Seasonal Distribution of the Cocoi Heron (Ardea cocoi) and Great Egret (Ardea alba) in Southern South America: Evidence for Partial Migration
Abstract
The Cocoi Heron (Ardea cocoi) and Great Egret (Ardea alba) occur throughout South America with scant evidence of long-distance migration. We analyzed the seasonal distribution of these species based on 89,048 eBird records of the Cocoi Heron and 149,343 eBird records of the Great Egret south of 20° S from 1937-2022. Although individuals of both species occurred year-round at the highest latitudes (> 50° S), the proportion of records during winter (June to August) decreased significantly as latitude increased for both species, suggesting that the southernmost populations are partially migratory with some individuals migrating northward during fall and returning during spring. Further studies are needed to confirm whether these species migrate long distances in southern South America.
Key words: Argentina, Chile, eBird, migration, Neotropical austral migrants, Neotropics, Patagonia.
Introduction
Bird migration within the Neotropics is complex (Jahn et al. 2020). Species that breed in the temperate latitudes of South America and migrate northward after the breeding season are referred to as austral migrants, Neotropical migrants, or Neotropical austral migrants (Chesser 1994, Hayes et al. 1994, Hayes 1995, Joseph 1997, Jahn et al. 2004, Cueto and Jahn 2008, Capllonch 2018, Jahn et al. 2020). Although the number of studies on these birds has increased in recent decades (Jahn et al. 2020), much remains to be learned about the seasonal movements of birds in southern South America.
Two species of Ardea herons breed in southern South America (Fig. 1). The Cocoi Heron (Ardea cocoi) breeds throughout much of South America (Kushlan and Hancock 2005, Martínez-Vilalta et al. 2020). Post-breeding dispersal has been documented southward to the Strait of Magellan and to larger continental islands off the coast of South America, and some birds in the extreme south of its range are thought to migrate northward during winter (Capllonch et al. 2008, Capllonch 2018, Martínez-Vilalta et al. 2020). The Great Egret (Ardea alba) is a cosmopolitan species breeding throughout much of North America, South America, Eurasia, and Africa (Kushlan and Hancock 2005, McCrimmon et al. 2020). Although post-breeding dispersal occurs in all populations and migration is well documented in breeding populations of North America and Eurasia, migration is not known to occur among breeding populations of South America and Africa (McCrimmon et al. 2020), although it is suspected to occur in South America (Capllonch et al. 2008, Capllonch 2018).
In the past two decades, eBird, an online citizen science database, has provided researchers with an enormous quantity of distributional records of birds that can be used for studying the seasonal distribution of birds (Sullivan et al. 2009, 2014, Wood et al. 2011). In this study, we use eBird data to examine the seasonal distribution of the Cocoi Heron and Great Egret within South America.
Methods
We downloaded all eBird records of the Cocoi Heron and Great Egret submitted and vetted by 21 October 2022 from south of 20° S in southern South America. Each record was defined as an observation of one or more individuals at a given locality (size of area highly variable) on a given date. For each degree of latitude south of 20° S, we calculated the percentage of records occurring during the winter months of June to August for each species. For each degree of latitude south of 45° S we calculated the percentage of records occurring during each month. A Cochran-Armitage test (χ2 statistic; Cochran 1954, Armitage 1955) was computed using on online calculator (epitools.ausvet.com.au) to test for a linear trend in the proportion of winter records for each degree of latitude. Google Earth (earth.google.com) was used to estimate the approximate coordinates of the southernmost published records and eBird records, which were often imprecise. An online calculator (nhc.noaa.gov) was used to measure the approximate distances between the southernmost breeding colonies and the southernmost records.
Results
We obtained 89,048 eBird records of the Cocoi Heron from south of 20° S, ranging from 13 December 1937 to 30 September 2022. More than a quarter of the records at each degree of latitude occurred during winter to the north of 30° S and less than a quarter occurred during winter to the south of 30° S, with the proportion of winter records decreasing significantly as latitude increased (slope = -0.0035, χ2 = 147.94, df = 1, P < 0.001; Fig. 2). The Cocoi Heron occurred during all months of the year at latitudes > 50° S (Table 1). The southernmost eBird record was observed by Tom Ostrand from a bus while traveling along Route 3 from Ushuaia towards Parque Nacional Tierra del Fuego, Tierra del Fuego, Argentina (~54° 50′ S, 68° 25′ W; Fig. 3), and described as “standing at streamside, unmistakable large blue-gray heron, very similar to Great Blue Heron [Ardea herodias],” on 25 January 2017 (ebird.org). The southernmost eBird winter record was an adult photographed by Julio Llanos at Río San Juan, Magallanes, Chile (~53° 39′ S, 70° 57′ W; Fig. 3), on 07 August 2020 (ebird.org).
