Sunday, 21 February 2016

Play in gulls

I usually check for ringed Common Gulls in my local park. Yesterday the flock, about a hundred strong, converged around someone providing food. While I was checking, a boy run towards the gulls flushing them. A few settled on a puddle nearby, and a second winter one was ringed! I approached and took some photos to try and see the code. No luck, the light was wrong and the gull too active. I decided a video could work as I might be able to get a sharp screen-grab. The gull found a pink object (visible on the left side on the photo above) and started to pick it up and drop it repeatedly, the object bobbed up on the water every time and the gull seemed quite interested, until, after a while, it walked away. I had ended up recording a session of gull play behaviour! I walked away and then thought I wanted to check the object, just in case it was some food item, but no, it was a spongy piece of plastic, probably torn from a soft ball. You can watch the clip here:

 I had never thought gulls played until I started following Ralph Hancock's wonderful blog on the birds on his daily walk around Hyde Park and Kensington Gardens. He describes and documents gulls (Herring, Common and Black-headed) playing drop-catch with various objects: sticks, leaves, plane tree seed heads, rubbish, and stones, and his observations made me pay more attention. He also pointed to this photo sequence of a young gull playing drop-catch.
 How can we know that a particular behaviour is play? Usually play is invoked when a behaviour appears to serve no adaptive purpose. Jennifer Gamble and Daniel Cristol, from the College of William and Mary, Virginia, studied drop-catch behaviour in Herring gulls to discount alternative hypothesis to play. Their Herring Gull population often dropped clams they find on the mud flats at low tide over hard substrates - a tarmac road nearby - to break the shells and eat them. Could drop-catch serve be a practical behaviour? They tested two alternative explanations: the 'kleptoparasite hypothesis' where drop-catch behaviour could allow a gull to asses the chances that other gulls that might be nearby, be ready to steal the clam as it hit the ground, or the 'reposition hypothesis' where the gull might drop the clam and then catch it to reposition it before a proper drop. In contrast, if drop-catch is a play behaviour they expected:
1) It would be carried out more frequently by juveniles.
2) Objects other than clams would be used.
3) It would be performed over soft substrates.
Their results rejected the alternative hypothesis and fitted the predictions of the play hypothesis: younger gulls engaged in more drop-catching than older gulls, that drop-catching was more commonly done over soft surfaces (the mud flats, as opposed to the road and gulls were less likely to drop-catch a clam (9%) than a non-clam object (62%). The conditions at which drop-catch happened were also more favourable to play behaviour: Drop-catch happened more often at warmer temperatures (when the gulls are less cold-stressed) and when wind was stronger (and flight is less costly).
 Play has been much less studied in birds than in mammals, and most of the research in birds has been done with corvids. Gulls are quite opportunistic, long-lived species and play could potentially have long-term beneficial consequences: perfecting the development of the coordinated flight behaviour required to succeed in kleptoparasitic attacks (gull often chase other gulls carrying food and if the attacked gull drops the food, individuals able to catch the dropped object in the air might have a higher chance of success of catching it first) or possibly practice the clam dropping behaviour.
 I have kept the bit of pink plastic, but I haven't yet managed to read the code on the ring, it starts by JP...

More information
Gamble, J. R. & Cristol, D. A. Drop-catch behaviour is play in Herring gulls, Larus argentatus. Animal Behaviour 63, 339–345 (2002). Here.

Monday, 15 February 2016

A pair of bullfinches

Bullfinches are generally shy and unobtrusive, they tend to stay hidden in vegetation and show little. It helps to recognise their sad contact note. I see them around regularly enough, but good views are rare. That's why I got quite excited that a pair of Bullfinches, Pyrrhula pyrrhula, recently started to frequent the local wildlife garden, giving me an opportunity to become more familiar with their behaviour. I first noticed a female, funnily enough at the Big Garden Birdwatch. I heard a specially mournful version of their call, possibly she had lost track of her mate.
The hard to photograph female amongst the bushes
The next day, both were around, skulking in the trees and hedges, occasionally coming to the ground to feed. They stayed close to each other, but still called regularly. They appeared to be feeding on buds and Guelder Rose berries.
The male feeding on Guelder Rose seeds
Yesterday, the pair was about, feeding together on Sycamore seeds, the last remaining ones on a tree, and had to stretch and balance precariously on the branches to get at them. Despite their stockiness, they are quite nimble. They peeled the seeds using only their parrot-like bills, unlike goldfinches that have to hold the seed with their feet.

 Bullfinches will feed on buds in spring. In autumn they feed on tree seeds, like Ash keys, Birch and Sycamore. Bullfinches are seed predators of berry producing trees: they discard the flesh of the berry and feed on the seed itself.

Monday, 8 February 2016

The Mute Swan

I recently purchased The Mute Swan, by Mike Birkhead and Christopher Perrins, after a visit to a local fishing lake where a pair of swans holds territory year round. It has been an interesting, informative read, especially the historical aspects, although I would have liked to read a little bit more on behaviour. Some aspects are dated, for example, lead weights have been banned since 1987, and the mute swans populations have recovered, probably also helped by milder winters (BirdTrends). Mute Swans are the heaviest British birds, with males over 10 kg in weight and 2.20 m wingspan.

