Saturday, 19 October 2013

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.

Tuesday, 15 October 2013

The Crow and the Squirrel

At this time of the year, Grey Squirrels are very busy burying seeds for the long winter. Crows also cache food, and there is an interesting dynamics between both species. Crows are known for eavesdropping on each other, and they avoid caching food if they see other crows around that might steal their food if they know where is stored. But Carrion Crows are good at stealing squirrel food as well. They walk nonchalantly nearby, while the squirrel buries some food, and later they retrieve it and either they eat it or store it somewhere else. Well, the squirrel seems to realise it is just not working for her, and I have often watched how the squirrel goes nuts and starts chasing the crows all over the place (top shot).
Carrion Crow storing food
Crow having a good look at where the squirrel is hiding its food.
These photos are today's, but I managed to get a video of a squirrel chasing the crows last winter.

Goldfinches pecking for grit

I found this flock of Goldfinches pecking the mortar between bricks to get grit. Most bird species that feed on plant materials contain grit in their gizzards. Seed-feeding birds aid digestion by incorporating insoluble grit (silica) into their diets, so that itscours the seed shell and exposed the nutritive inside to digestive enzymes or just helps with grinding the seeds. Soluble grit, containing calcium carbonate (limestone or grit of organic origin) is also actively sought for in preparation for egg laying, as it is a component of egg shells. I guess the mortar has the right type/size of grit for goldfinches, as I have found several photos showing similar behaviour (see here and here). As mortar contains both soluble (lime) and unsoluble (sand) grit, it is unclear which one the goldfinches are obtaining from it.

Wednesday, 2 October 2013

Canada Geese taking off decisions

ResearchBlogging.orgThis morning, the 60 strong flock of Canada Geese that seemed to have roosted in the park were restless. There were continuous loud grunts and honks and, after a crescendo in which more and more individuals joined in the calling, part of the flock took off in a coordinated way. The vocalising geese were making a decision, with individuals deciding to join or not a party leading the departure, maybe to quieter feeding grounds. It is unclear if the individuals felt hungry or if the several dog walkers had made then nervous (or both!).
  Flocking birds have to travel together to stay in a group to search for new feeding or roosting patches, to avoid predation or to migrate. When the individuals differ in their motivations to move away (some might be hungrier than others, for example), there might be a period in which an a consensus is reached about leaving or staying involving communication of the individuals intentions. An individual that is not hungry might change its mind and join in if most of the flock appear willing to move, so as not to become isolated.
  Dennis Raveling, in a paper in 1969, reported on the behaviour before flock taking off in Canada Geese. He observed a large flock in their natural range, in which 77 geese had been marked and radio tracked, including 10 families. Flock departure was preceded by a ceremony, with the neck stretched, there are quick head tossing movements with the bill pointing up and repeatedly, and the white head patch conspicuously displayed - communicating an intention to fly. Geese often spread and flap their wings and start to walk in the intended direction of flight for a few steps (this video illustrate this behavior). Ganders (adult male geese) were more successful at recruiting his family than any other family members, as a shorted time elapsed from his initiation of head-tossing until the family took flight, although all family members initiated head tossing at some point. In a couple of occasions when an excited immature took flight but the rest of the family did not follow, it flew in a circle and returned with the family shortly.
 Other than the cohesive function of the head-toss ceremony for family members, group vocalisations and wing flapping serves to synchronise the whole flock. The coordination of the flock during take off included the presence of an invisible 'starting line': individuals run until they arrived at the same position at which the individual before them had just taken off (instead of taking off where they were). Raveling hypothesized that the contrast between the white tail coverts and the black tail served as a signal to optimise the position of individuals during flight, quite important in such large birds. Families tended to keep together during flight and their vocalisations then changed from grunting to a more trumpet-like honking.

More information

Dennis G. Raveling (1969). Preflight and Flight Behavior of Canada Geese The Auk, 86, 671-681 DOI: 10.2307/4083454