There are few published studies about Black Skimmer breeding in the Neotropics, although enough information exists to compile some basic patterns.
The reproductive cycle of this species is linked to the annual rhythm of precipitation and flood cycles. As described by Preston (1962), near the equator the nesting season is determined by the behavior of rivers: “The birds nest while the River is falling, the sandbanks emerging and the time for rising water so far off that there is time to lay and incubate the eggs and to raise the young to the flying stage”. Skimmers and terns will thus take advantage of low water levels, quickly occupying newly exposed sandbars.
The basic pattern is for skimmers to breed inland when water levels are lowest, which occurs at different times depending on the location, and then move to coastal areas when river flooding occurs (Murphy 1936). For example, Murphy (1936) reported that skimmers breed on the Río Paraguay from July to September during the low water period, and then migrate downstream to the coast (although the precise location on this major river of south-central South America was not specified). Then after the winter, once water levels begin to lower again, skimmer populations move upstream again in preparation for the next nesting season.
The phenology of breeding varies latitudinally. For example, in subspecies interdecens, colonies form during spring (September to November) in Brazil, and but form later during summer (January to March) farther south in Argentina (Mariano-Jelicich and Madrid 2014).
The recent study of skimmer migration in Peru (see Distribution) made intriguing insights about variation in the movements of this species after breeding (Davenport et al. 2016). Skimmers breeding in the tropical Amazon then migrate south, spending the nonbreeding season at temperate latitudes. They noted that this is the reverse pattern followed by most austral migrants, and by Black Skimmers breeding in coastal North America, who migrate to lower latitudes in their nonbreeding seasons. The pattern found in Peruvian skimmers is also opposite to the behavior of African Skimmer (Rynchops flavirostris), which breeds only within the tropics of southern Africa (i.e., Okavango and Zambezi systems) and then migrates north, to lower latitudes, for the austral winter.
The upshot is that these new findings open up new questions about the factors that control the timing and patterns of skimmer breeding and migration.
This species nests in colonies — usually at the same sites annually if they have proven successful — and then gathers in large flocks when not breeding. General details about this and all other aspects of the breeding biology of this species can be found in Gochfeld and Burger (1994).
The two South American subspecies breed mainly along sandbars of rivers that are exposed during the dry season, compared to the marine coastal breeding colonies of northern niger (see Geographic Variation and Distribution).
Typically, Black Skimmer nests in the same colonies with more aggressive terns. In North America this can include Common Tern (Sterna hirundo) at the northern distributional limit of the skimmer, while in the south co-nesting species include Least (Sternula antillarum), Forster’s (Sterna forsteri), and Gull-billed (Gelochelidon nilotica) terns, and Laughing Gull (Leucophaeus atricilla); see details in Gochfeld and Burger 1994). Black Skimmer nestx with Royal Tern (Thalasseus maximus) at selected sites along most of the Pacific coast of Mexico (Mellink et al. 2007). In South America, skimmers usually nest with Yellow-billed (Sternula superciliaris) and Large-billed (Phaetusa simplex) terns (Preston 1962, Krannitz 1989, Zarza et al. 2013).
In the Brazilian Amazon, important habitat features for breeding include landscape characteristics and social attraction: nesting sites within an archipelago tended to be found on larger beaches that were exposed to open water, on islands with little vegetation cover, remote from river margins, distant from other islands, and closer to large colonies of other skimmers (Zarza et al 2013).
Krannitz (1989) measured a nesting site on the Trombetas River in Brazil and reported that at lowest ebb, the river was about 500 m wide where a colony of terns and skimmers was located on a sandbar. The sandbar was ca 4 km long x 1 km wide, up to 2 m high, and exposed during the dry season (June to December). The sand was coarse (1-2 mm in diameter), with no plant growth. The skimmer nests were shallow, 30-cm wide depressions in the sand, placed an average of 42.3 m from the river. Similarly, another river colony in Amazonia had “scrapes” that were an estimated 7-10 cm deep and and 20-25 cm wide (Preston 1962).
The following data are from 36 nests at the Krannitz site in the Brazilian Amazon. (Refer to the following section on Demography for measures of reproductive success).
• Incubation period: 20 days (n = just 2 nests, due to missing start of the laying period), shorter than for North American birds which can hatch after 21–23 d, but typically 23–25 d due to disturbance (Gochfeld and Burger 1994).
• Hatching: median hatch date was November 7, and the hatching period lasted a mean of 3.8 days for clutches of 3 eggs, and 3 days for a clutch of two eggs.
• Chicks: semi-precocious, left the nest at 1-3 d post hatch.