Birds exhibit remarkable variation in plumage patterns, involved in mate choice, social signaling, camouflage, and many other functions; melanin-based patches have been identified as having a principal role in plumage patterns that mark intraspecific social dominance (Mason and Bowie 2020). Melanins are the most common pigments in birds and can be distinguished in two forms, depending on concentration and distribution within the feather, eumelanin is responsible for black, grey, and/or dark brown colors, whereas phaeomelanin is responsible for warm reddish brown to pale buff (van Grouw 2021).
Birds with atypical plumage often call the attention of birders but receive little attention beyond reporting one-off sightings by the scientific community (Aguillon and Shultz 2023). There is a wide nomenclature for these aberrations with decrease, absence, or altered distribution of melanin, which are not always used appropriately, but the commonest are Albino, Brown, Dilution, Ino, Leucism, Melanism, and Progressive Greying (van Grouw 2021).
The Rufous-collared Sparrow Zonotrichia capensis has one of the largest distributions of any Neotropical passerine, from southern Mexico to Chile; with broad habitat tolerances, such as coastal, paramo, humid forest, and urban areas, from lowlands, at sea level, to highland areas (Rising and Jaramillo 2020). There are 27 subspecies recognized, that vary mainly in depth of coloration, head pattern, and size (Op. cit.). Broadly, Z. capensis has a grey crown with black lateral, malar, and postocular stripes, a grey superciliary, a tawny or rufous nuchal collar that becomes wider on the sides of the neck, white throat, foreneck with a usually incomplete black band; back varying from sandy to dark brown streaked with black; breast greyish-white, becoming whitish below, laterally washed with brownish; two white or whitish wing-bars, tail feathers brown with rufous-brown edging; iris reddish-brown; bill horn-colored to greyish, with black culmen and tip; legs brownish-pink; sexes are alike in color, the female measuring slightly smaller; the most dissimilar subspecies are at the extremes of their distribution, Z. c. insularis from the Caribbean has a complete black band in the foreneck; whilst Z. c. australis from the further south hasn´t the black lateral stripe in the crown (Chapman 1940; Rising and Jaramillo 2020).
We identified and re-identified the Z. capensis aberrations following van Grouw (2021): Leucism (white feathers are present typically on the face, the primaries, and belly, often in bilaterally symmetrical patches); Progressive Greying (in an early stage white feathers are often randomly spread about the plumage); Albino (entire plumage is all-white, with red irides and pink feet and bill); Brown (black plumage becomes brown, rufous is unaffected, feet and bill coloration is slightly lighter); Ino (black becomes very pale brown/cream to dark brown; rufous shows paler; irides, feet and bill are pinkish); Dilution (black becomes silvery-grey; rufous turns buff/cream in Isabel, or unaffected in Pastel); Melanism (an increase of black in eumelanised plumage; of reddish brown in phaeomelanised, or presence of an altered pattern).
Zonotrichia capensis is a well-known bird in the Neotropics, but photographic records of its aberrations are not always readily diagnosable and so to facilitate the identification and comparison of abnormal plumage coloration types we present a plate illustrating the commonest aberrations and their variants (Fig. 1), based on van Grouw (2021) descriptions and online photos (Table 1).
There are previously published records for 29 individuals with color aberrations in Z. capensis (Chapman 1940; Fuentes and González-Acuña 2011; Urcola 2011; Cadena et al. 2015; Meyer and Crozariol 2020; Tiravanti et al. 2021; do Amaral 2022; Buitrón-Jurado et al. in press), most of them (n=11) reported as Leucism, but the aberrations have at times been erroneously identified. Chapman (1940) reports “A Bolivian mutant. Except for a slight tint on the nape, the rufous color is lacking, and the black neck-marks have entirely disappeared”, it is a case of Melanism, where there was switching from phaeomelanin to eumelanin (Fig. 2A); Melanism is also evident when the normal pattern and pigment distribution is changed, but the plumage is not necessarily darker (van Grouw 2021).
