Thursday, 13 July 2017

Individual human recognition in crows

Corvid researchers have ample anecdotal evidence that crows recognise individual humans: crows are unusually alert and often direct mobbing calls to researchers that have previously ringed them, while ignoring other researchers. Other crows follow and harass individuals who feed them in order to obtain food, something that Ralph Hancock has described to me in comments to this blog (see previous post), and amply illustrated in his blog (Charley and Melissa are the pair of crows that often pester him for food). Unlike predators, who tend to be dangerous in general, people can be either very dangerous, mildly dangerous, neutral or good to crows. Some people, e.g. farmers who treat them as pests persecute and kill crows (see photo below) and crows might learn to be cautious when they see them and fly away before being shot. Other people are plain annoying, as bird ringers who trap them, and it may pay to learn who they are so that the trap is avoided next time. Yet others people are good to crows, such as people who feed them, and it may pay to be confiding and even harass them to get more food. It is not surprising that intelligent birds like crows pay attention and learn to discriminate between 'good' 'neutral' and 'bad' people and act accordingly.
Two Carrion Crow carcasses on the cows feeder of a local farm.

 John Marzluff, a researcher at the University of Washington has studied crows for many years, and part of his research has involved trapping and ringing crows in Seattle. Marzluff was aware of the crow's mobbing behaviour towards the specific researchers that had trapped and ringed them but he wanted formal proof. He incorporated ringing into an experiment to test if crows recognise individual humans. He worked with American crows, Corvus brachyrhynchos, a close relative of the Carrion Crow with a similar social organisation. The research included urban populations that included previously trapped and ringed individuals, and also 'naive' populations that had not been ringed before. Experiments included a single negative event: the setting a trap (a net launcher) to capture feeding crows in 5 different sites. The people activating the trap, capturing and ringing the birds wore specific masks and/or hats (a 'dangerous' mask). Masks were used so that they were interchangeable and effects due to inadvertent researcher behaviour, outfit or body size could be removed. During transects around the crow territories, researchers (wearing either the dangerous mask or a 'neutral' mask) assessed crow activity towards them before and after the trapping event. They in particular noted if crows displayed the 'scolding' response to them (defined as a harsh, alarm ‘kaw’ directed repeatedly at the observer, and accompanied by agitated wing and tail flicking) or were indifferent. The experiments were repeated by people unaware of the experimental design or objectives, which were asked to walk around the crows territories wearing or not the mask used during the trapping event or a different, neutral masks. This experiment removed any subjective bias by the experimenters themselves.
 The experiments show that crows rapidly learn to recognise the 'dangerous mask' used during the trapping in their local site, with more scolding in response to the dangerous mask than to neutral mask during and after the trapping event, but no differences before the trapping event. In addition, the percentage of local crows scolding the person wearing the dangerous mask increases from 26% steadily with time over 2.7 years to 66%, suggesting that the local crows learn from each other about the dangerous person. The masks used in one of the experiments were of ordinary people and the crows learned to mob the specific mask associated to the trapping and not similar masks.
The crows today calling and wing-flicking.

  Today I walked around the park today with my camera. Three crows at the end of the park, in the same area where they mobbed and followed me three weeks ago immediately flew to the top of a tree and cawed harshly looking in my direction, wing-flicking, then one of them flew towards the tree by which I was standing. I took a video that shows how it is scolding me and records the call. The top shot is a screen gan of the video. As I left the park they stopped cawing. I seem to have become the not so good people to the local crows, maybe I will wear a mask next time.

And flying towards me

More information
Marzluff, J. M., Walls, J., Cornell, H. N., Withey, J. C. and Craig, D. P. Lasting Recognition of Threatening People by Wild American Crows. 699–707 (2009).

Monday, 19 June 2017

Mobbed by crows

In my way to work as I crossed the park I saw a pair of crows with a fledgling. The young one hopped to one adult and was fed. I took my camera out and started taking photos, but all feeding and begging stopped. The young one started looking around for tidbits on the grass. One of the adults, after eyeing me (the one in the background above), flew to the top of the tree under which I was standing, and cawed loudly and repeatedly. The other adult followed. I didn't think much about this until I carried on with my walk and the crows flew from tree to tree above me, calling all the way, sounding pretty angry. It took that for me to realise that I was being mobbed! I thought about the episode by Konrad Lorenz when his tame jackdaws apparently mistook his wet swimming trucks, which he was holding, by a dead jackdaw and actually attacked his hand, drawing blood. I saw my black camera with dangling cap and immediately put it away. I do hope the crows don't remember me tomorrow!
 Curiously, but not surprisingly, this call is completely different to the mobbing call uttered when chasing sparrowhawks or gulls away (the call that gave name to this blog), which is a dry rattling. Instead is a long, harsh caw. I found a video of a crow attacking a fox and using the same call.
The adult, on the right, eyeing me. 
The young crow stops feeding, as the adults caw above me. 
The young crow is taking everything in, looking at the parents reaction.

