Molt strategy and pattern

Birds were banded, allowing for identification and tracking of individuals over time. I took detailed notes about plumage, body molt, flight feathers molt, molt limits, shape of external retrices, iris color categorized following ^{Smithe´s color guide (1975)} and skull ossification (^{Ralph et al. 1996}, ^{Pyle 1997}). Ossification was determined following the methodology of Pyle (1997) assigning a percentage of the total area of the skull. In this methodology, 0 % means a skull with a single layer of bone and 100 % means a skull with two layers and complete pneumatization. The terminology used for plumage and molt follows ^{Humphrey and Parkes (1959)} modified by ^{Howell et al. (2003)}.

Ageing, molt and plumage description

The bird's age was determined with criteria such as: the presence/absence of brown streaks on the chest of juveniles (^{Miller 1961}), molt limits, shape of flight feathers, sharp in juvenal feathers, broad in individuals with basic feathers, growth bars, aligned in juvenal flight feathers and not aligned in basic feathers (^{Pyle 1997}). Additionally, I took notes describing other useful age characteristics of Z. capensis like iris color and skull ossification. I used the mentioned criteria to assign every bird to an age category, following the molt cycle-based age-classification system - WRP (^{Wolfe et al. 2010}, ^{Johnson et al. 2011}).

Molt strategy and molt pattern

I banded 102 Rufous-collared Sparrows and recaptured 42 during the sampling year. I found that besides the basic plumages, Z. capensis counts with a formative plumage. Also, I did not find traces of alternate plumage, as it will be discussed later. Thus, the molt strategy of Z. capensis in Bogotá is a Complex Basic Strategy (CBS; sensu^{Howell et al. 2003}). The prebasic molts present a complete molt pattern, i.e. molting all the feathers in the same molting process, and the formative plumage is the result of a partial molt in which flight feathers, primaries, secondaries, tertials, retrices and primary coverts, are not molted. Nevertheless, some captured individuals with a formative plumage had molted tertial feathers (30 %) or central retrices (17 %).

Ageing, molt and plumage description

In Bogotá, I categorized Z. capensis in eight age classes within the WRP system (^{Wolfe et al. 2010}, ^{Johnson et al. 2011}). The first category, first-cycle prejuvenal molt (FPJ), occurs in the nest when the bird substitutes the natal down plumage for the juvenal. I did not sample nests and I do not have information about the FPJ. The first-cycle juvenal (FCJ) birds are the easiest to identify. They have a juvenal plumage which highly contrasts with posterior plumages. Juvenal plumage presents cryptic colors, brown streaks on stomach and breast and the absence of the specie's characteristic rufous collar (Fig. 1a). Moreover, 90 % of FCJ birds showed a skull ossification between 0 and 20 % and a raw amber iris color (Table 1; Fig. 2a).

Figure 1 Plumages of Rufous-collared Sparrow (Z. capensis costaricensis) in Bogotá, Colombia. a. Juvenal plumage, b. Formative plumage, and c. Basic plumage. Pictures by Diego Cueva. 

Table 1 Distribution of the captured birds in the categories of iris color and skull pneumatization. Bold numbers highlight the higher percentage values at each age class. Some percentages may not reach 100 % because I could not conduct a proper inspection of some captured birds (n). i.e. molting shades may difficult the observation of the ossification. 

Figure 2 Iris color variation throughout different age stages. Colors according to ^{Smithe (1975)}. a. Color 223 Raw umber, b. Color 32 Chestnut, c. Intermediate between 32 and 140 colors, and d. Color 140 Pratt’s Rufous. Pictures by Diego Cueva (a, b) and Julian Avila-Campos (c, d). 

After FCJ, Z. capensis reaches a first-cycle preformative molt (FPF). The molt in FPF age does not include flight feathers. FPF birds presented molting body feathers, brown juvenal feathers on head and remaining streaks on chest, skull ossification under 40 % with a majority under 20 % (Table 1) and a raw amber/chestnut iris color (Fig. 2b). The acquired formative plumage named the next age class, first-cycle formative (FCF; Fig. 1b). As expected (^{Miller 1961}, 1962), formative plumage is superficially similar to later basic plumages (Fig. 1c). Thus, body feathers became useless to classify individuals into an age class. FCF Rufous-collared Sparrow molted all secondary coverts but not primary coverts, they exhibited molt-limits between greater coverts and primaries coverts. Juvenal primary coverts had an external buffy (Fig. 3a) color that almost disappears in posterior moltings (Fig. 3b). Also, external retrices (juvenal) were narrower shaped, and flight feathers presented aligned growth bars. The preformative molt extent is similar to the one described for Z. capensis in other regions (^{Pyle et al. 2015}) and for other Zonotrichia species (Pyle 1997). Furthermore, 77 % of captured FCF individuals had ossification between 20 and 60 %, and 54 % of the total FCF birds did not get over 40 % ossification. Finally, 77 % FCF individuals presented a chestnut colored iris, and 23 % presented an intermediate iris color, between chestnut and definitive Pratt's Rufous color (Table 1; Fig. 2b).

Figure 3 Color variation in primary coverts of Z. capensis in Bogotá. a. A FPF individual with buffy color on the external border (arrow) of the retained primary coverts and, b. a DCB individual without the buffy border after molting the same group of feathers. 

After FCF, Z. capensis turned into a second-cycle prebasic molt (SPB) individual. At this stage, 64 % of the individuals still have skull windows, most of them with skull ossification between 80 and 95 %. Also, 68 % of the birds presented an intermediate iris color between chestnut and Pratt's Rufous (Fig. 2c). Finally, juvenal primaries, secondaries, retrices and primary coverts can be identified before they are completely molted, according to the descriptions of the FCF birds. After second prebasic molt, Z. capensis enters into the second-cycle basic (SCB) age class. Birds in this stage did not show differences with subsequent plumages. Nevertheless, some individuals, presumably at the beginning of the SCB, presented skull windows and iris that did not reach a Pratt's Rufous coloration yet (Fig. 2c) permitting age identification. However, at this stage, Z. capensis develops the final iris color, Pratt's Rufous, and completes skull ossification, making birds indistinguishable from birds in definitive-cycle basic (DCB). This forces categorization as after first cycle (FAJ), following ^{Wolfe et al. (2010)} and ^{Johnson et al. (2011)}.

Finally, definitive prebasic molt (DPB) birds differ from previous molt stages because the primaries, secondaries and primaries coverts are not juvenal and do not present the characteristics described in the FCF section. Furthermore, the individuals have a Pratt's Rufous iris (Fig. 2d) and a completely ossified skull. The definitive-cycle basic (DCB) birds have a plumage that will not change in posterior cycles. Moreover, they present a Pratt's Rufous iris color and 100 % skull ossification.

Additionally, I captured several times an individual 7.4 years old, first banded by the Ornithology Group of Universidad Nacional de Colombia (GOUN) on the 20^th Sep 2006 and last time captured on the 16^th Feb 2014. This bird allowed me to confirm the characteristics of DCB. Also, outstanding captures were two FCF females with high feather wear, category three over four, and developed brood patch. Moreover, seven males with suspended molt (P2-3), all with a cloacal protuberance. These captures highlight that in Bogotá Z. capensis reach sexual maturity and can reproduce at an early age class, during the FCF. Finally, the seven males are evidence of molt suspension due to overlapping with reproduction. Similar cases were found in Tucumán, Argentina (^{King 1972}, 2009).