Behavioural and morphological correlates of variation in the extent of postjuvenile moult in the SiskinCarduelis spinus

Abstract

Siskins Carduelis spinus show great variation in the acquisition of adult plumage, so that yearling birds can be classified as either “delayed” (i.e. still showing yearling plumage) or “advanced” (i.e. with an adult plumaged appearance). The extent of moult in males is related to the size of their black bib, which in turn is highly correlated with their social dominance rank. Autumn male body mass is higher in advanced than in delayed moult birds, but by winter the relationship is reversed, suggesting a trade‐off between investment in moult and subsequent body condition. Results were similar in Spain and Britain. A possible cost of advancing moult is suggested by an analysis of aggressive interactions at bird feeding tables which showed that adult males discriminate between delayed and advanced birds, directing most aggression towards adult‐looking yearling males. The results suggest that the variation in the extent of postjuvenile moult is not only related to energetic constraints but has other important behavioural and ecological implications.

The signaling function of plumage characteristics in female tree swallows and implications for reproductive success and life-history decisions.

“…Their finding suggests that a lack of testosterone was responsible for DPM. With respect to tree swallows, McCarty and Secord (2000) My results suggest that there may be inter-and intrasexual costs to displaying higher amounts of adult-coloured plumage in SY female tree swallows, as has been found in other species (Senar et al 1998;Greene et al 2000). Yet what SY females are potentially communicating and to which potential receiver(s) has not been tested here and needs to be …”

“…If females with subadult plumage are perceived as less threatening to other females, the behavior of conspecifics may also depend on variation in this plumage trait, and SY females with higher amounts of iridescent blue-green plumage may experience more aggression from other females. In species where SYs have varying degrees of subadult plumage, individuals with 54 more adult-like plumage are commonly targets of aggression from older individuals (Hill 1989;Greene et al 2000) and under certain conditions can have decreased body condition and increased mortality rates compared to birds with less adult-like plumage (Grant 1990;Senar et al 1998). If more iridescent blue-green SY females are experiencing increased harassment from ASY females, this may account for the reduced reproductive success (Table 3.2) and body condition (Figure 3.2) of these females in my study.…”

“…In other species, younger birds that exhibit more adult-like plumage experience increased aggression from older conspecifics, potentially because of their increased ability to attract mates (Hill 1989;Senar et al 1998). SY female tree swallows that exhibit higher amounts of DPM may be displaying their age and inexperience more honestly to older individuals and therefore could face less harassment compared to more adult-coloured SY females.…”

“…On the other hand, DPM has been found to be beneficial to SY females by reducing aggression from males (Stutchbury and Robertson 1987a) and females (Lozano and Handford 1995; Chapter 2) during the breeding season. Perhaps there are social consequences of displaying more iridescent bluegreen plumage for SY females (Senar et al 1998;Greene et al 2000). If iridescent bluegreen plumage is a signal of breeding experience in female tree swallows, this plumage colouration may elicit more aggression from ASY females and may represent an evolutionary motivation for the maintenance of DPM in SY female tree swallows.…”

The use of mass and fat reserve measurements from ringing studies to assess body condition

“…Increased mass may allow for more rapid moult, or may be traded off against moult duration and frequency so that there is no direct relationship between mass and fitness (eg Senar et al 1998, Bojarinova et al 1999, or may be reduced during moult to compensate for reduced flying ability (Lind & Jakobsson 2001, Senar et al 2002.…”