Parasites of the Shiny Cowbird, Molothrus bonariensis , and the Austral Blackbird, Curaeus curaeus , (Passeriformes: Icteridae) in Chile

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Introduction
Parasitism is ubiquitous in nature and although recent advances have revealed important details of some host-parasite associations (e.g. Doña et al., 2019), we are still far from understanding in depth key points of host-parasite dynamics. The relatively vast avian diversity and even richer diversity of gastrointestinal and external parasites make the study of parasite transmission dynamics an obscure subject. For example, transmission of ectoparasites generally happens through direct contact between individuals, either between adult birds (horizontal transmission) or between parents and their offspring (vertical transmission) (Eichler, 1963;Marshall, 1981;Darolova et al., 2001). In the case of gastrointestinal parasites, because they often have complex life cycles and generally need a fecal-oral route to infect, transmission also occurs from ingestion of eggs or other infective stages of the parasite, often within an intermediate host (Atkinson et al., 2008). The Shiny Cowbird, Molothrus bonariensis (Gmelin, 1789), and the Austral Blackbird, Curaeus curaeus (Molina, 1782), are two locally common icterids that have a similar size, behavior, and diet (Fraga, 2011). Although the Shiny Cowbird and Austral Blackbird are largely distributed in South America and Patagonia, respectively, they are sympatric only in Chile, where they share a large part of their distribution range and are often observed in mixed flocks foraging together (Fraga, 2011;Couve et al., 2016). This close proximity between these two species might suggest interspecific gastrointestinal and ectoparasite exchange. However, to date, the few studies of parasites of Shiny Cowbirds and Austral Blackbirds were centered on taxonomical descriptions based on morphological traits of new species instead of the host-parasite dynamics (see Table 1 and references therein).
In the present study, we examined the external and gastrointestinal parasites of sympatric Shiny Cowbirds and Austral Blackbirds in Chile, with the aim to determine whether sharing environment and diet could influence interspecific parasite transmission between these hosts.
In Shiny Cowbirds, the Brueelia lice found (Figure 2A, 2B) corresponded to the classic descriptions of this genus in New World Icterids (for details, see Cicchino & Castro, 1996). In particular, the macule on the pulvinar edge of the frontoclipeal suture placed these parasites in the amazonae subgroup. Within this group, males of B. bonariensis and B. americana Cicchino & Castro, 1996 could be very similar, however, it was possible to clearly discriminate from the latter by its head's shape, the II-IV brownish tergites, and lack of the postespiracular setae in the IV tergite  (Cicchino & Castro, 1996). From each bird host, we collected lice that were classified within the Philopterus-complex ( Figure 2C-E) due to their e.g., preantennal head very narrow, marginal carina with indentation on median side, and terminal segment of female abdomen with paired pseudostyli (more details in Mey, 2004). The lice from this complex are specialized to live on the head of their hosts and are usually found on perching birds (Mey, 2004). Although these lice found shared mutual similarities, they represented two distinctive morphotypes, clearly differing morphologically from one another (see Price et al., 2003), and did not correspond to any known species.
In the Austral Blackbird, a louse showed morphological characteristics that suggested a good fit to males of the genus Myrsidea ( Figure 2F), according to Waterston (1915). Unfortunately, this louse was distinctive enough not to fit any of the nearly 350 described species for Myrsidea (Valim et al., 2011;Valim & Weckstein, 2013). Nevertheless, this goes in line with the ideas by the latter authors, suggesting that to date only a small fraction of species in the genus Myrsidea have been described. Shiny Cowbirds, 37% of individuals (10/27), were infested with three feather mite species of the family Proctophyllodidae (Table 2a and Figure 3): Amerodectes molothrus (Mironov, 2008) (Pterodectinae), and two morphotypes of Proctophyllodes (hereafter Proctophyllodes sp. 1, and Proctophyllodes sp. 2) (Proctophyllodinae). On Austral Blackbirds, 53.6% of birds (15/28) had feather mites (Table 2b and Figure 3), belonging to the species Amerodectes sp. and Proctophyllodes sp. 3. Amerodectes mites found on both bird species ( Figure 3G-I) met the morphological criteria of the genus (see Valim & Hernandes, 2010). The only female found on the Shiny Cowbird generally corresponds to the description of A. molothrus, described from this bird species in Brazil (Mironov et al., 2008), in having similar shape of the dorsal shields, lobar region, and the pattern of ornamentation on the hysteronotal shield. However, in contrast to the original description, our specimen lacks the rudiments of humeral shields and has noticeably shorter terminal appendages that could probably be a manifestation a geographic variability of this mite species widely distributed on Molothrus species. Females of the genus Amerodectes found on the Austral Blackbird have different shape of the lobal region (with much shorter opisthosomal lobes than On one Austral Blackbird carcass, we found a male specimen of flea (Figure 4) that, according to Hastriter & Schlatter (2006), was classified as Dasypsyllus (Neornipsyllus) cteniopus Jordan & Rothschild, 1920. This represents a rare finding on host, because fleas often abandon their host once dead. These fleas are commonly found in bird nests (Lewis & Stone, 2001). Dasypsyllus species often parasitize passerine birds, however, is not uncommon to find them on hosts of other avian orders, and even on mammals, although the latter hosts have been reported as rare and sporadic (Hastriter & Schlatter, 2006). In Chile, this parasite has been found in nests of the Magellanic Tapaculo Scytalopus magellanicus (Gmelin, 1789), Chucao Tapaculo Scelorchilus rubecula (Kittlitz, 1830), Thorn-tailed Rayadito Aphrastura spinicauda (Gmelin, 1789), Rufous-Collared Sparrow Zonotrichia capensis (Müller, 1776), and in specimen from the Shiny Cowbird), and the hysteronotal shield entirely covered