Territorial advantage
MARCH is a month when our resident birds are getting ready for a new breeding season. Some will already have territories and partners, while others, especially youngsters, are still in the process of preparation. In order to set up home, a male bird will have to establish a territory, which he will do with an ever-increasing amount of song. His territory must be large enough to provide enough food for two adult birds and their young, and must also contain a potential nest site, suitably protected. Only by finding an area which fulfils all of these criteria will he ensure he is adequately attractive to a mate.
Many birds will have song posts, often along the border of the territory, and from these the male will proclaim what is his, defending it vigorously. Studies have shown that birds which have the best songs are the ones defending territories of the highest quality. If song is not enough of a deterrent, then a male may confront an intruder aggressively. European Robins, which are infamous in their defence of territory, will use their orange breasts as a show of aggression.
The size of a territory is critical. If it is too large, it will be difficult to defend, and if too small, it will not provide all of the necessary benefits. Most garden birds have smaller territories than their counterparts in natural woodland. The shape of a territory may follow natural boundaries, like a hedgerow or stream. Territories of most birds in the wider countryside will be roughly circular, but species like Common Kingfisher will have a long, narrow territory which follows the path of the river it lives on. In times of high population density, territories may be smaller than years when fewer birds are present.
Birds which feed on animals, such as insects, tend to have larger territories than those which feed on plants. A Goldfinch or Greenfinch has a small territory around its nest and some may even nest together in small groups. Their plant food supplies are localised and plentiful so lots of them can feed together with little competition. A Chaffinch, which feeds more on insects, needs a much larger territory that contains its food, and so will defend it from other Chaffinches.
When populations of a species are high, some may be forced to breed in less-than-optimal habitats. Great Tits, for example, breed best in woodland, but may have to settle for nearby gardens if all of the best territories in woods are taken. Outside the breeding season many will move away from woodlands into gardens to survive the winter, with supplemental feeding helping them to get through colder weather. This may mean that they cannot reclaim a woodland territory later as others which stayed in the wood may have claimed it first. With high population density it is difficult for young birds to establish a territory and many will fail in their first year.
Most territorial aggression occurs between individuals of the same species, but it may also take place between differing species. Interspecific territorial disputes can take place for several different reasons. Closely related species which occupy the same habitat are more prone to this than unrelated species, probably due to competition for food and other contested resources. Birds of a similar size are more likely to dispute ownership, compared to birds of greatly differing size, almost certainly for similar reasons. Species which share a nest type, especially cavity nesters, may have limited nest holes and so have to defend them from other unrelated cavity nesters.
EUROPEAN Robins are well known for their fierce territorial behaviour, but a new study has cast light on how the sound of traffic influences levels of aggression in the species.
Previous studies have shown that urban robins are more inclined to get physical in defence of their territory than birds in the countryside, adapting their song, approaching the rival with swaying visual displays, then going in for an attack if necessary.
A 3D-printed replica of a robin was placed within a city park in Istanbul near busy roads, while another was situated in a quiet wood on the outskirts of the city. Scientists from the UK’s Anglia Ruskin University and Turkey’s Koç University equipped the plastic bird with a speaker playing song of the species, while another speaker in the area played the sound of traffic noise.
Dr Çağlar Akçay, senior lecturer at Anglia Ruskin University and lead author of the study, said: “In normally quiet surroundings, we found that additional traffic noise leads to rural robins becoming more physically aggressive, for instance approaching the model bird more closely.”
By contrast, playing extra traffic noise in the city environment did not increase levels of aggression. Instead, the birds reduced their singing, hinting that they had learned to cope with brief increases in noise pollution.
Dr Akçay said: “The chronic high levels of noise that exist day and night in urban habitats, such as from traffic or construction equipment, may permanently interfere with the efficient transmission of acoustic signals and this is likely to be the key reason why urban robins are typically more aggressive than rural birds.”
The team suggested that increased aggression would not make day-to-day life easy for the robins, and could even put individual birds at greater risk of predation as they focus on and move in on a rival bird.