There is an unusual colour variety called immutabilis or 'Polish swans' in which the young are white when downy and moult into white feathers, instead of the usual brownish feathers of the juvenile. The feet of the immutabilis form are pinkish grey instead of dark grey or blackish. Read more here.
A normal juvenile with its parents
An immutabilis juvenile (pink bill) with parents.
An immutabilis cygnet (left) with its normal sibling.

Mute Swans get their name from their relative lack of calls, especially in flight, and in stark contrast to their vocal relatives the Trumpeter, Whooper and Bewick. Although relatively quiet, Mute Swans are not mute. They have a repertoire of vocalisations, which, although weak and not carrying long, are quite obvious when at close quarters: they snort, a pig-like call; they hiss loudly like geese, a threat call to predators or people approaching too much, especially when young are about, and they have a short, quiet two note call reminding of rooks'.
A hissing swan
Their wingbeats, however, are loud and make a whistling noise that is audible, it is said, from over a mile. This sound might have replace the flight contact calls of the other swan species.
A pair of Mute Swans in flight.

Non-breeders migrate to safe waters to moult in the summer, often congregating in large numbers. Moulting in breeding pairs is different. They moult when they are breeding: the female moults first, and then the male, once the female's feathers are fully grown. This way, there is always an adult with grown wings, which play an important role in the cygnets defence.
A moulting flock of non-breeding swans at Hornsea Mere

A semi-domestic bird?
At some point in history most if not all adult British swans, and most likely those from other European countries, were pinioned, and therefore, unable to fly. They were a heavily managed species, as it was regularly eaten, and it is likely that this management saved it from extinction from overhunting. The spectacle that is the sight of flying swans would have been much rarer, and restricted to movements involving young birds. In the early XX century, the custom of eating swans disappeared, and, management in most areas was much reduced. In a way, we can see swans as a once domestic, now feral species.

Food and the swan's neck
Swans have a record number of bones (vertebrae) in their necks for a bird: 25. Their long necks allow them to exploit aquatic plants and tubers much deep than other water birds. They use several feeding techniques, shallow feeding in which they keep their neck folded to up-ending, when they use their body and neck to reach deep levels, about a meter deep. They also use their feet in a paddling movement to disturb the bottom sediments and being able to filter out small particles or loosen roots and tubers. Swans might migrate some distances if their territories ice up in the winter.
These feeding swans attracted numbers of coots and various ducks, most likely benefiting of the swans disturbing and pulling deep aquatic plants.
A swan uses its long neck to feed from a hole in the ice

I have never seen the courtship of the mute swan, which appears to have nothing to envy to the Great Crested Grebe's (watch a video here). It involves the pair facing each other with smoothed, dipped wings, and engaging in synchronised, repetitive head dipping and flank rubbing, swimming in circles and, after copulation, both partners paddle and raise their bodies together, with their necks arched and bills pointing down.

Territorial defence
A pair of breeding swans defend their territory, in a lake or river, fiercely: sometimes over the winter if enough food is available and there is no ice. They will chase other swans, with flapping wings being used as a threat, and use a threat display, busking, which I have covered at The Rattling Crow before, in which they swim with strong pushes of both feet together towards intruders, their secondary wing feathers kept high up, neck arched and neck feathers ruffled.
an approaching busking swan.
high intensity busking just before wing flapping and chasing of an intruding juvenile.
Both males and females participate in territory guarding and defence. This pair approached a juvenile busking, chased it away and then swam around the perimeter of their lake.

Nests are often very visible as they are built from surrounding plants, creating a gap in reed beds or river margins. Swans lay 4-7 eggs that only the female incubates while the male keeps guard nearby.
Incubating female swan.
Parental behaviour
Like geese, which are the closest relatives of swans, both parents are involved in caring and defending the young. Adults will carry the young downy cygnets on their backs, often only the cygnet's heads showing over the parents wings, to suitable feeding areas. The male is more involved in territorial and brood defence, often chasing other birds like mallards or coots away. When little, the cygnets are unable to reach aquatic plants at the bottom of the lake, but parents will often disturb the water with their legs, or pull vegetation for the young.
Male swan (cob) chasing a mallard away.
Female with cygnets
Mute swans often swim with one leg, keeping the other dry out of water (cygnet on the left). The cygnet on the right is feeding by up-ending.
Up-ending adult pulling food for cygnet
Normally young disperse in September, once they have grown their first year plumage. Then they leave their parents territory and join a flock of non-breeding birds. However, some young stay with their parents until the spring. The local lake couple seem very tolerant of their young, and they often let them stay until the spring.
Last years first year swan moulting into adult plumage (23/1/15) still in its parents territory.
Non-breeding flocks
Mute Swans do not normally breed until they are 3-4 years old. Young swans unable to secure a breeding territory are more sociable and form non-breeding flocks.