Thursday, 16 February 2012

Builder Wren and counting hens

ResearchBlogging.orgWrens start singing early. They have been doing it occasionally since the first days of January, their powerful quick song cheering up the dark winter days. Today they seem to have gone for it on earnest: I heard four different males singing on my way to work. They have lots to do and there is so little, precious time. They have to start building their nests before females are ready to lay. Yes, I said nests, not just one, but many, lots, as many as he can possible make before the females start visiting. And also, I said females, as wrens are polygynous, with one male mating with between one to nine females per season. Each spring, male Wrens, Troglodytes troglodytes, build several nests in their territory in quick succession. They are called "cock nests" and are spherical, with a side entrance, placed in suitable locations and offer the structure of the finished nest (the female will line it with hair and feathers). It takes between half a day to five days to make a nest. Although Wren's songs are incredible powerful for their minute size it is the nests the females will judge before settling for a male, in particular, the number of nests. The male will display to entice the female to each vacant nest, singing and excitedly guiding her to the nests, to advertise what an accomplished builder he is. The more empty nests a female is shown the better. The next graph shows the positive relationship between the number of nests built by a male and the number of females making breeding attempts - and therefore the number of fledglings that the male will sire.
(from Evans and Burn, 1996)
 Males vary in their ability to make nests - or in their ability to defend a territory where the nests can be built. Age makes a difference, with older males better at making nests. Habitat structure is also important, with denser vegetation indicating a better territory, where nests are less likely to be predated. But the most important explanatory factor is male condition: heavier males at the beginning of the season will be able to make more nests, so the number of nests is a measure of male quality. Females benefit from mating with males demonstrating their good condition, so it pays them to carry out a nest count before settling for a male. Experimental manipulating of number of nests present on each male's territory carried out by Mathew Evans and Joe Burn showed that the actual number of nests is the mate-choice cue that the female uses to assess male quality. The making of multiple nest by the male wren as an ornament, an extended phenotype result of the same factor than the tail of the peacock, a signal of quality selected by sexual selection.

Evans, M. (1997). Nest building signals male condition rather than age in wrens Animal Behaviour, 53 (4), 749-755 DOI: 10.1006/anbe.1996.0311

Evans, M., & Burn, J. (1996). An experimental analysis of mate choice in the wren: a monomorphic, polygynous passerine Behavioral Ecology, 7 (1), 101-108 DOI: 10.1093/beheco/7.1.101

Friday, 10 February 2012

Bold as a coot

ResearchBlogging.orgCoots are the odd ones out in their family. Unlike other rails, they are bold, noisy and aggressive birds. Instead of searching for cover, they are all out, even their nests are usually placed in the open water and they are as prominent that you cannot miss them. Their squabbles amongst themselves and with other birds may carry out through the winter, as some birds keep their territorial behaviour year round. Coots have ashy-black bodies and black heads where a strikingly contrasting white frontal shield and beak is one of their most distinctive features. Unlike the long, thin toes of the Moorhen, coots have lobulated toes with side extensions - similar to a Grebe's - that allow them to dive in search of the algae and underwater plants that form a good portion of their diet.
The Coot "flippers"
But lets go back to the frontal shield. The size of the shield changes through the year depending on the bird reproductive status. Both males and females have frontal shields although the males' are larger, but both change in parallel, getting bigger in size as the start of the reproductive season approaches, and peaking in February -March.
(from Visser 1988)
Birds holding a territory have thicker and larger shields than birds not defending one.  Experiments on the American Coot, a close relative of the Eurasian Coot, showed that frontal shields enlarged when coots were injected with testosterone pellets, while they become smaller when injected with estradiol. At the same time, the birds climbed up in the pecking order when injected with testosterone due to an increasingly "pugnacious attitude" in the words of Gordon Gullion. The size of the frontal shield in European coots is also correlated with gonadal size. The frontal shield is apparently used for individual recognition and it is an important in territorial disputes or fights. When an intruder enters a territory, the resident bird will lower its head, raise its wings and prominently displaying their shield to the intruder. Like so...

A typical posture of an aggressive coot

I have much more on stock on Coots, but this will have to do for today.

More information
Visser, J. (1988). Seasonal changes in shield size in the Coot Ardea, 76, 56-63

Gordon W. Gullion (1951). The Frontal Shield of the American Coot The Wilson Bulletin, 63, 157-166

Thursday, 9 February 2012

Snowy birds

It has been cold and snowy in the last few days. Berries that a couple of weeks ago were plentiful - are now in short supply, as the Blackbirds, Woodpigeons and thrushes have dug in. Flocks of thrushes are moving south to avoid the cold temperatures and entering parks and gardens. Birds used to feed on the ground have the added disadvantage of having their food under a layer of snow, and the ground hard and frosty. Blackbirds and Carrion Crows will persist if the snow is not too thick. They push the snow aside with their beaks to uncover their hidden morsels. The robin on the photo above looked a bit perplexed though.
Carrion crows searching on the snowy grass
This blackbird has dug himself a hole