A Social Bird Species Ending Solitary: Flocking and Breeding Aspects of Greater Rheas (Rhea americana) in the Brazilian Dry Forest Region

Introduction

Populations of numerous bird species have been declining due to factors such as habitat loss and modification, hunting and capture [1, 2, 3, 4]. This involves the reduction of flock size of social species in contemporary landscapes of all continents [5, 6]. Among the South American social birds is the Greater Rhea Rhea americana (Linnaeus, 1758), a species considered Near Threatened, mainly due to hunting and expansion of agriculture [7]. This is a large and omnivorous species that relies on grasslands, savannas, and other open habitats [8, 9, 10].

Greater Rheas occur widely in South America, and are found mainly in open landscapes from Bolivia and Argentina to northeastern Brazil [10]. In Argentina and northeastern Brazil, its geographic distribution and populations have been substantially reduced [11, 12, 13]. Studies with radio-tracked birds showed that they use a range of native and modified habitats within agricultural landscapes in Argentina [14, 15, 16, 17]. In the Pantanal wetland, the density of Greater Rheas was higher in open fields than in open savanna, indicating that they can be favored by deforestation and the implementation of exotic pastures [18]. In the Brazilian Cerrado, Greater Rheas use a range of modified habitats in non-protected landscapes, and their occurrence is strongly associated with municipalities with low human abundance [19]. They breed between August and April, depending on the region [9, 10, 19]. Nests are round shallow depressions in the ground and usually have 20-30 eggs, eventually reaching up to 80, as polygyny is a remarkable characteristic of this species; incubation and chick-care are done exclusively by male [9, 10, 20].

In Brazil, Greater Rheas are commonly found in the Pantanal, the Cerrado and the Pampa ecoregions [18, 19, 20, 21, 22]. They were widespread and common in the Caatinga - the dry forest region of northeastern Brazil - by the mid twentieth century, when groups of 20-30 individuals were commonly found in its northern portions [8]. However, populations of the Caatinga have declined substantially since the 1960s, mainly due to hunting [23, 24, 25]. As result of this current scarcity in the Caatinga, Greater Rheas have been recorded only eventually and by a few recent avian inventories [26, 27, 28]. As a result, studies on aspects of its biology are lacking in this ecoregion.

We considered that the knowledge on the biology of Greater Rheas could be increased through the use of citizen science data, as recently occurred for its geographic distribution in Brazil and its breeding in the Cerrado [13, 19]. The objective of this study was to examine flocking and breeding aspects of Greater Rheas in the Caatinga through the use of data gathered by citizen scientists and deposited in the WikiAves platform. Results were discussed in relation to their flock size, reproduction and conservation in this dry forest and other South American ecoregions.

Results

A total of 112 photographic records of the Greater Rhea have been produced by citizen scientists in the Caatinga, and deposited in the WikiAves platform (Appendix, Table 1). Most of them have occurred in central and eastern Caatinga, and were obtained in 52 municipalities of six states between 2007 and 2023 (Figure 1). Municipalities had 1-14 records, and those with more records were Riachão do Jacuípe and Ourolândia, with 14 and seven records, respectively. On the other hand, most municipalities (n = 33; 63%) had only one record (Appendix, Table 1).

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The number of adult Greater Rheas shown in the photographic records ranged between 1 and 5 (Figure 2), excluding those records showing evidence of breeding activities (Figure 3). When considering the information brought by photographs, most records (n = 77; 74%) had a single adult, 23 records (22%) had a pair of adults, while a minor portion (4%) showed three or five adults. Of the

104 records involving only adults, 45 (43%) had the flock size confirmed by authors of the photographs (Appendix). Among them, 17 records (38%) had a single adult, while 9 records (20%) had a pair of adults. Seven (15%) records had only three adults, while records with more adults were even less common. Three (6.7%) records had 20-25 adults (Figure 3, Appendix).

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Figure 2: Photographic records of the Greater Rhea (Rhea americana) obtained by citizen scientists in the Caatinga, Brazil: (A) a solitary adult walking on a dirty road at Cabaceiras-PB; (B) a pair of adults in a patch of second growth caatinga at Canudos- BA; (C) three adults on a cultivated field at Riachão do Jacuípe-BA; (D) two adults on a dirty road at Ibipeba-BA; (E) a nest with six eggs adjacent to shrubs at Petrolina-PE; (F) four young and an adult at Riachão do Jacuípe-BA. Photo authors: Nailson Junior (A), Ruana Souza (B), Jaelson Gomes (C), Tiago Moreira (D), Adelmo Queiroz (E), Jaelson Gomes (F). Records were gathered in the WikiAves platform, and included here with permission by their authors.

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Figure 3: Number of adults found per record (flock size) of the Greater Rhea (Rhea americana) obtained by citizen scientists in the Caatinga, Brazil. Results were shown for to situations: (1) flock size shown on photographs (n = 104); (2) flock size informed by authors of the photographs through email messages (n = 45). Records of families made up of adult(s) and small young were not included here. Records were gathered in the WikiAves platform in April 2023.

