LC
Least Tern Sternula antillarum



Justification

Justification of Red List Category
This species has an extremely 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). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to 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.

Trend justification
The overall population trend is decreasing, although some populations are increasing or have unknown trends (Delany and Scott 2006). In North America, this species has undergone a small or statistically insignificant decrease over the last 40 years (data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007).

Distribution and population

The Least Tern breeds along almost the entire coast of North America, excluding Alaska and Canada, on the northern coast of Central America and locally on the northern coast of South America. It also breeds inland along rivers in central North America. It is migratory, wintering on the southern coast of Central America, and the northern and Atlantic coast of South America as far south as central Brazil (del Hoyo et al. 1996).

Ecology

This species can be found on lakes, rivers and estuaries, strictly on the coast in some regions (e.g. California) but inland in others (e.g. Florida). It feeds on small fish fry, shrimps, marine worms and occasionally flying ants and other insects. Prey is usually caught by plunge-diving up to 10 m, preceded by prolonged hovering, and occasionally by surface-dipping and aerial hawking. The breeding season begins between April and mid-June depending on locality. It breeds in a large variety of habitats, from barren sandy beaches to parking lots and roof tops. Individuals form colonies between 5 and 200 pairs. It is a highly migratory species, though some populations in the north of South America, and on the Pacific coast of Mexico may be year-round residents (del Hoyo et al. 1996).

Threats

The Least Tern is vulnerable to human disturbance. Patagial tagging has been shown to cause a highly significant increase in nest desertation (Brubeck et al. 1981), likely to cause a loss of reproductive success through late-nesting birds experiencing lower hatching success due to predation (Mendillo 2009). Recreational activities have been shown to cause a significant increase in brooding terns flushing from the nest; however, this does not seem to cause a significant decrease in reproductive success (Mendillo 2009). The Least Tern is also vulnerable to habitat loss, having already lost large areas of habitat to flooding (Gochfeld et al. 2018), human alteration of waterflows (Farnsworth et al. 2017), and beach-side residential and tourism developments. It has shown itself to be flexible in terms of habitat use, moving to sandpits when sandbar habitat becomes available (Farnsworth et al. 2017). Since the early 1950s, the Least Tern has nested on gravel rooftops in areas such as the Florida Panhandle (Audubon Florida 2017, Warraich et al. 2012) and it is unsure what the effect of the shift to non-gravel roofs in these areas will be (Warraich et al. 2012).

Acknowledgements

Text account compilers
Calvert, R., Martin, R., Butchart, S., Stuart, A., Ekstrom, J.


Recommended citation
BirdLife International (2021) Species factsheet: Sternula antillarum. Downloaded from http://www.birdlife.org on 24/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 24/10/2021.