We obtained 149,343 records of the Great Egret from south of 20° S, ranging from 13 December 1937 to 30 September 2022. About a quarter of the records at each degree of latitude occurred during winter to the north of 33° S and less than a quarter occurred during winter to the south of 33° S, with the proportion of records during austral winter decreasing significantly as latitude increased (slope = -0.0035, χ2 = 307.11, df = 1, P < 0.001), even though more than half of the records (n = 118) from 50-51° S occurred during winter (Fig. 2). The Great Egret occurred during all months of the year at latitudes > 50° S (Table 1). The southernmost eBird record was an adult photographed at the mouth of the Río Santa María, Magallanes, Chile (~53° 41′ S, 70° 58′ W; Fig. 3), by Sebastián Saiter Villagrán on 09 April 2022 (ebird.org) and by Santiago Imberti on 18 May 2022 (ebird.org). The southernmost eBird winter record was observed by Melissa Carmody and Santiago Imberti at the mouth of the Río Agua Fresca, Magallanes, Chile (~53° 24′ S, 70° 59′ W; Fig. 3), on 07 June 2020 (ebird.org, ebird.org).
Discussion
In southern South America, the Cocoi Heron breeds during September to December and the Great Egret breeds during October to January (De la Peña and Montalti 2014). There have been three band recoveries of the Cocoi Heron up to 600 km to the south-southeast of where they were banded as chicks in Santa Fé Province in central Argentina, with the most distant recovery in January (Olrog 1965, 1968a). And there have been 12 band recoveries for the Great Egret up to 1,200 km to the north and 600 km to the south, mostly within 6 months of being banded as chicks in Santa Fé Province in central Argentina (Olrog 1965, 1968a, 1969, 1973). Most of these band recoveries of the Great Egret, including the most distant, were to the south during February to March and to the north during April to August. The seasonal patterns of dispersal demonstrated by these band recoveries are consistent with long-distance latitudinal migration by Neotropical austral migrants (Capllonch et al. 2008, Capllonch 2018), but do not conclusively demonstrate a cyclical pattern.
The seasonal movements of waterbirds in southern South America are also influenced by hydrological regimes (Beltzer and Neiff 1992, Hayes 1996). For example, the abundance of both the Cocoi Heron and Great Egret is negatively correlated with water level along the Paraguay River (Hayes 1996) and Parana River (Beltzer and Neiff 1992). The herons disperse toward major rivers when water levels are low, when concentrated food resources occur in smaller pools and mudflats. And the herons disperse away from major rivers to other wetlands when water levels are high and food sources are more dispersed. This cyclical pattern of dispersal, in contrast with unidirectional dispersal, represents a localized and poorly documented form of migration.
Although many species of birds are expanding their ranges southward in Argentina in response to climate change (Capllonch et al. 2020), there is no evidence that the Cocoi Heron and Great Egret are expanding their breeding ranges southward. The southernmost known breeding colony of the Cocoi Heron is at Islote de los Pájaros, Río Negro, Argentina (~41° 27′ S, 65° 02′ W; Fig. 3), where it was first recorded breeding in 1995 (De la Peña and Montalti 2014, De Lucca et al. 2022), and the southernmost known breeding colony of the Great Egret is at Punta Lobería, Chubut, Argentina (~44° 35′ S, 65° 24′ W; Fig. 3), where it was first recorded breeding in 1982 (De la Peña and Montalti 2014). It is possible that new breeding colonies farther south have been overlooked.
Individuals of both species wander far south of their breeding colonies, with the southernmost eBird record of the Cocoi Heron about 1,507 km south of the southernmost known breeding colony and the southernmost eBird record of the Great Egret about 1,085 km south of the southernmost known breeding colony. Both species occasionally disperse shorter distances (> 1,100 km for Cocoi Heron, > 750 km for Great Egret) from their nearest breeding colonies to the Falkland Islands (Wace 1921, Olrog 1968b, Woods 1975). These data demonstrate long-distance dispersal by both species in southern South America, indicating that they are capable of long-distance latitudinal migration.
