We obtained 336,143 occurrences for the Americas from 1930s to 2020. Some authors suggest the Cattle Egret presence in South America since the end of the1800s and the beginning of the 1900s (e.g., Palmer 1962, Wetmore 1963, Crosby 1972, Arendt 1988). Even, there is a report of the species in in 1933 in Providence Island (Arendt 1988). Nevertheless, until 1937, the first specimen was collected in Buxton, Guyana (Blake 1939). From these records, three different invasion fronts appeared. The first front came by the expansion of the Cattle Egret following an insular pathway: the Antilles (Arendt 1988) with the first record in Providence Island (Arendt 1988) in the decade of 1930; subsequently in Aruba (Drury et al. 1953), Puerto Rico and Jamaica in 1940 (Bond 1957, Arendt 1988); the Antigua Island (Holland and Williams 1978); and Trinidad and Tobago (Ffrench 1980). This expansion continued to Florida (US) in the 1950s (Rice 1956). A second invasion front started in Guyana and continued in other South American countries, such as Venezuela (Phelps 1944); Colombia (Haverschmidt 1953, Wetmore 1963); and Peru (Stott 1957) in the 1950s; Chile (Post 1970) and Argentina (Petracci and Delhey 2005) in the 1970s; as far as Uruguay in 1980 (Vaz-Ferreira et al. 1981). The third invasion front followed Central America, towards Costa Rica (Slud 1957), Panama (Eisenmann 1955) and Guatemala (Land 1963) in the 1950s. This front continued towards Mexico, central and western US, to southern Canada in the following decades (Denham 1959, Dickerman 1964, Hubbard 1966, Zimmerman 1971).
We observed the presence records of the Cattle Egret in regions that were not considered previously by Hengeveld (1989). For example, in the decades of 1970 and 1980, we encountered the first records in central and western Mexico and the US; likewise, in Chile and central-eastern Brazil. Although in the most recent decades (1990 to 2020), the records have become more frequent in the latter regions suggesting the invasion completion, which currently includes from the southeastern of Canada to the extreme south of South America.
Regarding to the insular invasion, Arendt (1988) described the invasion process in the Caribbean region from 1930 to 1970. However, adding information to the insular front, we found, based on the bibliographic review, that in 1966 the species was recorded in the San Andres Island on the coast of Nicaragua (Paulson et al. 1969); and in 1968 in the Farallon Islands, US (Richardson et al. 2003). Then, it was recorded in 1976 in the Bay Islands, Honduras (Udvardy 1975); and in 1977 in the Chesapeake Bay Islands, US (Williams et al. 2007). In South America, the island invasion process included the South Georgia and the South Sandwich Islands, UK, in 1979 (Prince and Payne 1979); in 1980 the Vila dos Remédios Islands, Brazil (Nunes 2010); and the Falkland (Malvinas) Islands, UK-Argentine. In 1986, the species reached the Tierra del Fuego archipelago (Argentina) and Cabo de Hornos, Chile (Clark 1986, Chebez and Gómez 1988); and in 1987 in the Easter island, Chile (Marin and Caceres 2010).
There are records in diverse islands in the Americas that do not coincide with the expected geographic and temporal continuous invasion process observed for the Cattle Egret. However, this lack of correspondence may be due to the absence of local recordings rather than a lack of continuity during the species' dispersion. These records correspond to that in 1973 in Vancouver Island, Canada (Kragh 1982); in 1979 in the Channel Islands, US (Stewart and Kovach 1982); in 1988 in Revillagigedo Islands, Mexico (Howell and Webb 1995); in 1999 in Newfoundland and Labrador, Canada, and the Archipelago Bocas del Toro, Panama (Cooper 1999, Montevecchi et al. 2003); in 2000 in the Desventuradas Islands, Chile (Aguirre et al. 2008, Flores 2014); in 2003 in the Nueva Esparta islands, Venezuela (Hilty 2002); and in 2007 in the Swan Islands, Honduras and islands of the gulf of Panama (Aceituno and Medina 2007, Angehr and Kushlan 2007). Finally, in 2014 the species was recorded in the Chriqui Gulf Islands, Panama (Angehr et al. 2014).
On the other hand, the PCA showed that the first three components explained 80% of the variance. In the PC1 and PC3 the most important variables were those related to temperature, conversely to the most important described by the PC2 related to precipitation variables (Table 1). The invasion process in environmental space showed that from 1930s to 1960s the majority of the climate range currently used by the species was covered. Nevertheless, from the beginning of 1970 to date, we did not observe a significant increase in the occupation of new climatic conditions (Fig. 2).
The seasonal patterns described by the Cattle Egret includes its presence from southern Canada to Patagonia between March and November. However, during December, January and February, months that correspond to winter in the northern hemisphere, there is a lack of records in southern Canada and most of the US (Fig. 3). This temporal pattern suggests the existence of migration behavior in the populations from these regions. Such behavior can be associated with the climate limits described by the three principal components representing environmental space. We observed that the range of the values defined by the records from the migration regions are separated from those of the year-round records from other areas. The minimum temperature of the migration regions ranges between -25 to 0°C and the maximum between -10 to 20°C. In the case of the precipitation, we observed lower values in the migration regions in contrast to the rest of the continental records. Conversely, in the southern hemisphere, we did not observe any migratory behavior pattern across the different seasons of the year.