REFERENCE
Önsal, Ç, Yelimlieş, A, and Akçay, C. 2022. Aggression and multi-modal signaling in noise in a common urban songbird. Behavioral Ecology and Sociobiology 76.7: 1-10.
DOI: https://doi.org/10.1007/ s00265-022-03207-4
Q I have sent some photos of a Taiga Bean Goose at Weybourne, Norfolk. Any views on the identity of its friend would be gratefully received. It seems to have characteristics of Tundra and Taiga Bean Geese and the head shape seems to vary depending on the view. Robin Chittenden via email A Dan Owen, BirdGuides news team member, replies:
Sometimes it’s impossible to categorise a bird, and bean geese have a good knack for testing boundaries. Despite such goodquality photos, there are aspects of the bird in question that should perhaps remain that way. For avoidance of doubt, the larger ‘gander’ is acceptable as a Taiga Bean Goose in my opinion. Firstly, the sheer size of the bird, and the way it ‘swaggers’ through the sea of Pinkfooted Geese is a good indicator. Furthermore, the overall paleness: at a distance Taigas are almost more likely to be discounted as ‘Pink-feet’ than their smaller and swarthier counterparts. Not to mention that long neck! It’s perhaps also important to note that not only do Tundra and Taiga Bean Geese hybridise, but it is thought that this is a semi-regular occurrence. Furthermore, some populations of Taiga Bean Geese show signs of inbreeding. It’s all a bit of a minefield. Essentially, identification shouldn’t be as straightforward as judging the bird based on the easier-to-identify bird it’s paired with.
The smaller bird of the pair, presumably the female (based on sexual dimorphism), is certainly interesting and falls into the ‘tricky’ category. In fact, the bird shows features of both Taiga and Tundra Bean Goose. Firstly, bill structure far outweighs pattern in its importance, which can vary considerably between the two. Structurally, the bill looks better for Taiga:
• The upper mandible is convex, dipping in about 50% along the length, before straightening out towards the tip;
• The lower mandible appears relatively straight although slightly thicker at the bill base. It is also partly covered by the upper mandible towards the tip;
• The tomium appears fairly straight, reducing the ‘grinning patch’ to something more suitable for Taiga.
While the above points are backed up by most photos, some show slight differences, although this is dependant on posture and angle. The flight shots show the neck’s maximum length, and the neck also appears thinner than the Taiga above it. Yet in the image of the birds ‘on the deck’, the story changes and the bird appears woolly necked and more akin to a Tundra. The bird’s body does appear long (pro Taiga), yet also plump and seems to ‘sag’ between the legs, which are also thickset. The overall colour of the bird isn’t too dissimilar from the Taiga behind, but the bird has a richer, more gingery tone to the breast, and head is just a shade darker.
I think you have to make more exceptions to fit it as a Tundra rather than a Taiga, yet something feels off for the latter. The smaller size is explainable due to the bird being a female, although a little extreme, but the stoutness isn’t quite right. The bill looks good enough in most pictures too, but with a few other features being ‘off-centre’ you’re left wanting better. It probably does just about fit into the small, female, end of the fabalis spectrum, but you have to wonder about the hybrid option.
Anyhow, the bird is not a safe Tundra Bean Goose in my view but sits too close to the overlap zone to commit to Taiga. Q
I was pleased to see a new gardening section in the February edition. Matt Phelps mentioned that dead plant stems are good for Goldfinches. They take the nyger seed we put out readily and sometimes we have 15 birds on the feeders. Often a pair stays until the spring but we have never had them nesting in the garden, even though we have some big hedges at the end. What could we do to encourage them to nest? Penny Shaw, via email
A Matt Phelps replies: It sounds like you already have everything that Goldfinches could possibly want! Unfortunately, there is no artificial nestbox option for the species, but you could try and make sure that there’s a plentiful supply of nest material on offer in your garden in the spring. Rake up some moss from your lawn and leave a few bits lying about. You could also strew some strands of wool and hair in obvious spots around the garden.