There is no evidence that the Cocoi Heron and Great Egret are expanding their non-breeding ranges southward. There are numerous published records, including winter records, of the Cocoi Heron south of 50° S during the past century (Venegas 1973, 1982, Venegas and Jory 1979, Clark 1986, Imberti 2003, 2005), with the southernmost eBird record (in 2017) only about 125 km south of a record from nearly a century earlier (in 1922) at Cabo Peñas, Tierra del Fuego, Argentina (53° 51′ S, 67° 33′ W; Venegas 1973), and about 72 km south of an undated record prior to 1986 at Lago Cami, Tierra del Fuego, Argentina (54° 34′ S, 67° 24′ W; Clark 1986). Venegas and Jory (1979) stated that most of these records were of solitary individuals during June and July. There are also many published records of the Great Egret south of 50° S (Olrog 1948, Venegas 1982, Venegas and Jory 1979, Clark 1986, Darrieu et al. 2008) and a few of these are farther south than the southernmost eBird record of Great Egret (in 2020), including Río Grande, Tierra del Fuego, Argentina (~53° 48′ S, 67° 45′ W; Clark 1986) and Río Ewan, Tierra del Fuego, Argentina (somewhere between 54° 07′ S, 67° 09′ W and 54° 24′ S, 67° 22′ W; Fig. 3; Clark 1986). Because these records are so close to the southern tip of South America (about 100 km from the southernmost record of Cocoi Heron), there is little space for non-breeding individuals to move farther southward, although the number of individuals dispersing south of 50° S may be increasing as a consequence of climate change.
The eBird data demonstrate that some individuals of both species linger throughout the winter south of 50° S and obviously do not migrate northward during fall, although they must eventually fly northward to breed. Both species appear capable of finding sufficient prey to survive harsh Patagonian winters. However, the proportion of winter records decreased steadily as latitude increased for both species, suggesting that some individuals in the southernmost populations are partially migratory with some individuals migrating northward during fall and returning during spring. Records from eBird reveal that some individuals in migratory populations of Great Egret in North America and Eurasia are capable of surviving for extended periods during winter at even higher latitudes, with the northernmost record at Kuljunlahti, Raahe, Finland (64° 38′ N, 24° 26′ E), from 2 January to 14 February 2019 (Kari Varpenius; ebird.org, ebird.org). Further studies of herons that are banded, tagged with light-level geolocators, or outfitted with satellite transmitters are needed to confirm whether some individuals of the Cocoi Heron and Great Egret exhibit long-distance latitudinal migration in southern South America.
Acknowledgments
We thank the contributors of eBird for posting records on eBird and the editors and administrators for curating the records.
Literature Cited
Armitage, P. 1955. Tests for linear trends in proportions and frequencies. Biometrics 11: 375.
Beltzer, A. H. and J. J. Neiff. 1992. Distribución de las aves en el valle del río Paraná. Relación con el regimen pulsátil y la vegetación. Ambiente Subtropical 2: 77-102.
Capllonch, P. 2018. Un panorama de las migraciones de aves en Argentina. Hornero 33: 1-18.
Capllonch, P., D. Ortiz and K. Soria. 2008. Importancia del litoral fluvial argentino como corredor migratorio de aves. Insugeo, Miscelánea 17: 107-120.
Capllonch, P., F. E. Hayes and F. D. Ortiz. 2020. Escape al sur: una revisión de las aves que expandieron recientemente su rango de distribución en Argentina. Hornero 35: 111-126.
Chesser, R. T. 1994. Migration in South America, an overview of the austral system. Bird Conservation International 4: 91-107.
Clark, R. 1986. Aves de Tierra del Fuego y Cabo de Hornos. L.O.L.A., Buenos Aires.
Cochran, W. G. 1954. Some methods for strengthening the common χ2 tests. Biometrics 10: 417-451.
Cueto, V. R. and A. E. Jahn. 2008. Sobre la necesidad de tener un nombre estandarizado para las aves que migran dentro de América del Sur. Hornero 23: 1-4.
Darrieu, C. A., A. R. Camperi and S. Imberti. 2008. Avifauna (non Passeriformes) of Santa Cruz Province, Patagonia (Argentina): annotated list of species. Revista del Museo Argentino de Ciencias Naturales (nueva serie) 10: 111-145.
De la Peña, M. and D. Montalti. 2014. Nidificación de las aves argentinas. Comunicaciones del Museo Provincial de Ciencias Naturales “Florentino Ameghino” (nueva serie) 18: 1-136.
De Lucca, E. R., B. M. Bertini and J. Ferrada. 2022. Reproducción del Jote Cabeza Colorada (Cathartes aura) y Jote Cabeza Negra (Coragyps atratus) en el Golfo San Matías, Río Negro, Patagonia Argentina. Nótulas Faunísticas (segunda serie) 334: 1-11.
Hayes, F. E. 1995. Definitions for migrant birds: what is a Neotropical migrant? Auk 112: 521-523.
Hayes, F. E. 1996. Seasonal and geographical variation in resident waterbird populations along the Paraguay River. Hornero 14: 14-26.
Hayes, F. E., P. A. Scharf and R. S. Ridgely. 1994. Austral bird migrants in Paraguay. Condor 96: 83-97.
