Demography and Populations

For Black Skimmer in the Neotropics, information on breeding and nonbreeding localities, migration schedules, and size of different populations has been scarce (Naves and Vooren 2006), although more studies recently are accumulating.

Measures of Breeding Activity

The following data are from 36 nests at a site on the Trombetas River in the Brazilian Amazon (Krannitz 1989):

• Clutch size: skimmers laid from 1 to 4 eggs (mean 2.83 eggs/clutch), and lost no eggs or nests to flooding, but abandoned 3 nests (seven eggs). This is a smaller clutch size than what has been reported for northern birds (e.g. mean 3.55 eggs in Virginia, Erwin 1977; Gochfeld and Burger 1994 called it a “typical 4­-egg clutch”).

• Nest survival: daily probability of survival during incubation was 0.983, was almost identical to that estimated for skimmers in Texas (0.98486, Custer and Mitchell 1987).

• Reproductive Success: an estimated 1.66 chicks fledged per skimmer nest.

Population status

Investigations of sex ratios have found that some populations are skewed towards females: strongly in Argentina with a ratio of 4.9 females per male (Mariano-Jelicich et al. 2007), and less so in Brazil with 1.8 females per male (Scherer et al. 2013).

On the Solimoes River (Upper Amazon) above Manaus, Preston (1962) documented a colony on an extensive sandbar that formed during the the dry season (August to November) in 1961 when the water dropped to “abnormally low levels”. It contained ca 100 pairs each of Black Skimmer and Large-billed Tern (Phaetusa simplex). At other locations in the Brazilian Amazon, one site on the Trombetas River had 37 nests (Krannitz 1989), while another at the Anavilhanas Archipelago, Amazonas State supported a total of 30 breeding skimmers (Zarza et al. 2013).