Monday, 17 May 2010

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 obsercations 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. here.

Saturday, 1 May 2010

Worms, quick, quick!

Blackbird fledglings are out and about. Their urging 'quick, quick' calls to their parent to bring food make them easy to track. If the female is not ready to lay a second clutch, the pair of Blackbirds will divide the care of the brood amongst for up to ten days after the young leave the nest. If the female has already started laying, then the male takes care of the whole brood, running back and fro in search of food for his demanding young. This behaviour is a trade-off: the male taking charge allows shorter intervals between broods - and therefore higher chances that more chicks are produced in a season - but the more young he is feeding, the lower the chances they will all reach independence.
Yesterday, a male sang while it flew toward his charge, like he didn't want to waste a minute - a stressed, multitasking parent. Then, landing nearby, caught a large earthworm and banged it against the ground several times while the fledgling called insistently. The male offered the worm to the young, and he swallowed it voraciously.

Gradually the young will learn to catch prey or find food by themselves, and by 2-3 weeks after leaving the nest they will reach full independence.

More information:
PHILLIP J. EDWARDS (1985) Brood division and transition to independence in Blackbirds Turdus merula. Ibis, 127: 42-59. here.

Nest Calling Woodpigeons

Woodpigeon pairs are already formed and nest building is underway. For quite a while I have become accustomed to an unusual song of the Woodpigeon. It starts like it is going to deliver the usual five-syllable song, but instead it stalls and it only delivers two long notes, 'coo, coooo' with the second note dragging on - it reminds me or a revving motorbike. After a minute or so, the two notes are repeated, and it can go on like that for quite a while. I have heard it mostly when the pigeon is concealed from view (inside a cypress) and in one occasion I saw it delivered by a pigeon sitting on its nest. A pair of Woodpigeons have made a nest on a fork in a large weeping willow and the pigeon in the photo was calling in such a way. This call has been named 'the nest calling' and is uttered by the male showing the female a nesting site. According to Murton and Isaakson:

Nest calling is a submissive display, mainly shown by the male and is used to attract the female to the nest or potential nest site. Caressing can occur at the same time as nest calling or away from the nest, and possibly evolved from the food begging movements of the nestling. It usually precedes courtship feeding. On the physical level the display is used to stimulate ovulation in the female, so that when used for long periods in the absence of courtship feeding it appears as a ritualized display.

And according to Cramp:

From early January, and often soon after the pair is definitely formed, the male begins to mark suitable nest sites by Nest Calling. He crouches on the proposed site (which may be a bare fork or branch, or an old nest) and makes short, sharp, downward pecking motions, 
accompanied by the special nest call—a deep, double note, `coo, coo'.
The male may `nest-call' at several sites, and I imagine that the female normally makes the final choice, for the nest is by no means always built on the site where her mate has `nest-called' most persistently. The first nests are seen normally towards the end of March, and the majority start to sit in April.

More information:
Cramp, S.(1958) 'Territorial and other Behaviour of the Woodpigeon', Bird Study, 5:
Murton, R.K., A. J. Isaacson (1962) The functional basis of some behaviour in the Woodpigeon Columba palumbus. Ibis: 104: 503-521. here.