The Behaviour of the Robin.—Part I. The Life‐history, with special reference to Aggressive Behaviour, Sexual Behaviour, and Territory. Part II. A Partial Analysis of Aggressive and Recognitional Behaviour

Summary.1  Each breeding pair of Robins own a territory of between 2000 and over 10,000 square yards in size; the unshared autumn territory varies between 800 and 6000 square yards. 2  The owner of a territory sings and attacks other Robins only within its territory, but commonly trespasses when feeding. 3  The Robin's posturing is aggressive, not courtship, and the red breast is a threat colour. 4  In the male, song and fighting are prominent in both spring and autumn; after pair‐formation song declines but aggressiveness increases. Some females hold autumn territories with fighting and song; after pair‐formation female song is rare but fighting occurs. 5  Juvenile Robins are occasionally attacked by adults, as are foreign species, especially Dunnocks; such fighting is too sporadic to assist food territories significantly. 6  When a Robin fights to displace the owner of a territory, the fighting is often “formal,” but is occasionally much more serious. 7  Males obtain mates between mid‐December and March. Nest‐building occurs near the end of March, followed after a few days by copulation and the male feeding the female. A succession of broods follow until June. 8  In pair‐formation the female enters the territory of a male. How the unmated male distinguishes a potential mate from trespassing Robins is not known. Pair‐formation is accompanied by no display apart from sporadic aggressive posturing. 9  During the pre‐nuptial period, which lasts up to fifteen weeks, neither member of the pair displays sexually. 10  In copulation the female invites the male. The male has no pre‐nuptial display. 11  Apart from copulation, the only “courtship” of the Robin is the male feeding the female; this possibly helps to maintain the pair. It is unconnected with copualtion. 12  The mated male recognizes his mate individually, but has copulated with a stuffed specimen. 13  Parents do not readily distinguish their own fledgelings from others, and fledgelings do not distinguish their own parents from other Robins. 14  The spring territory of the Robin seems important as assisting in pair‐formation, in maintaining the pair, and perhaps in rapid feeding of the brood. It is doubtful if it limits the breeding density, and it seems unlikely that it insures an optimum population density. 15  The autumn territory seems functionless, and is perhaps a partial revival of the spring behaviour associated with a similar physiological state.Summary.1  The Robin shows aggressive behaviour not only towards intruding Robins but, to a varying extent, towards a stuffed adult Robin, foreign species (especially in flight), living and stuffed juvenile Robins, and a stuffed red breast. The external situation eliciting aggressive behaviour seems partially separable into a flying‐away movement eliciting pursuit‐flight, Robin‐shape eliciting striking, a red breast eliciting posturing, and song eliciting song, but these divisions are not complete, for occasionally a Robin will posture at a specimen lacking the red breast, or strike a red breast, or sing at a silent Robin. 2  The Robin's own mate, which possesses all the above four elements in the external situation eliciting aggressive behaviour, is not normally attacked. 3  The external situation eliciting aggressive behaviour is not simply another Robin, but every Robin (excluding the mate) in a particular area, the territory, and none outside it. There is one exception, a male without territory may attack, and sometimes ejects, a male with territory. 4  The internal state promoting aggressive behaviour in the male Robin varies seasonally, with a spring and an autumn maximum. The female has a shorter spring period and a much more variable autumn period. The fluctuations do not follow those of sexual behaviour, nor do they completely coincide with those of song. 5  In spring an unmated male is moderately aggressive, after pair‐formation extremely aggressive, whether pair‐formation occurs in mid‐December or May. The unmated male in autumn is as aggressive as the spring‐mated male. 6  The internal state shows marked variations in intensity among different Robins, and may vary somewhat from day to day in the same Robin. At the appropriate seasons its strength is sufficient for every intruding Robin to be vigorously attacked, but rarely a living intruder has elicited no attack, or only an ineffective one. Sometimes, on the other hand, it is so strong that foreign species, usually tolerated, are attacked. Further, the internal state cannot be regarded as a unity, since it seems different for striking as compared with posturing, and perhaps for the different types of posturing. 7  Experiments with caged and stuffed birds show that aggressive behaviour weakens with repetition of a similar external situation producing it under artificial conditions. But during, or immediately after, attack, aggressive tendencies may be temporarily heightened, for several Robins have then mildly attacked their mates, and one vigorously attacked the empty space formerly occupied by the specimen. 8  Rarely, aggressive behaviour, including posturing, is transferred to man as an object. 9  The position of the red breast in posturing is closely related to the position of the intruder, being such that the greatest possible area of red is presented.