Robins are known for being friendly and, in general, fearlessly approaching human. They don’t think twice about going next to gardeners (or animals such as wild hogs) in search of bugs in the recently turned up soil. Nonetheless, this species is considerably more aggressive than one would expect. Sedentary robins keep their territory throughout the whole year and use a number of strategies to prevent other birds (especially rival robins) from claiming it for themselves.
A not-so-nice-if-you-are-a-bird song
Birds sing for many reasons: to communicate mid-flight, to attract potential mates, just because… It’s an area of study with many questions yet unanswered. One of its most important use cases, however, is to defend a bird’s territoryHow do territorial songbirds convey aggression? - British Ornithologists' Union. Robins, as they keep their territory in the winter too, are easy to hear even outside of breeding season. For us humans, this song is usually pleasant, but for other birds it’s a warning not to get too close.
Girls can fight too!
Historically, the study of birdsong has been centered on males. Such a costly signal (after all, it’s a very small bird making a lot of noise) would be explained by Darwin’s theory of sexual selection, where it would signify a male’s aptitude—both to attract mates and to establish itself above other males.
A study by the Royal Society researched the females’ song, finding that—in the case of robins—both sexes’ songs increased in intensity when they felt threatenedThe parameter measured by the paper is “vocal performance”, which is defined as the degree of vocalization close to the respiratory limit of the bird. Vocal performance in birdsong is an aggressive signal in both females and males: experimental evidence from a field study in European robins.. The conclusion that the researchers drew from this was that song was perceived as equally threatening in both cases.
It also showed that females tended to sing shorter songs but with an increased vocal performance. It’s hypothesized that this is a strategy to cope with sex-specific consequences of a sustained high vocal performance.
Getting physical
When singing isn’t enough, birds end up physically fighting each other. Robins get to this limit exceptionally quickly.
A terrifying mortality rate
A significant percentage of adult robin deaths is caused by territorial disputes. An article published by the Royal Society for the Protection of Birds (RSPB) estimates that up to 10% of adult deaths are caused by fights meant to protect a robin’s territoryThe RSPB: Robin: Territory (archived). Even though the original page is no longer available and I haven’t been able to find this information elsewhere, given their territorial nature, such a high mortality rate doesn’t seem too far-fetched
Cities worsen the issue
Birds living in cities (including robins) are much more prone to aggressive behaviors and physical fights than their rural counterparts. The step before attacking is to warn the rival bird, for which both visual and auditory (i.e. song) signals are used. Nonetheless, cities are full of noise, and noise makes auditory signals less effective. Thus, birds in urban environments escalate fights sooner than ones in rural environments. They also favor the usage of visual signals, as they become relatively clearer.
Interestingly, when introducing ambient noise, rural robins became more aggressive than urban ones— possibly because the urban ones have adapted to use less energy in territorial disputes when there’s noiseHow do urban birds settle fights under noise? - British Ornithologists' Union.
- ↑ The parameter measured by the paper is “vocal performance”, which is defined as the degree of vocalization close to the respiratory limit of the bird.
- ↑ How do territorial songbirds convey aggression? - British Ornithologists' Union
- ↑ Vocal performance in birdsong is an aggressive signal in both females and males: experimental evidence from a field study in European robins.
- ↑ The RSPB: Robin: Territory (archived)
- ↑ How do urban birds settle fights under noise? - British Ornithologists' Union