with ornamentation of small lacunae. Males of this mite belong to the species group having long filiform setae h3, and by the shape of dorsal shields and the length of the genital apparatus they are most similar to Amerodectes xanthocephali Mironov and Overstreet, 2015 described from the Yellow-headed Blackbird Xanthocephalus xanthocephalus (Bonaparte, 1826) in Oregon, USA (Mironov & Overstreet, 2015). Males of Proctophyllodes sp. 1 ( Figure 3B) have the genital apparatus not extending to the level of setae ps3 and the opisthogastric shield split into two longitudinal pieces that formally allows to refer this mite to the musicus species group by Atyeo & Braasch (1966). However, extremely long, narrow and widely separated pieces of the opisthogastric shield and narrow terminal lamellae in males give evidence that this mite could belongs to the thraupis species group (phylogenetic clade) (Klimov et al., 2017). Proctophyllodes sp. 2, also found on the Shiny Cowbird ( Figure 3C, 3D), has an H-shaped opisthogastric shield and wedge-shaped genital sheath with a strongly sclerotized basal ring in males that univocally refers it to the pinnatus group (Atyeo & Braasch, 1966). Males of Proctophyllodes sp. 3, found in the Austral Blackbird ( Figure 3F), have extremely long whip-like genital sheath extending far beyond the posterior end of the body of males. Therefore, following the revision by Atyeo & Braasch (1966), this mite could be formally referred to the glandarinus species group. However, taking in attention the structure of the opisthogastric shield represented by a pair of extremely long and thin sclerites bearing setae g and ps3, and narrow ribbon-like terminal lamellae, this mite definitely belongs to the thraupis species group as interpreted by Klimov et al. (2017). Within this group, Proctophyllodes sp. 3 is most similar to Proctophyllodes longiphyllus Atyeo and Braasch, 1966 associated with the Baltimore Oriole Icterus galbula (Linnaeus, 1758) and Hispaniolan Oriole Icterus dominicensis (Linnaeus, 1766) in North America. House Wren Troglodytes aedon (Vieillot, 1809) (Turienzo & Di Iorio, 2013;Beaucournu et al., 2014). We argue that the present record is possibly an accidental finding, because Austral Blackbirds have been reported predating on nests of the Thorn-Tailed Rayadito (Vergara, 2007). Thus, we cannot confirm the present finding as a natural parasite-host association.
The two nematodes found in Shiny Cowbirds agreed with descriptions by Macko et al. (1977) and Zhang et al. (2004) for D. nasuta, andof Sandground (1928) for male T. paucispina. In the case of acanthocephalans, we identified only gravid females of M. papillosus, and two females of one fusiform species, presenting a lacunar system with reticular anastomosis, long cylindrical proboscis, with numerous small hooks at the base, and a terminal genital pore. These features agreed with characteristics of the genus Plagiorhynchus (Yamaguti, 1963). In the Austral Blackbird, the Capillaria specimens found were males (terminal anus, membranous caudal alae, long and thin spicule with spiny sheath) and females (vulva without a flap or protrusions), but their morphology did not match any previously known species described for related host (Yamaguti, 1961). Only one specimen of cestode was found, a female of Anonchotaenia sp. This parasite did not have a rostellum or hooks, had very short proglottids (except for the posterior one), and longitudinal muscles arranged in two distinct layers (Yamaguti, 1959). Mediorhynchus papillosus was also present in the Austral Blackbird but at higher prevalence and intensity than in the Shiny Cowbird. As typical for acanthocephalans, M. papillosus requires an invertebrate as intermediate host to complete its cycle (Atkinson et al., 2008). Therefore, it is possible to conclude that both birds share at least one invertebrate species in their diets. We did not find Anonchotaenia longiovata (Fuhrmann, 1901) in the birds sampled, the only helminth previously reported in the Austral Blackbird (Rausch & Morgan, 1947).

Shared parasite fauna
Host shifts are widespread across parasite taxa (Sieber & Gudelj, 2014). Accordingly, we found that most genera of ectoparasites on Shiny Cowbirds and Austral Blackbirds were shared by these birds. However, the chewing louse genera Brueelia and Myrsidea were specific to Shiny Cowbirds and Austral Blackbirds, respectively. Although ectoparasites are generally highly species-specific, lice tend to be very mobile, with many accidental reports, e.g. in birds of prey (see discussion on Heteromenopon macrurum in Valdebenito et al., 2015). Therefore, it is interesting that these phylogenetically closely related icterid birds (subfamily Agelaiinae) (Remsen et al., 2016) did not share a higher number of ectoparasites. Bush et al. (2019) recently showed that feather lice require a relatively low number of generations to develop adaptations to a new host, suggesting a high plasticity from lice in case of host shifts. However, these are only speculations and we need further studies to prove these ideas on these icterids, since factors that determine success or failure of these events are complex and strongly dependent on, for example, ecological processes (climate, geographical, vectors) and host immune mechanisms (Moens et al., 2016;Sieber & Gudelj, 2014). Gastrointestinal parasites are the most likely to be transmitted between hosts because, contrary to ectoparasites, their life cycle allows indirect transmission. Interestingly, only M. papillosus was found in both bird species. Perhaps, only this parasite had the ideal conditions for the transmission to occur, which may include sharing similar diets (to ingest the intermediate host) and physiological conditions to allow its development and further infestation (Holmes, 1987;Hurd, 1990).
The present study contributed to the biodiversity of parasites of Chile, establishing new parasite-host associations. We also showed that, although these birds live in sympatry and share similar behaviors and diets, only one species of gastrointestinal parasite was shared. Parasite-host interactions of icterids of South America are widely understudied. Future studies involving representative samples across the distribution range of both icterids could provide more thorough description on the composition and ecology of their parasite fauna.