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

Bird Watching (UK) - - Contents -

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

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