Incidence of eccentric molt in first-year Wrentits increases with fledge date

Abstract

In some passerines, the extent of preformative molt varies among individuals. Wrentits (Chamaea fasciata) undergo either a complete preformative molt or an eccentric (i.e., incomplete) preformative molt where some juvenile remiges are retained through the first cycle. Factors that influence the incidence and extent of molt are largely unknown. Using a 10‐yr data set from the Palomarin Field Station in central coastal California, we quantified the incidence of eccentric molt and the degree to which variation in the incidence was associated with fledging date and weather. From 1999 to 2009, 159 Wrentits were banded as nestlings and subsequently recaptured. Of these, 21% of first‐year Wrentits underwent eccentric molt. We used logistic regression and an information theoretic approach to compare models with fledging date, weather (annual precipitation and breeding‐season temperature), and a random effect of year as predictors of the incidence of eccentric molt. Our top model included a random intercept term for year and a fixed effect for the effect of fledging date; birds that fledged later in the season were more likely to undergo eccentric molt. Although the proportion of individuals that underwent eccentric molt varied among years, models with breeding‐season temperature and annual rainfall showed little to no support. Our results suggest that the incidence of eccentric molt is more strongly associated with fledging date than with annual variation in weather. The absence of a correlation with weather suggests that weather does not impose an energetic constraint on molt or, if it does, that birds are constrained in their ability to respond to changes in weather by adjusting the extent of their preformative molt. Other factors, such as nestling condition, may provide alternative explanations for year‐to‐year variability in the incidence of eccentric molt.

Muda Preformativa Excéntrica en Un Espécimen De Colección De La Tangara Azul Y Amarilla (Pipraeidea Bonariensis Darwinii Bonaparte, 1983) (Aves: Thraupidae)

“…Moreover, we cannot eliminate the possibility of this molt pattern being prevalent for other subspecies. Previous studies reporting more extensive molt patterns for other taxa suggest possible associations with age-sex groups (Pyle, 1997a(Pyle, , 1997b(Pyle, , 1998Guallar et al, 2016;Díaz et al, 2018), fledge date (Elrod et al, 2011;Crates et al, 2015), latitude and migration (Pyle, 1997a(Pyle, , 1997bHowell, 2010;Johnson, 2013;Morganti et al, 2013), and occupancy of open habitats and high levels of solar exposure for long-distance migrants (Pyle, 2008;Pyle & Kayhart, 2010). Molt patterns are among the most important tools used by bird banders to determinate the age of birds (Pyle 1997a(Pyle , 1997b.…”

Defining Moults in Migratory Birds: A Sequence-based Approach

“…Remigial replacement during preformative moults is likewise fixed, following the same sequences described above for prebasic moults, despite varying nodal positions among primaries and a proximal wave from S1 appearing to be suppressed in many passerines exhibiting eccentric moults (Pyle 1997, Gargallo 2013, Jenni and Winkler 2020. In these and other species exhibiting partial but not eccentric preformative moults, distal replacement from a node at the second tertial appears to be prioritized among secondaries, with for example S6 through S4 being replaced distally and at decreasing rates of frequency (Pyle 1997, Gargallo 2013; replacement extent during eccentric moults has been correlated with hatching date within the breeding season (Elrod et al 2011).…”

Defining moults in migratory birds: a sequence‐based approach

“…In these and other species exhibiting partial but not eccentric postjuvenile/preformative moults, a proximal wave from S1 appears to be suppressed and the distal replacement from a node at the second tertial appears to be prioritized, with for example S6 through S4 being replaced distally and at decreasing rates of frequency (Pyle 1997, Gargallo 2013). Replacement extent during eccentric moults has been correlated with hatching date within the breeding season (Elrod et al 2011).…”