Phenotypic quality and molt in the barn swallow, Hirundo rustica

Phenotypic quality may determine the development and expression of secondary sexual characters. We studied the relationship between molt and several measures of phenotypic quality in the sexually size- dimorphic barn swallow (Hirundo rustica) in its winter quarters in Namibia. Males were in a more advanced stage of molt than females and juveniles, and the speed of molt as determined from the residual of the regression of the size of the gap in wings caused by missing and growing feathers on wing molt score (residual wing raggedness) was also higher in males than in females and juveniles. Male barn swallows with long and symmetric tail feathers had a more advanced stage of molt and molted at a higher speed than males with short and asymmetric tails. Long-tailed females had a delayed molt, and females with asymmetric tails had less advanced molt and lower rates of feather growth than females with symmetric tails. Molt of secondaries in juveniles also appeared to be less advanced if they had long tails. Adult barn swallows molted their tail feathers in an irregular sequence with the longest, outermost tail feather usually replaced before the second or the third outermost feathers. Good body condition was positively associated with a high molt score for some feather tracts and a rapid wing molt in adult females and tail molt in juveniles. Mallophaga were only weakly negatively associated with primary and secondary molt score in adult females and speed of wing molt in adult males. In conclusion, phenotypic quality of adult male barn swallows as reflected by the expression of their secondary sexual character during the previous molt reliably reflected stage and speed of current molt.