The bill of the blackbird

This morning I spotted this male blackbird sitting in a bush. It had the brightest orange-pink bill I have ever seen in a blackbird, and it prompted me to finish writing this post that had been for a little while in my drafts folder.
Carotenoids are pigments obtained from the diet in animals that give yellow to red hues to many sexually selected ornaments. Carotenoids are antioxidants and have a role in the immune system and if they are supplemented in the diet they often increase the condition, growth and survivals of organisms, therefore, their use as honest signals of body condition in a sexually selected context is one of their most studied roles: Females may assess male quality by using signals, if they are good indicators of the male body condition.
 Blackbirds are sexually dimorphic: males have a black plumage and orange bill and eye ring, while females are brown and their bills are brown or have a pale yellow base. There is intrapopulation variation in bill colour. Young males often have black streaks or a black bill in their first winter and adult bills range from yellow to bright orange. Several studies have investigated the role of sexual selection in bill colour variation. Results show that male blackbirds with orange bills are heavier and larger, have less blood parasites and pair with females in better condition than males with yellow bills. Females in good condition start breeding earlier and make more breeding attempts during the breeding season, which correlates with the number of fledglings produced per year. The correlation between female condition and her partners bill colour appears not to be due to direct female mate choice, that is, as no preference in females for more orange bills has been found in two studies. It is possible that this correlation is due to males with brighter bills being able to secure better territories during male-male conflict, and the female choice happens on territory quality. Although blackbirds are socially monogamous and form long-term pair bonds, there is a certain amount of extra-pair copulations and it is not known if polymorphism in bill colour determines how many extra-pair copulations can males achieve.
 It is surprising how much there is to learn from such a common bird.
Here are some photographic examples of bill colour polymorphism in male blackbirds.

A young male subsinging (31/Jan/2010)

Another young male with brighter bill but streaks of black (18/Jan/2014)

An adult with bright orange bill

An adult male with yellow bill.

More information
  B. Faivre, M. Preuault, M. Theury, J. Secondi, B. Patris & F. Ceuzilly (2001). Breeding strategy and morphological characters in an urban population of blackbirds, Turdus merula Animal Behaviour, 61, 969-974 DOI: 10.1006/anbe.2000.1669

Bright A, Waas JR, King CM, & Cuming PD (2004). Bill colour and correlates of male quality in blackbirds: an analysis using canonical ordination. Behavioural processes, 65 (2), 123-32 PMID: 15222961

Builder Wren and counting hens

Wrens 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.

References
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