Fork tails evolved differently in swallows and swifts.

Area of Science:

• Evolutionary Biology
• Ornithology
• Animal Behavior

Background:

• Ornamental traits can be driven by sexual selection or viability selection, but distinguishing between them is challenging when traits have dual functions.
• Deeply forked tails in birds are a classic example of a sexually selected trait that may also enhance aerodynamic performance.

Purpose of the Study:

• To investigate the evolutionary drivers (sexual vs. viability selection) of deeply forked tails in aerial foragers.
• To compare the evolution of forked tails in swallows (Hirundinidae) and swifts (Apodidae), two distinct groups of aerial insectivores.

Main Methods:

• Employed a phylogenetic comparative approach to analyze the evolution of tail morphology.
• Examined the relationship between tail fork depth and the lengths of central and outermost tail feathers across species.

Main Results:

• Found that central tail feather length decreased with increasing tail fork depth, particularly in swifts, supporting the role of sexual selection over viability selection.
• Observed that the increase in outermost tail feather length relative to tail fork depth was greater in swallows than in swifts, suggesting differential evolutionary costs.
• Evidence supports an optical illusion influencing the relative importance of tail feather lengths in sexual selection.

Conclusions:

• Sexual selection appears to be the primary driver for the evolution of deeply forked tails in swallows and swifts.
• The evolution of this ornamental trait varied between swallows and swifts, potentially due to differences in the costs associated with tail morphology and aerodynamic function.