Yule blog: robins exposed

A helping of seasonal science in which Oxford's Joseph Tobias exposes the scandalous life of the robin:

OxSciBlog: What, in evolutionary terms, explains the robin's aggressive territorial behaviour?
Joseph Tobias: Robins are a classic example of a species which survives by defending resources. They feed largely on ground-dwelling invertebrates, a food supply that can quickly be exhausted if too many individuals forage over the same patch of ground.

To maintain their food supply, they defend it aggressively against all-comers, much as we might get a little stroppy if someone was pilfering our fridge. Only in robins the stakes are higher because a cold snap and an impoverished food supply means death, and you can't get more non-adaptive than that.

OSB: What benefit/use is its distinctive red breast?
JT: The red breast appears to be an honest signal of competitive ability. It is fluffed out in displays between rival males, and between courting pairs. It contains information about the genetic quality of the individual, and so functions as a signal mediating conflict and courtship.

As with many such signals in the natural world, the display of honest signals can settle contests one way or the other without the need for direct aggression. This can save a lot of time, energy and injury. When resource value is high and robins equally matched, neither individual backs down and they will famously fight to the death.

OSB: Do we know what impact global warming is having on robin populations?
JT: No. The picture is highly complex. British robins will probably be in favour of global warming as many of them die during harsh winter weather. Spanish robins will probably be a little less enthusiastic as their summers may get too hot and dry.

In most winters a fair proportion of British robins migrate to France or even Spain, while a good number of Scandinavian immigrants swell the ranks of British residents. Warming may therefore lead to shifting patterns of migration, and a contraction of the southern boundary of the species' range.

OSB: How does food availability affect relations between male and female robins?
JT: Pairs of robins form very early, in January, if the food supply is sufficient for two individuals to share a territory. From January onward the female is silent and the male takes charge of territory defence; in April she builds the nest alone, and the pair collaborates to incubate the clutch and feed the offspring; in August the adults moult, the pair bond breaks down and the male and female go their separate ways. So far, the species is typical of many birds, except for the early pairing date.

However, from September onwards, the female experiences a large influx of testosterone, and she changes character. She breaks into song, and she competes aggressively with males for a winter territory. In effect, for a few short months, females become males and both sexes compete on an equal footing for space and resources. This period of the year perhaps explains why both sexes show the red breast. It also makes the robin a remarkable creature, with a highly unusual approach to territoriality. In no other British passerine do females sing, or defend their own territories. And very few British birds are so strongly territorial during the winter.

From January onward, female robins join forces with a male, and it is the timing of this switch-over that is governed by food supply. In high quality territories with plenty of food this can happen shortly after Christmas, but where food is scarce it is not possible for two birds to forage over the same space until February or March. Every year, 20% of male robins advertise their territories in vain, and fail to find a mate. Consequently, this places a high premium on defending the best possible patch to maximize chances of attracting a female early in the New Year.

OSB: Any other tidbits about how robins survive the British winter?
JT: For a species so intimately linked in our minds with cockiness and aggression, one of their most surprising habits is communal winter roosting. Up to twenty individuals, who have fought fiercely all day over the boundaries of their territories, will gather at dusk in a dense shrub to sleep. This behaviour is very rarely observed, as robins gather at their winter roosting sites at dusk. While it is not fully understood why they do this, they presumably gain some anti-predator or thermoregulatory benefit by abandoning their daytime disputes and roosting communally.

OSB: Where do your current research interests take you?
JT: My early forays into robin biology, involving bracing winter fieldwork in Cambridge Botanic Gardens, led to a fascination with the tropics, where things just seemed a whole lot warmer. I've been working there ever since, tackling a range of questions in evolutionary biology and conservation biology. Perhaps the biggest and most challenging of these is Darwin's "mystery of mysteries" - what processes cause species to multiply and to assemble into communities.

This puzzle takes me to the richest rainforests of Peru, where up to 500 bird species sometimes occur in a square km. Here, with studies of song, behavioural interactions and phylogenetics, I am finding out some fascinating things about how signals mediate their co-existence, and in turn how co-existence has influenced the evolution of signals. With a bit of luck and a lot of comparative genetics, I also hope to discover the key to that old chestnut: how they all got there in the first place. 

Dr Joseph Tobias is a postdoctoral researcher at Oxford's Department of Zoology.