The eleven individuals previously reported as Leucism (Fuentes and González-Acuña 2011; Urcola 2011; Cadena et al. 2015; Tiravanti et al. 2021) have white feathers randomly spread, do not have the bilaterally symmetrical pattern, then actually there are Progressive Greying cases. Even one individual reported by Tiravanti et al. (2021), was recorded ten months later in the same locality of Plaza de Yanque, Arequipa, showing more white feathers, as is diagnosable for Progressive Greying. The two individuals reported as Dilution Pastel (Urcola 2011; Amaral et al. 2022) fit more accurately with Brown because feathers are mostly brown and bare parts are slightly lighter. The individual reported as schizochromism in Cadena-Ortiz et al. (2015), is Ino actually, because the original black and rufous feathers are very pale; feet and bill are yellowish. Schizochroism is an unsuitable term that does not distinguish between different mutations (van Grouw 2021). Finally, the individual reported as Dilution Isabel (Meyer and Crozariol 2020) fits with Brown, already bleached further, because has no dilution of its phaeomelanin and its bare parts are slightly lighter.
In online databases (eBird and iNaturalist) we also found photos of 28 individuals of Z. capensis with color aberrations; we classify the kind of aberrations based on van Grouw (2021). Two individuals (https://ebird.org/checklist/S34804144 and https://ebird.org/chile/checklist/S74353499) appear to show increased carotenoids, but we classify them as Melanism, because, sometimes, carotenoid mutations, like Xanthochroism, Flavism, and Erythrism, are often applied to birds that are afflicted by a melanin mutation, and not due to an increase of carotenoids; any reduction in melanin results in the underlying carotenoids becoming clearer (van Grouw 2021), but normally Z. capensis, has rufous only on its neck and back, so the reddish brown in the face of these individuals is due to an increase in phaeomelanin—which is why both of these individuals more correctly represent cases of Melanism with phaeomelanised plumage.
In summary, 29 individuals aberrant of Z. capensis are in the literature (Chapman 1940; Fuentes and González-Acuña 2011; Urcola 2011; Cadena et al. 2015; Meyer and Crozariol 2020; Tiravanti et al. 2021; do Amaral 2022; Buitrón-Jurado et al. in press) and 28 individuals in online databases, in addition to these 57 records (Table 1), we present two new cases, involving five individuals, a family of two adults and two young, all with Melanism and one with full white plumage probably Leucism or Progressive Greying.
Phaeomelanised family. - On 14 May 2022, DP encountered two melanistic Z. capensis on a farm 8 km north of the town of El Chaco, Napo province (0°16´0” S, 77°51´31” W; 1730 m). On 18 May 2022, LS photographed these individuals (Fig. 2B, C). The birds were seen feeding in two different groups with at least five normal-colored individuals. These aberrant birds were identified as Melanistic Rufous-collared Sparrows because they both displayed an abnormally black head, breast, and belly, characters that fit with Melanism eumelanised plumage (van Grouw 2021). The only difference between the two adult individuals, on 18 May 2022, was that one individual had pin feathers on its head (Fig. 2C).
On 3 Dec 2023, HC et al. went to the site and found that both melanistic individuals, looked almost identical and they were feeding two young individuals (Fig. 2D), that showed yellow gapes, pale grey bills, and dark brown in their plumage, particularly on their faces (without evidence of malar and postocular stripes, and obvious streaks on its breast, and its belly, as is common even in young birds). On 11 Jan 2024, DP returned to the site and recorded that both adult melanistic individuals kept feeding two young individuals, young individuals now do not have evident gapes, and all plumage was darker, particularly on the face (Fig. 2G).
Both young birds are also melanistic, on the first encounter (3 Dec 2023) they would be at least 21 days old. Fledging is at 10 days and fledglings stay hidden for 10 days more before they begin following their parents out in the open (Miller and Miller 1968). Juvenile normal plumage shows ventral streaks that progressively disappear; all identifiable remnants of such plumage are lost by 80 days of age (Miller 1959).
Reproduction evidence in aberrant individuals is relevant because avian sexual selection is linked to coloration in most species (Galván and Møller 2009). Further, this kind of Melanism in Z. capensis could be dominant in inheritance as in Pyrocephalus rubinus (van Grouw and Nolazco 2012). Finally, this family could be an atavism case, but the ancestor of Zonotrichia is unknown, however, is known that Junco is the sister genera (Barker et al. 2013; Lougheed et al. 2013), and the male of Dark-eyed Junco Junco hyemalis also have a head pattern similar to this melanistic family.
