Assessing the usefulness of ptilochronology in the study of melanin‐ and carotenoid‐based ornaments in the Great TitParus major

Abstract

Ptilochronology is a method for assessing the nutritional condition of birds based on the width of daily growth bars on feathers. Wide growth bars reflect fast feather growth and as feather growth is costly, the width of the bars reflects the condition of a bird during moult (Grubb 1989). It is a very simple and inexpensive method, which makes it ideal for field research (Grubb 2006). In addition, as a sampled feather is soon replaced by a new feather, a process that would take place during natural moult, this method is also harmless to the bird. Ptilochronology has therefore become a popular method for assessing the nutritional state of birds in the wild (Grubb 2006). However, the efficacy of the method might differ, for example, between sexes (Grubb 1989, Takaki et al. 2001, Bostrom & Ritchison 2006) or age categories (Grubb et al. 1991). Kern and Cowie (2002) failed to find any relationship between the growth of different types of feathers taken from the same individual. Furthermore, other studies have concluded that the general validity of the method is unclear and that it can be used only under strictly controlled conditions (Murphy & King 1991). Despite these reservations, ptilochronology has been used in several studies of feather ornaments as an indicator of condition (Hill & Montgomerie 1994, Eeva et al. 1998, Keyser & Hill 1999, Doucet 2002, Senar et al. 2003, van Oort & Dawson 2005, Hegyi et al. 2007, Siefferman et al. 2008, Kimball 2009). The assumption is that these species moult body and contour feathers at the same time. Thus, if both ornaments and feather growth bars reflect condition (Griffith et al. 2006, Grubb 2006, Hill & McGraw 2006), then these two traits should covary. Carotenoid-based feather ornaments are expected to reflect a bird’s condition and there is evidence supporting this claim (von Schantz et al. 1999, Hill 2002, McGraw 2006a). Although melanin ornaments were thought not to reflect condition (McGraw 2006b), recent evidence suggests that they might be as condition-dependent as carotenoid-based ornaments (Griffith et al. 2006, Galvan & Alonso-Alvarez 2008). No clear-cut relationship between feather ornaments and feather growth has emerged from studies to date (see above). As ptilochronology is a very simple method and has great potential in field ornithology, we examined the relationships between both carotenoidand melaninbased ornaments and feather growth in a large sample of individuals of a wild passerine. We chose the Great Tit Parus major because expression of its carotenoid-based (Horak et al. 2000, Tschierren et al. 2003, but see Fitze & Richner 2002) and melanin-based ornaments (Fitze & Richner 2002, Galvan & Alonso-Alvarez 2008) is known to depend on condition during moult and feather growth. Thus, if feather growth also reflects condition during moult, we expected a positive correlation between the width of feather growth bars and the expression of both carotenoidand melanin-based ornaments.