This presumably has the function that a predator that catches a jackdaw receives such a severe punishment. The memory of the offender is imprinted into all the members of the flock participating in the rattling reaction and will persist for years and form the basis of local traditions that will be passed down generations. As the rattling reaction was elicited when Lorenz held feathered young for ringing, he decided to dress up with the only outfit he had at hand - a devil costume -  and climb onto his roof to ring the young jackdaws. A sight that shocked some of the people of his village one day.

A pecking order
The jackdaw flock has a precise stable hierarchy. There is a pecking order in which "very high caste jackdaws are most condescending to those of lowest degree and consider them merely as the dust between their feet" according to Lorenz. Males, which are larger than females are higher in the rank. High ranking jackdaws always takes the weaker side when intervening on disputes, which helps maintaining the cohesiveness of the colony. The way Jackdaws assert their dominance is by raising the neck pale feathers.

A pair of jackdaws basking in the early morning sun

Lifelong partners
Jackdaws are socially monogamous and pair for life. Pairs form on the individuals' first spring, although they are unlikely to breed until their second spring due to a shortage of nesting sites. The partners in a jackdaw pair are together all year round and support each other loyaly in any dispute. The rank of a new female partner automatically rises correspondingly in the female's rank order, a refinement of Lorenz discoveries explained in the later monograph on Jackdaws by A Roell. Lorenz said he was surprised how quickly all jackdaws in the colony knew of the rise in rank of the new partner of the recently arrived alpha male. But not only that, the female also knew that now she stood over everybody else! The male will show the female a nest site by calling her with "zink" calls. The bond between the pair is shown by allopreening, usually by the female which will preen her partner's neck, particularly his silvery feathers. The female also will beg for food and the male will courtship feed her as he will in time feed the young. Both male and female share the building and defence of the nest and the rearing of the young but only the female incubates. During incubation, the male will feed the female.

The few pairs of local Jackdaws now sitting on their nests remind me of Lorenz entertaining book.

More information
Konrad Lorenz (1954). King Solomon's ring: new light on animal ways Methuen, London Other: 0415267471

Röell, A. (1978). Social Behaviour of the Jackdaw, Corvus monedula, in Relation To Its Niche Behaviour, 64 (1), 1-122 DOI: 10.1163/156853978X00459

Extraordinary feral pigeons

When I make bird lists I do not often tick feral pigeons (Columba livia). They are always there, but I like to take photos of their behaviour.

The local flock sitting on the roof, waiting for the sun to rise in the morning,

 the flock scared by a movement or noise, suddenly taking off and flying in a few elegant circles before settling back; a few of them feeding on the bread that some old lady dutifully brings to the same corner of the street each morning; males in display flight holding their wings high in a V pattern and gliding in a circle.

Their leisure times, having a bath...

...or enjoying the sun on a cold day

Despite their ubiquity, feral pigeons are still extraordinary birds. They are descendants from the Rock Dove, a wild bird of river canyons and sea cliffs, domesticated millennia ago. Some have now have returned to the wild, although still quite attached to humans. Some feral pigeons (above) have very similar plumage patters to their wild ancestor: bluish grey, with a double black wing bars and iridescent neck. Many also keep a trademark white patch in their rumps (like the one bathing on the photo above).

Charles Darwin extensively bred many domestic pigeon breeds and dedicated a sizeable section of the first chapter of the Origin of Species to domestic pigeons. He used them to illustrate how domestication had generated distinct races through breeding together animals with certain preferred variations, producing a wide array of diverse shapes, colours, plumages, flight ability and voice from the original ancestral species, the wild Rock Dove. He stressed that this "selection by man" was essentially the same mechanism that his "natural selection".

 Male pigeons court females by cooing and bowing to them while walking alongside her with puffed feathers, inflated necks and fanned tails, pirouetting occasionally. The courting male, on the left, is also showing his white rump patch.

 A quick copulation may ensue...

 As other pigeons, Feral Pigeons lay two eggs per clutch. They nest on ledges inside abandoned buildings, inside roof spaces where they can gain access, or outside, under bridges. Fledged squabs, the young birds (below), have dark eyes, underdeveloped operculum - the white swelling a the base of the beak - and have a look of naivety about them. Both siblings often move about together.

Although mortality, even in town, is high. According to research by Alberto Pelleroni and colleagues, the wild-type blue plumage with a white rump affords a survival advantage to feral pigeons, as it decreases the chances of success in Peregrine Falcon attacks. When pigeons with and without the white rump patch had their feathers swapped, the success of the Peregrines reversed accordingly. The presence of Peregrines in the area increased the frequency of the feral pigeon wild type in the area. As for the possible reason for the increased failure of Peregrines in capturing wild-type feral pigeons:

All feral pigeons perform the same evasive roll during predation by falcons. The protective white patch may disguise the initiation of the pigeon’s evasive roll by contrasting conspicuous (white patch) and cryptic targets (grey wings and body). A fast flying falcon primed to a conspicuous target centered on the roll might fail to detect the dodge initiated by the cryptic wings as the predator closes from behind.