The Blanquito.- On 10 Jun 2023, RN recorded an abnormally white Z. capensis in an urban (“Bonanza”) Park in the city of Quito, Pichincha province (0°07´53” S, 78°28´46” W; 2870 m). HC returned on 11 Jun when the bird was photographed (Fig. 2F, G). It was identified by its size and behaviour within a group of at least four normal-colored Z. capensis individuals. Nowadays, the individual is seen often by RN, it remains feeding with at least 10 normal plumaged individuals. The abnormal bird’s plumage was completely white. Its iris was dark brown, and its bill and feet were pink, which are characteristics that fit with full Leucism but also with Progressive Greying in a late stage (van Grouw 2021). So for parsimony, we count it as Progressive Greying. This individual has been seen for nine months it is relevant because aberrant individuals usually have a lower survival rate; may suffer harassment by conspecifics and can have deficiencies in how they reflect solar energy; lack of pigments also affects the structural properties
of feathers; have a higher risk of predation due to their conspicuousness (Colombo et al. 2018).
There is a total of 62 cases of Z. capensis with color aberrations. All kinds of the most common aberrations detailed by van Grouw (2021) were represented in this species (Table 1), even the most variants of each aberration (Fig. 1). One individual that could not be identified with certainty but the most probably it is most likely a variant of Dilution (Fig. 6m in Buitrón-Jurado et al. In press). In Progressive Greying, we found individuals with pink bills and feet (n = 6) and also with normal-colored bills and feet (n = 33). In Leucism individuals one shows normal colored bills and feet and the other one pinkish bills and feet. In Melanism, two of the three ways proposed by van Grouw (2021): (1) any individual with all of the plumage darker; (2) two individuals with normally dark markings bolder and noticeably ‘overrun’ their typical boundaries (Fig. 2A, and https://ebird.org/argentina/checklist/S70684198); and (3) six individuals showed the normal pattern and pigment distribution is changed, but the plumage is not necessarily darker (Fig. 2B-E, https://ebird.org/checklist/S34804144 and https://ebird.org/chile/checklist/S74353499).
To summarise aberrations as grouped as Progressive Greying (n= 39), Brown (n= 8), Melanism (n= 8), Leucism (n= 2), Albinism (n= 2), Ino (n= 2), and Dilution (1). Records come from nine countries: Ecuador (n= 25), Chile (n= 11), Argentina (n= 10), Peru (n= 7), Brazil (n= 3), Costa Rica (n= 3), Bolivia (n= 1), Guatemala (n= 1), and Uruguay (n= 1). Finally, 34 records come from rural habitats in contrast to 28 from urban (Table 1).
Consistent patterns of atypical plumage patches could provide insight into the evolution of color patterning (Aguillon and Shultz 2023), as five individuals have been reported as Leucistic from different countries, showing white heads (Fuentes and González-Acuña 2011; Cadena-Ortiz et al. 2015; Tiravanti et al. 2021), from all list, 20 individuals identified as Progressive Greying, show some to all white head (Table 1); also our melanistic family showed an abnormal head pattern (Fig. Fig. 2B, C), similar to a male of Dark-eyed Junco Junco hyemalis, which is a sister genus (Barker et al. 2013; Lougheed et al. 2013). So the head could be an important feature for mate choice, and then in the evolution of Z. capensis. Whereas plumage patterns on the other parts of the body could be essential for other functions, for example, it has been suggested that the dorsal patterning and coloration in Eremophila alpestris, closely similar to Z. capensis, is associated with environmental conditions to avoid visual detection by avian predators (Mason and Bowie 2020).
The frequencies of birds with plumage aberrations are related to genetic factors, especially in populations with a high degree of endogamy (Bensch et al. 2000), that would not apply to Z. capensis, which is common and abundant (Rising and Jaramillo 2020). On the other hand, urban or highly polluted environments would seem to favor the greater presence of aberrations (Møller and Mousseau 2007), although Z. capensis that showed aberrations recorded a similar number in urban as well as rural habitats. We highlight that it is important to continue publishing these kinds of records to show patterns if they exist.