Pinkfeet together

At East Park this morning we came across a Greylag flock by the cafe lawns. I spotted the Pinkfoot, and then another, which surprised me. There has been a resident Pink-footed goose in East Park for a year now, a young bird flocking with Greylags. In November, I had spotted a different bird (and blogged about it, see here), also with Greylags at Pearson Park. Could these be these two birds that had got together?
 I looked carefully at my photos and saw that they have distinct bill markings, one with a very small mark close to the nail, the other with a larger U shape mark. The older bird, with the U shape mark (I'll call her Stripe) appears to the the East Park resident. The younger bird, still with tiny body feathers has a small bill patch and matches the bill marks - and tiny white feathers around its bill - of the one that was at Pearson Park. Despite being younger, this bird was bigger and has a larger head, with a Roman profile and a thicker neck and a heavier head, so it might be well a male. Since last saw both birds I was impressed how much more grown up they look. Their neck stripes are better delineated, their head shapes less gentle, and they have a more silvery backs as they have moulted their juvenile feathers. The Pinkfeet stayed near each other in the flock, but one lunged at the other when it got too close. At some point the flock got wary of a radio controlled car, and after a crescendo of cackles, they flew off.

'Dot' Pearson Park 20/11/2014

East Park, today. A male?

'Stripe' East Park 14/2/2014

East Park, today, a female?

Dot's juvenile feathers very noticeable in the flanks, with some adult feathers, darker and silvery just over his legs.

Stripe seems to have a full adult plumage now.

In the wrong flock

Although I wouldn't consider Pink-footed Goose (also known as Pinkfoot) to be an urban bird, it is a species that I regularly see. Skeins of this migratory geese, announced by a chorus of high pitched calls, regularly fly over the city in October and March, on their way to and from their breeding headquarters in Iceland and Greenland. Occasionally, however, a straggler will turns up with Greylags in local parks, allowing a closer look.
  On my way to work, walking through the park, a flock of Canada Geese and another of Greylag geese were about. While counting them I noticed a much smaller goose, a Pinkfoot, amongst the greylags. They have small bills, marked with pink, much darker head, contrasting with the slaty back. I think this individual was a young bird born this past summer, as you can see two types of feathers in its flanks and back, as the larger adult feathers, with a pale edge, have started to grow. The furrows that adorn the neck of adult geese are not yet sharply defined. The goose wasn't welcome and was often lunged at by the Greylags. Surprisingly, it was quite relaxed in the park, despite proximity to people, and it got very close to me as it followed the other geese across the lake and then to the grass to feed.
 I other occasions when had previously seen other Pinkfoot with greylags, it was also young birds. I wonder if leaving with your own species as the flock departs takes some learning. Geese need to reach a consensus before leaving as a flock, a decision that takes place by increasingly noisy vocalisations and movements of the birds of the flock when they are ready to leave. If a young bird misses the cues, it will fail to take off might miss the flock altogether, becoming stranded possibly with another geese species. Geese have long lives and move about, so most likely, while following the greylag flock in its travels, it will eventually meet a Pinkfoot flock and the young bird will be reunited with other members of its species.

A Greylag drives the pinkfoot away from its partner

Walking towards the grass. The difference in size between both species is evident here.

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