Eight (7%) of the total number of Greater Rheas records showed evidence of breeding activities (Table 1). They involved a nest with an incubating adult, a nest with exposed eggs, and six records of families with 1-8 small young and 1-2 adults (Figure 2). These breeding records usually occurred between December and April, with the exception of a record obtained in July. Most of them were gathered during rainy periods (Table 1).

Discussion

Apparently, this study is the first to examine in detail aspects of the natural history of Greater Rheas in the Caatinga. This is because previous studies examined only its geographic distribution [13]. Other information results from inventories of bird communities that reported its presence or absence in particular localities; of 51 avian inventories conducted in the Caatinga during the last three decades (1990-2019) [28], only eight recorded Greater Rheas [23, 26, 27, 36, 37, 38, 39, 40]. Thus, citizen scientists have been more effective than researchers in recording Greater Rheas, as they found it in more than 50 municipalities (this study).

Olmos [23] reported the occurrence of Greater Rheas in the northern portion of Parque Nacional da Serra da Capivara in the early 1990s, but a more recent inventory in this same region reported its absence in the early 2010s [25]. Thus, it is probable that Greater Rheas do not persist in the localities reported by researchers and citizen scientists some years ago. For example, the local extinction of Greater Rheas has been mentioned for some protected areas found in the Caatinga, including the Floresta Nacional de Negreiros [24], and the Estação Ecológica do Seridó [41]. Further, a recent evaluation of its regional distribution in the Caatinga indicated its absence at several localities of previous occurrence, with a resulting range collapse [13]. These and other studies Olmos F [8, 42] mention hunting as the major cause of its population decline in the Caatinga since the 1960s. Despite not dealing with causal factors, our study suggests that populations of Greater Rheas in this dry forest region have been facing reduction of flock size.

Recent studies of Greater Rheas in the Caatinga just mention their occurrence, and do not mention aspects of their flocking behavior [13, 23, 26, 27, 36, 37, 38, 39, 40]. Thus, our study is the first to examine this aspect of its biology in this ecoregion. Information gathered by citizen scientists since the late 2000s indicates that Great Rheas tend to occur solitary or in flocks of 2-3 birds in the Caatinga. This pattern substantially contrasts with the reported commonness and the often encounters of flocks with 20-30 birds in its northern portion until the 1950s [8, 43, 44].

Flock size shown in the photographic records was substantially smaller than that informed by their authors. Major differences included (1) a higher frequency of solitary rheas, and flocks with 2-3 birds, in photographic records than in the set of messages provided by authors, and (2) the absence of flocks with six or more rheas in photographs. Probably, these differences happen because some citizen scientists might focus 1-2 particular rheas, to show them in detail, while other members of the flock become out of frame. Thus, the information obtained through messages sent by citizen scientists should be that considered when examining Greater Rheas flocks in the Caatinga and elsewhere.

The set of breeding records obtained by WikiAves citizen scientists indicate that, apparently, the breeding season of Greater Rheas occurs, at least, between December and July in the Caatinga. Records with small young occurred until April, while a record with more developed juveniles occurred in July. Thus, the breeding season in the Caatinga is delayed in relation to other Brazilian ecosystems. This is because its breeding season occurs between August and June in the Pampa [21], between June and March-April in the Cerrado [19], while young are found between September and December in the Pantanal [22]. Most breeding records obtained in the Caatinga occurred during rainy periods, while a single one was obtained in a dry period. Due to our small set of breeding records, further studies based on citizen science data and observations by researchers are necessary for a better understanding of the breeding phenology of the Greater Rhea in the Caatinga.

It is not known if the photographed Greater Rheas of our study refer to naturally free-living birds or individuals released from captivity. Reintroductions by IBAMA have occurred in some localities, such as Estação Ecológica do Seridó in the early 1990s [41], but information on how they happened throughout the Caatinga is apparently not available. Reintroduction has been proposed for protected areas such as Parque Nacional da Serra da Capivara [42]. We suggest that reintroductions occur in protected as well as unprotected areas throughout the Caatinga regions where it originally was found. Greater Rhea has five subspecies, and R. americana americana is the one that occurs in the Caatinga [10]. Thus, we suggest that reintroductions involve individuals of this subspecies. These actions should be accompanied by educational programs to mitigate hunting pressure in localities with current and future reintroduced flocks.

Authors Contributions

Dárius P. Tubelis [Idealization, conceptualization, data analysis, interpretation and writing].

Filipe M. O. Costa [Idealization, conceptualization, data collection and analysis, interpretation and writing].

Conflicts of Interest

The authors declare that there are no conflicts of interest involving this publication.

Acknowledgements

We thank Universidade Federal Rural do Semi-Árido [UFERSA] for logistic support. We are grateful to all citizen scientists that deposited their photographs of the Greater Rhea in the WikiAves database. We especially thank Adelmo Queiroz, Jaelson Gomes, Nailson Junior, Ruana Souza and Tiago Moreira for giving permission to include photographs of their authorship in this paper. Two anonymous reviewers and the Editorial Committee improved this manuscript with numerous suggestions.