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The article focuses on a multiple-factor breeding-threshold hypothesis ( BTH) proposed by M. V. Studd and R. J. Robertson to explain the general pattern of delayed plumage maturation ( DPM) in North American passerine birds. Topics include their finding that, everything else being equal, longer-lived species should be more likely than short-lived ones to delay the maturation of their plumage and DPM is evidenced by a distinctive, dull subadult male plumage in first potential breeding season.