Neighbours from hell
The bigger the tail, the better chance you have of breeding if you’re a Swallow – but those who don’t measure up could well resort to foul play in order to find their mate
Dominic Couzens reveals the darker side of the Swallow and the tactics used to find a mate
IF YOU BELIEVE the theme song of a well-known Australian TV soap (7,590 episodes and counting), everybody needs good neighbours. Apparently, they can become good friends. Few of us would deny that neighbourliness is an admirable quality, and most of us appreciate it when next door is friendly. But that’s humankind. If you are a Swallow, neighbours are an absolute nightmare. For this reason, a good many Swallows nest as a single pair, perhaps in a barn or under a bridge, and these are the lucky ones. By contrast, the Swallows that breed in colonies go through a breeding season that festers with misery. They might ‘need good neighbours,’ but Swallows are never friends. They are frequently much worse. One of the by-products of proximity is, of course, rivalry. If you live next door to somebody, it is difficult to avoid comparison. In human conditions this can be quite trivial – our neighbour’s lawn is, for example, invariably better maintained than our own, which is embarrassing – but it can also be uncomfortable if, for example, next door’s shiny car quite clearly reflects a disparity of income. In Swallow society, such comparisons are far from trivial. In the last 30 years, scientists have unearthed a great deal of information about how Swallows can judge each other, by how they look and how they sound, and the information resounds around the colony. Nobody has any secrets, and the differences really matter. Swallows, in particular, have tails that tell tales. In fact, it is worse than that; they have tails that tell the truth. It is proven that, when it comes to selecting a mate, female Swallows prefer males with long and symmetrical tails (scientists clipped the tails of successful males, with the result that their popularity immediately plummeted). Various studies have shown that the longertailed males are healthier and have enhanced breeding success, so the tail length is a proven (‘honest’) indicator of male quality. It has recently been discovered that females with longer tails are also superior to their rivals, and that the trait is heritable. It must be bad enough flying around knowing that you come up short in the tail department. What is much worse is that, in a colony, your deficiency is such public knowledge. You cannot help but be judged by neighbours. It gets worse. Not only is tail length important; so are the white spots at the base of a tail. Males with larger white spots have higher breeding success than those with smaller spots, and this is another test of male quality. So is a male’s song rate. In fact, it takes only a glance or a caught fragment of song for birds to make an honest assessment of rivals. In a Swallow colony, nobody is ever in any doubt about how ‘fit’ a given individual is. Not surprisingly, this has fundamental consequences. Swallows are far from strictly faithful to their partners, and high-quality birds attract attention from all sides. Males with long tails are routinely invited to mate with their female neighbours, while those with shorter tails are not. So, not only do high-quality birds flaunt their assets, they use them, as well. In a colony, as opposed to an isolated breeding pair, opportunities abound for this sort of thing. Parading their charms and enacting ‘infidelity’ are not the only dirty tricks that neighbouring Swallows visit upon their colleagues. They also carry out a practice that can have an equally devastating effect. Some females lay eggs in the nests of their neighbours, too. During the egg-laying period, these individuals
choose a moment when nobody is attending the next-door nest, and they quickly lay an extra egg to add to their neighbour’s brood. If in mid-clutch, the parasitized female is highly unlikely to reject it. In one study, about 16% of nests were interfered with in this way. At first sight, this might not seem so terrible. A Swallow looks after a neighbour’s genetic material – so what? Nobody dies. In fact, an extra mouth to feed can exact extreme strain on a Swallow parent, and it can genuinely compromise the survival of its brood-mates. The parasite in the nest may not actually kill its step-siblings, but its very presence brings hardship. Besides, the potential damage to a brood from a parasite, even of the same species, is reflected in the sheer amount of effort a pair of Swallows put in to avoid it. For example, neighbours often behave very aggressively to each other during the egg-laying and incubation period, and closely guard their broods. They will try to avoid nesting too close, if possible. But the most telling fact is that, where possible, neighbouring pairs try to avoid a complete overlapping of the breeding cycle. So, for example, if a neighbour is in the midst of egg-laying, a suspicious pair will literally hold back in beginning their brood. This is an extraordinary effort to make when weather conditions and other factors are taken into account. Breeding success is always something of a lottery, so to hold back is a very strong reaction to the threat of parasitism. It might be hard to believe, but some Swallows treat each other in a still more extreme and unpleasant way even than brood parasitism. Although rare, it is shocking, and it occurs under great pressure. The perpetrators are males that have failed to pair up and breed. The victims are usually young males that have managed to pair up, but have done so late in the season. Stakes are high and time is running out. The febrile environment breeds infanticide. In some colonies, there are many unpaired males coping with a failing breeding season. Such birds are opportunists. They strike not during the egg-laying period, but when the chicks have hatched out. At a moment when the nest is unguarded, these birds steal in and simply remove the young from the cup, dropping them down to the ground. The youngsters are irretrievable and invariably they perish. And, against any kind of natural justice, the bird that has killed the youngsters quite often pairs up with the very female whose brood he has destroyed. At this sharp end of the breeding cycle, neighbourliness is meaningless. There are so many disadvantages to breeding in colonies that you might ask: why do Swallows tolerate neighbours, anyway? Why don’t they all breed as single pairs? The answer to this question is not entirely established, but colonies often aggregate near excellent feeding sites, presumably making it easy to find food – location, then, is important. Colonies, with many pairs of eyes, are also better at detecting predators. Furthermore, males, especially young birds, stand a much better chance of finding a mate at a colony rather than alone, and many later-arriving birds simply need to find their way to a colony to stand any chance of breeding at all. It seems, therefore, that many Swallows attend larger colonies simply because they are making the best of bad circumstances. And that’s a very long way from the idealised version of ‘colonial’ life that we see on the TV.
HEAD TURNER Would you not be impressed by this display of long tail streamers? WHITE TAIL SPOTS Research has found that larger tail spots mean a better chance of breeding success