Imberti, S. 2003. Notes on the distribution and natural history of some birds in Santa Cruz and Tierra del Fuego provinces, Patagonia, Argentina. Cotinga 19: 15-24.
Imberti, S. 2005. Aves de Los Glaciares: Inventario Ornitológico del Parque Nacional Los Glaciares, Santa Cruz, Patagonia, Argentina. Aves Argentinas y Administración de Parques Nacionales, Buenos Aires, Argentina.
Jahn, A. E., D. J. Levey and K. G. Smith. 2004. Reflections across hemispheres: a system-wide approach to New World bird migration. Auk: Ornithological Advances 121: 1005-1013.
Jahn, A. E., V. R. Cueto, C. S. Fontana, A. C. Guaraldo, D. J. Levey, P. P. Marra and T. B. Ryder. 2020. Bird migration within the Neotropics. Auk 137: 1-23.
Joseph, L. 1997. Towards a broader view of Neotropical migrants: consequences of a re-examination of austral migration. Ornitología Neotropical 8: 31-36.
Kushlan, J. A. and J. A. Hancock. 2005. The herons. Oxford University Press, Oxford, U.K.
Martínez-Vilalta, A., A. Motis and G. M. Kirwan. 2020. Cocoi Heron (Ardea cocoi), version 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie and E. de Juana, eds.). Cornell Lab of Ornithology, Ithaca, New York, U.S.A. [online] Accessed 25 April 2023.
McCrimmon Jr., D. A., J. C. Ogden, G. T. Bancroft, A. Martínez-Vilalta, A. Motis, G. M. Kirwan and P. F. D. Boesman. 2020. Great Egret (Ardea alba), version 1.0. In Birds of the World (S. M. Billerman, ed.). Cornell Lab of Ornithology, Ithaca, New York, U.S.A. [online] Accessed 25 April 2023.
Olrog, C. C. 1948. Observaciones sobre la avifauna de Tierra del Fuego y Chile. Acta Zoológica Lilloana 5: 437-531.
Olrog, C. C. 1965. El anillado de aves en la Argentina. 1961-1964. Cuarto informe. Hornero 10: 202-208.
Olrog, C. C. 1968a. El anillado de aves en Argentina. Quinto informe. Neotrópica 14: 17-22.
Olrog, C. C. 1968b. Las Aves Sudamericanas, Una Guía de Campo. Universidad Nacional de Tucumán e Instituto Miguel Lillo, Tucumán, Argentina.
Olrog, C. C. 1969. El anillado de aves en la Argentina, 1961-1968. Sexto informe. Neotrópica 15: 82-88.
Olrog, C. C. 1973. El anillado de aves en la Argentina. 1961-1972. Octavo Informe. Neotrópica 19: 69-72.
Sullivan, B. L., C. L. Wood, M. J. Iliff, R. E. Bonney, D. Fink and S. Kelling. 2009. eBird: a citizen-based bird observation network in the biological sciences. Biological Conservation 142: 2282-2292.
Sullivan, B. L., J. L. Aycrigg, J. H. Barry, R. E. Bonney, N. Bruns, C. B. Cooper, T. Damoulas, A. A. Dhondt, T. Dietterich, A. Farnsworth, D. Fink, J. W. Fitzpatrick, T. Fredericks, J. Gerbracht, C. Gomes, W. M. Hochachka, M. J. Iliff, C. Lagoze, F. A. La Sorte, M. Merrifield, W. Morris, T. B. Phillips, M. Reynolds, A. D. Rodewald, K. Y. Rosenberg, N. M. Trautmann, A. Wiggins, D. W. Winkler, W. K. Wong, C. L. Wood, J. Yu and S. Kelling. 2014. The eBird enterprise: an integrated approach to development and application of citizen science. Biological Conservation 169: 31-40.
Venegas C., C. 1973. La Garza Mora, (Ardea cocoi Linne) en Magallanes. Anales del Instituto de la Patagonia 4: 275-279.
Venegas C., C. 1982. Suplemento a la guía de campo para las aves de Magallanes. Anales del Instituto de la Patagonia 13: 189-206.
Venegas C., C. and J. Jory H. 1979. Guia de Campo para las Aves de Magallanes. Publicaciones del Instituto Patagonia, Punta Arenas, Chile.
Wace, R. H. 1921. Lista de aves de las Islas Falkland. Hornero 2: 194-204.
Wood, C., B. Sullivan, M. Iliff, D. Fink and S. Kelling. 2011. eBird: engaging birders in science and conservation. PLoS Biology 9: e1001220.
Woods, R. W. 1975. The Birds of the Falklands Islands. Compton, Salisbury, U.K.