Justification of Red List Category
This species has a very large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be stable, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
The overall population trend is stable, although some populations have unknown trends (Wetlands International 2006). This species has undergone a small or statistically insignificant increase over the last 40 years in North America (data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007) Note, however, that these surveys cover less than 50% of the species's range in North America.
The Royal Tern is found in the Americas and the Atlantic coast of Africa. In Africa is breed from Mauritania to Guinea, ranging in winter from Morocco to Namibia. In the Americas it breeds from southern California (USA) to Sinaloa (Mexico), from Maryland to Texas (USA), through the West Indies to the Guianas and possibly Brazil, on the Yucatan Peninsula, in south Brazil, Uruguay and north Patagonia (Argentina). It winters from Washington (USA) south to Peru on the western coast, and from Texas to south Brazil on the eastern side (del Hoyo et al. 1996).
Behaviour This species undergoes post-breeding dispersive movements northwards before migrating southwards for the winter (del Hoyo et al. 1996). It breeds between April and June (Richards 1990) in dense colonies of 100-4,000 pairs often near colonies of Laughing Gull Larus atricilla and Sandwich Tern Sterna sandvicensis (del Hoyo et al. 1996). The species may also nest singly amidst colonies of other tern species (del Hoyo et al. 1996). It usually feeds singly or in small flocks and roosts gregariously even outside of the breeding season (del Hoyo et al. 1996).
Habitat Breeding For breeding the species shows a preference for inaccessible sites including barren sandy beaches, islands in saltmarsh, dredge spoil and coral islands surrounded by shallow water and with a high degree of visibility, no mammalian predators and little vegetation (del Hoyo et al. 1996). It also forages along estuaries, in lagoons and in mangroves during this season, mostly within 100 m of the shore but up to 40 km from the breeding colony (del Hoyo et al. 1996). Non-breeding Outside of the breeding season the species forages within 100 m of the land along sheltered coasts in estuaries, harbours and river mouths, sometimes also foraging a short distance inland along broad rivers (del Hoyo et al. 1996). Diet Its diet consists predominantly of small fish 3-18 cm long as well as squid, shrimps and crabs (del Hoyo et al. 1996). Breeding site The nest is a simple scrape (del Hoyo et al. 1996) in sand (Urban et al. 1986) in inaccessible sites surrounded by shallow water near the mouths of bays with a high degree of visibility, no mammalian predators and little vegetation (del Hoyo et al. 1996). Management information The preferred breeding sites of this species are often vulnerable to flooding (del Hoyo et al. 1996).
This species may be affected by human disturbance on the dredge-spoil islands on which has nested since the degradation and development of the barrier islands that previously held colonies (Buckley and Buckley 2002). Egg collection and hunting have historically caused declines and, although some subsistence harvesting still occurs in the West Indies (Gochfield et al. 2018, Buckley and Buckley 2002), it is not thought to be causing significant declines in the population (Buckley and Buckley 2002).
Text account compilers
Calvert, R., Malpas, L., Martin, R., Stuart, A., Ekstrom, J., Butchart, S.
BirdLife International (2020) Species factsheet: Thalasseus maximus. Downloaded from http://www.birdlife.org on 09/08/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 09/08/2020.