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A once-in-a-century severe winter revealed selection for short-tailed Pacific Swallows

Natural selection on wing and tail morphology in the Pacific Swallow. Hasegawa, M. & Arai, E. Journal of Ornithology. 2017. DOI: 10.1007/s10336-017-1446-7. VIEW

Severe climates provide excellent opportunities for studying natural selection. Because swallows can forage only when weather conditions allow flying insects to be active, there are several classic examples to show selection on swallow morphology in cold and rainy weather (e.g., Brown & Brown 1998). However, these studies focused on the breeding sites of migratory birds. Therefore, the relative importance of foraging, energy reserves after migration, and mating effort, is unclear.

We studied selection on morphology of a non-migratory bird, the Pacific Swallow, Hirundo tahitica, during the 2015–2016 winter on Amami Oshima Island. On Amami Oshima Island, there was severe weather during this period. There was snowfall in this subtropical region for the first time in 115 years, accompanied by the lowest air temperature recorded since records began in 1897 (4.4˚C, Japan Meteorological Agency 2016a]. Precipitation during the winter of 2015–2016 was 188% higher than the average for this region, and was the highest recorded since records began in 1947 (Japan Meteorological Agency 2016b).

Hasegawa shorttail fig 1Figure 1 Short tails of the Pacific Swallow, Hirundo tahitica © Masaru Hasegawa

Among other morphologies, the short tail length of the Pacific Swallow (Fig. 1) provides a rare opportunity to tackle a long-standing controversy: the importance of natural (and sexual) selection in the evolution of tail length in swallows, particularly the genus Hirundo (reviewed in Hasegawa et al. 2016). Long tails might have an aerodynamic advantage due to increased manoeuvrability, but may impair flight speed (Norberg 1994). Therefore, it is still unclear whether natural selection favours long or short tails. The Pacific Swallow has a short tail, which imitates the initial evolutionary stage of a tail ornament. Pacific Swallows, like other Hirundo, fly at low altitudes with high speeds, thus we can realistically study selection on tail length in the genus Hirundo under the natural flight behaviour of the birds in their natural environment.

We captured Pacific Swallows at night. During the study period (28 January to 6 February 2016), we also searched for dead birds, which were found under or inside the nests (Fig. 2). We measured wing, tail, keel and bill sizes of these birds. In total, we captured 36 live birds and collected 10 dead birds.

Hasegawa shorttail fig 2Figure 2 A dead Pacific Swallow found under the nest © Masaru Hasegawa

After statistically controlling for confounding factors such as sex, we found that survivors had longer wings and shorter tails than non-survivors. Because there were no significant differences in keel and bill sizes, selection on body size or foraging apparatus should be negligible.

We conclude that wing and tail length are important targets of selection during a severe winter. Long wings and short tails would be favoured in aerial foraging, particularly during high-speed, straight-line flight. During severe winter weather, large prey, but not small prey, are available (Turner 1983), the traits related to high-speed flight might be particularly favoured to capture large, fast-flying prey. This might explain why Pacific Swallows, which feed on large prey, have long wings and short tails.

Selection against long tails is intriguing, because it indicates that long tails have no selective advantage during foraging, at least during severe weather. This contrasts with a potential aerodynamic advantage (or, high manoeuvrability) of long tails during foraging (Norberg 1994). Our latest comparative study supports the foraging cost of long-tailed swallows, because egg size decreases with increasing female fork depth, as predicted if deeply forked tails entail foraging costs during the egg formation period (Hasegawa et al. 2017). To clarify the evolution of diverse tail length in the genus Hirundo and other swallows, detailed studies of each selection episode should be the focus of future studies.


References and further reading

Brown, C.R. & Brown, M.B. 1998. Intense natural selection on body size
and wing and tail asymmetry in Cliff Swallows during severe weather. Evolution 52: 1461–1475. VIEW

Hasegawa, M. & Arai, E. 2017. Egg size decreases with increasing female tail fork depth in family Hirundinidae. Evolutionary Ecology (in press) DOI: 10.1007/s10682-017-9895-2. VIEW

Hasegawa, M., Arai, E. & Kutsukake, N. 2016. Evolution of tail fork depth in genus Hirundo. Ecology and Evolution 6: 851–858. VIEW

Japan Meteorological Agency 2016a. Naze 2016 (tsuki goto no atai). Accessed 7 April 2017 (in Japanese). VIEW

Japan Meteorological Agency 2016b. Huyu no tenkou. Accessed 7 April 2017 (in Japanese). VIEW

Norberg, R.A. 1994. Swallow tail streamer is a mechanical device for self deflection of tail leading edge, enhancing aerodynamic efficiency and flight manoeuvrability. Proceedings of the Royal Society of London B 257: 227–233. VIEW

Turner, A.K. 1983. Time and energy constraints on the brood size of Swallows, Hirundo rustica, and Sand Martins, Riparia riparia. Oecologia 59: 331–338. VIEW

Image credit

Featured image: Pacific Swallow, Hirundo tahitica | Mike Prince | CC BY 2.0

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