 I wonder who ate this squab? Sorry if you are squeamish!

Never underestimate pigeons. They have been found to outperform university students in a range of probability calculation tests (see this post for an example). According to Walter T. Herbranson and Julia Schroeder:

Pigeons might not possess the cognitive framework for a classical probability-based analysis of a complicated problem [...], but it is certainly not far-fetched to suppose that pigeons can accumulate empirical probabilities by observing the outcomes of numerous trials and adjusting their subsequent behavior accordingly.

Update: see this post on pigeon convergence


References

Herbranson, W. & Schroeder, J. (2010). Are birds smarter than mathematicians? Pigeons (Columba livia) perform optimally on a version of the Monty Hall Dilemma. Journal of Comparative Psychology, 124 (1), 1-13 DOI: 10.1037/a0017703

Palleroni, A., Miller, C., Hauser, M. & Marler, P. (2005). Predation: Prey plumage adaptation against falcon attack Nature, 434 (7036), 973-974 DOI: 10.1038/434973b

Great Tit nestlings respond adaptively to different antipredator alarm calls

Young birds are vulnerable. While they are still in the nest they can easily fall prey to cats, snakes or predatory birds, and once they leave it they are still naive and clumsy and they can also be easy prey. Parents, however, can help: they are experienced and know what other animals represent a risk, and they could make a difference by communicating this to their young offspring by using alarm calls. In a recent paper Toshitaka Suzuki showed how Great Tits (above, a fledgling) produce different alarm calls depending of which predator approaches their nest, and how, crucially, nestlings use this information to behave in the most effective way to avoid predation. The main predators for Great Tits in Japan are the Japanese Rat Snake species and the Jungle Crow. Snakes fit into the cavities Great Tits use and can then kill the chicks, while crows can only snatch chicks approaching the nest entrance. Suzuki presented nesting Great Tits with either a stuffed jungle crow (11 nests) or a live snake (10 nests) in a transparent plastic box, and the parents readily responded to the predators with repeated alarm calls.

In response to a crow, they continually gave ‘chicka’ alarm calls that were composed of several different types of syllables, but these calls were rarely produced in the snake trials. Instead, when detecting a snake, parents produced ‘jar’ alarm calls that were composed of harsh syllables. Such ‘jar’ alarm calls were repeatedly given in response to the snake, but were never uttered for the crow.

He recorded the nestlings responses to the parents' alarm call using video cameras set inside the nest. When the nestlings heard the 'chicka' call, they crouched inside the nest, making less likely that a crow would be able to reach them from the entrance hole. In contrast, upon hearing the 'jar' call, all nestlings in the 10 nests tested with the snake jumped hurriedly out of the nest. Snakes can easily enter the nest, so early fledgling is the only chance of escape. The parents took care of the early fledged nestlings as normal for fledglings. Although early fledgling can mean lower chances of survival, it is a better option to an almost certain death when a snake enters the nest.

This study shows that parent offspring communication can be quite nuanced, and hints at how little we still understand important aspects of the behaviour of common bird species.

References
Suzuki TN (2011). Parental alarm calls warn nestlings about different predatory threats. Current biology : CB, 21 (1) PMID: 21215927

Goldfinches and dandelions

You don't need to get out much to enjoy wildlife. Literally, sometimes surprising things happen right at your doorstep. This morning I had to stop short of opening my front door as I realised a pair of goldfinches were standing next to it. One of them was grabbing a couple of dandelion heads - that had probably tumbled over with its weight - and feeding on the unripe seeds. This went on for quite a while. The photographs were taken through glass.

Goldfinches are specialist on small seeds, they benefit greatly from flocking when foraging as they need a lot of effort to obtain enough biomass from tiny seeds that have to be prepared and swallowed one by one and they can devote less time to scanning for predators when they are in a flock.

Gluck investigated these effects on Goldfinches feeding in orchards in Germany during the breeding season. He recorded Goldfinches feeding on milky ripe seeds from 20 plant species. Of these, most of his observations were on five species: Dactylis glomerata (Gramineae), Knautia arvensis (Dipsacaeeae), Senecio vulgaris, Taraxacum officinale and Tragopogon

pratensis (Compositae). The following graph illustrates the relationship between the number of dandelion seeds ingested per minute and flock size in Goldfinches, showing that even in small flocks of 5 birds each bird can almost double their seed intake per minute.

I am a big fan of dandelions and today's sighting is going to make me do even less weeding around the garden.

More information

Erich Glück (1986) Flock size and habitat-dependent food and energy intake of foraging Goldfinches. Oecologia, 71:149-155.