Thursday, 15 June 2017

Stock dove driving

Stock Doves are so intriguing. A combination of shyness and a superficial resemblance to feral pigeons makes them often pass unnoticed. Several times this spring I have watched them in flight, usually a pair, sometimes trios, with one of the doves flying right above another, their wings almost touching, but there didn't seem to be aggression in the behaviour. What are they doing?
 I might have found the answer today, reading an old article by Derek Goodwin. He describes a behaviour called 'driving' which is present in several members of the pigeon family, including the domestic pigeon. This is his description:
The driving cock follows his mate everywhere, often literally "treading on her tail". He pecks, usually in a gentle manner, but sometimes fiercely, at her head. If the hen takes wing he flies closely behind and above her. If she goes to the nest (domestic birds) the male stops driving.
Pigeon breeders thought that driving meant that the male pigeon wanted the female to go to nest, given that when the female goes to the nest the driving stops. Goodwin disagreed with this interpretation and thought instead that driving allowed the male to keep the female away from suitors, either on the ground or in flight.

In the two photos illustrating this post, a third pigeon was flying out of frame. I presume the male is the individual above, intercepting the female from the male.

Goodwin observations were consistent with this, as driving happened when other males were near the female, in particular during a period before egg laying and ended once the first egg has been laid, that is, a period when the female is sexually receptive, so that the behaviour is the way the male keeps the female away of competitors, and is effectively a form of mate guarding, as he put it 'It functions to prevent insemination of the female by males other than her own mate. Interference with copulating pairs is part of the same behaviour-complex as driving and has a like causation and function.' When, occasionally, driving was observed with just the pair involved, he thought that the male had succeeded in his driving: 
I have seen a pair fly up from some crowded feeding ground, and the male, who was at first driving hard in flight, swing out beside or in front of his mate as they got well away from others in the air, usually going into display flight as he did so.
More information
Derek Goodwin (1956) The Significance of Some Behaviour Patterns ofPigeons, Bird Study, 3:1, 25-37, DOI: 10.1080/00063655609475836.

Stock dove take over

Yesterday the insistent, zebra-like call of a stock dove just outside the house called my attention. It was atop the Tawny Owl box. We've got the box occupied by the owls since the beginning of the spring, and I presume that they fledged their young earlier as they have been quiet in the last couple of weeks. It's funny that the owls took over the repaired box from Stock Doves and now the doves are back. He was there just now, calling again. The doves should have enough time to raise at least a couple of clutches.

Wednesday, 14 June 2017

Carrion crow fledgling

The Carrion Crow pair in the local park have fledged two young. One of them sat on a carpet of poplar fluff, looking sleepy, and unable to fly very high. It looked like it had flown the nest a bit early. It was very tame, and allowed me to crouch by it and take his portrait. You can see that the chest and head feathers lack the metallic lustre of the adult plumage and the young's eyes are blue. The wing feathers are more metallic. A parent was nearby, furiously chasing a squirrel away. The sibling was much more active, and was intrigued by water, it opened it's bill to the water, like it was testing it or finding out what the best way of drinking was.
The inquisitive sibling. 

Wednesday, 5 April 2017

Encounters of the Sparrowhawk kind

I treasure encounters with Sparrowhawks as they are quite secretive and alert birds which don't like being seen. Today I managed some sightings of a pair in my local park. First I heard a call 'kikikikiki!' and I saw a female landing on a tree. The view was quite obstructed and unfortunately, she flew off as I tried to get a better view. 
Then, a while later, in the same area I spotted the male. Not sure how, possibly a blue tit alarm call alerted me. He had some prey, what looked like a small bird, and was plucking it.
 He flew off, carrying the prey in his talons, and I wandered after him towards some big trees where he had disappeared. Then I saw the pair together, high up the crown of one of the largest trees in the park, a poplar (top shot). Fortunately the beautiful light made up for the distance to take some photos. The male started preening. I couldn't see if the female had the prey now. Male Sparrowhawks hunt for the female while she incubates.
The male at the top, smaller and with reddish tones in his plumage. The female at the bottom, all alert. Soon tree foliage will completely obscure views of these birds from the ground.
 On my way back from work I looked longingly at the poplar. No Sparrowhawks to be seen there. But then, beyond the tree, high in the sky, the pair was circling together, playfully chasing and soaring.

Blackbird mob

Returning home in the light evening today I noticed Blackbirds 'chinking'. This is a very persistent call, often by several individuals, that I usually associate to summer evenings. The blackbirds, up to four of them, were in the tree where the Tawny Owl nest box is. The owls are indeed in occupation, although they were nowhere to be seen from the ground. They have been calling at dusk, and every few days I have been able to watch one of them as it called, sat on the box for a few minutes and then left to go hunting. Often, later at night, one calls with a lovely gurgling sound from inside the nest. Two of the blackbirds got close to the entrance, calling non stop, if there was an owl in the nest, they might be able to see it. At some point they calmed and left, but then one returned, started calling and the others joined him in the tree.
I wondered that the 'chinking' is not a roosting call, but a mobbing call to an owl, which is more likely to be encountered active as nights become lighter in the summertime. In a couple of weeks, the leaves would obscure the view and although a blackbird calling is evident, a quiet owl sitting inside a tree is not. This is a lovely clip of blackbirds mobbing a Tawny Owl demonstrating the call and also how fearless they are towards the owl.

UPDATE 7 April 2017. From the living room at 20:05, I heard a Blackbird suddenly starting to 'chink' and I looked at the nest box. The round silhouette of a Tawny was sitting atop the nest. It was light enough to get a low resolution photo.