Convergent evolution of the tradeoff between egg size and tail fork depth in swallows and swifts

Abstract

Convergent evolution provides strong evidence of the power of natural selection, particularly for distantly related taxa. Swallows and swifts are such distantly related taxa; both are specialised to feed on flying insects and have similar morphological features, such as long wings. These birds also exhibit deeply forked tails in some species, but their function remains unclear; some have argued that fork tails have evolved due to sexual selection to attract mates, while others claim that viability selection for efficient foraging favours fork tails. A recent phylogenetic analysis found the negative relationship between female tail fork depth and egg size in swallows perhaps due to foraging costs of fork tails during egg formation, but its generality remains unclear. Here, using swifts, which differ from swallows by foraging on weak‐flying insects, we found that egg size significantly decreased with increasing female fork depth. Because female fork depth was not significantly related to clutch size, clutch size would not compensate for the relationship between egg size and fork depth. The current finding using swifts, together with the previous finding in swallows, provide strong support for an evolutionary tradeoff between the female plumage ornament and reproductive investment, as predicted by sexual selection theory.

Fork tails evolved differently in swallows and swifts

“…tribe Chaeturini, which is out‐group of tribes Apodini and Collocaliini), for which tails might have different functions from those of other species (e.g. they use their tails as a prop when climbing: Thomas, 1997); thus, as in our previous study (Hasegawa & Arai, 2018), we focused on a monophyletic clade of the two tribes Apodini and Collocaliini, all of which have tails without spines, to test the function of forked tails (del Hoyo & Collar, 2016; Päckert, Martens, Wink, Feigl, & Tietze, 2012; Tietze, Wink, & Päckert, 2015). Whenever sex‐specific values could be obtained, we used those from males (though sexual dimorphism is small in most cases, which prevents meaningful analysis of each sex).…”

“…Because of the large variation in measurements within species, trait sizes were rounded down to the nearest integer to prevent overfitting the model (e.g. Hasegawa & Arai, 2017, 2018; Hasegawa et al, 2016).…”

“…Norberg, 1994;Thomas, 1993; see Introduction), we focused on relative fork depth, that is (fork depth)/(central tail feather length) as in previous studies (e.g. Hasegawa & Arai, 2017, 2018, although the use of absolute fork depth yielded similar results (see Results). In fact, relative and absolute fork depth were highly correlated (Pearson's product-moment correlation coefficient, r = .98, n = 72, p < .0001).…”

“…Friedman, McGraw, & Omland, 2013; Prager & Andersson, 2010). In fact, recent studies have shown that deeply forked tails have a reproductive cost in terms of a small egg size in swallows and swifts, as predicted by the foraging cost of deeply forked tails in these income breeders (Hasegawa & Arai, 2017, 2018; also see Hasegawa, Arai, and Kutsukake (2016) for a negative relationship between tail fork depth and prey size). However, because forked tails would enhance some components of flight performance (e.g.…”

“…egg size, here) does not clarify whether forked tails are costly or beneficial in total. For example, reduced reproductive output might be offset by enhanced viability due to predator escape (sensu Hasegawa & Arai, 2017, 2018). Therefore, it remains unclear whether sexual selection explains the evolution of fork depth.…”