Diet and Foraging

A fair amount of data about basic feeding ecology of Black Skimmer has accumulated for the southern portion of its range. Black Skimmer is piscivorous; as has been reported for the northern populations, in the Neotropics the skimmer feeds primarily on small fish (Naves and Vooren 2006).

At an estuary in southern Brazil, from December to August, the diet of nonbreeding skimmers contained 23 types of prey, spanning 12 families of fish, 2 of crustaceans, and 1 squid. The diet was dominated by Silversides (Odonthestes argentinensis and Atherinella brasiliensis), anchovies (mainly Lycengraulis grossidens and Engraulis anchoita), and mullets (Mugil spp.). Prey had an average length of 51.6 mm (range 9-212 mm), with a mass of 2.8 g (range 0.1-78.1 g). The diet analysis suggested that Black Skimmers fed mainly in a lower estuary, but also on an adjacent ocean beach (Naves and Vooren 2006).

Skimmers occasionally regurgitate pellets, which are clumps of indigestible, solid material. An analysis of summer and autumn diets in coastal Argentina detected fish in 100% of skimmer pellets, with insects in just 2.7% of them (Favero et al. 2001). Among 14 fish species identified, the most important prey items, in order of occurrence, were Argentine Anchovies (Engraulis anchoita), Silversides (Odontesthes spp.) and White Croakers (Micropogonias furnieri). The mean length of fish prey was 77 mm (range 15-169 mm) with a mass of 6.9 g (range 0.1-31.6 g). The distribution of fish prey varied by year and season, presumably in accordance with varying environmental conditions. Skimmer diets overlapped with those of three tern species in the same area: South American (Sterna hirundinacea), Royal (Thalasseus maximus), and Sandwich terns (Thalasseus sandvicensis).

Roughly 200 km further south on the Atlantic coast of Argentina, Mariano-Jelicich et al. (2008) also found that nonbreeding skimmers consumed a fish diet. The silverside Pejerrey (O. argentinensis) and Cornalito (O. incisa) were the main prey items (Mariano-Jelicich et al. 2003, Mariano-Jelicich and Favero 2006).

Other studies from the same Argentinian research group have compared diet composition of the sexes. Spontaneous regurgitations showed that male skimmers capture larger prey items than do females, consistent with the larger size of males (Mariano-Jelicich et al. 2007). The authors were unable to determine whether this was due to temporal variation in the diet or to different foraging strategies. Further, analysis of stable isotopes from blood samples showed that skimmers ate mainly marine prey, with some intake of estuarine fish. Males showed enrichment in 15N compared to females, with no differences in the 13C isotope. Overall, the results revealed a trophic segregation between the sexes, evident as different prey species and larger prey sizes consumed by male skimmers. This could have conservation implications, causing environmental changes to affect males and females differently (Mariano-Jelicich et al. 2008).

A recent study in the southern portion of the range in North America, in southwest Florida, used naturalists' photos from social media to evaluate the diet of Black Skimmer chicks (Forys and Hevesh 2017). Parents fed only fish to their offspring. Most were marine species, but almost 20 % were exclusively freshwater species, which showed that skimmers at four coastal colonies (Indian Shores, Marco Island, Sand Key, and St. Pete Beach) also foraged in freshwater ponds during the breeding season. Chicks consumed 22 different species of fish, 9 of which had not been documented in any prior studies. About half of the fish prey were from 3 genera, the most common in order of abundance being Kingfish Menticirrhus sp. (M. americanus and M. littoralis), Bluegill (Lepomis macrochirus), and Atlantic Needlefish (Strongylura marina). While there was no difference in the species distribution of fish fed to chicks of different ages, parents generally delivered smaller fish to younger (downy) chicks compared to older (feathered) chicks. One observer even noted that when a large fish was fed to a downy chick, the adult broke bones of the fish before feeding it to the chick (Forys and Hevesh 2017).

Skimmers seem to select feeding areas rather than individual prey items, typically in shallow and calm coastal waters where small fishes and juvenile forms are abundant. As their feeding is restricted to the top layer of water, it seems likely that skimmers opt to forage in shallow water because prey there are concentrated near the surface, making them easier to capture (Erwin 1977, Black and Harris 1983).

Black Skimmer hence is associated with fisheries on both coasts of South America, such as upwellings due to the Humboldt Current on the Pacific side, and the mouths of the Amazon and Río de la Plata on the Atlantic side. They also may seek low water conditions in freshwater systems, to find more concentrated prey (Davenport et al. 2016). Thus, Black Skimmers feed in freshwater during the breeding season, but also in brackish water and along coasts during winter (Mariano-Jelicich et al. 2003, 2008). Field studies of skimmers in Virginia showed that roosting and feeding sites must be close to each other, to make capture of small prey energetically cost-effective (Erwin 1977).

Although skimmers are active during the day, they also feed at night (e.g., Yancey and Forys 2010), presumably because their tactile feeding enables successful fishing in low light conditions (Gochfeld and Burger 1994). Murphy (1936) stated that they are mainly nocturnal feeders, suggesting that they might be more active at night because that is when aquatic organisms approach the surface more frequently. Other observers, however, regularly have documented feeding in daytime, at appropriate tide cycles, especially during the raising of chicks (Gochfeld and Burger 1994). While Black and Harris (1981) reported that nocturnal feeding is not prominent in Florida, another study found that light levels were the only environmental variable that influenced what time of day Black Skimmers depart on foraging trips, such that feeding peaked when light levels are low (Yancey and Forys 2010).

The unique bill and flight mechanics of Black Skimmer enables it to catch fish while flying (Zusi 1962, Withers and Timko 1977). Gochfeld and Burger (1994) provided evidence of fish capture via the skimming technique, and stated “a feeding skimmer flies low over the water with its bill open and its lower mandible slicing the surface. When the mandible touches a fish, the upper bill (maxilla) snaps down instantly to catch it”

Murphy (1936) reported is as a "fact that may be freely accepted that skimmers sometimes fish from a standing point of vantage ... striking, as better waders such as herons might to do, at the minnows which scurry across the shallows"; it is unclear whether this "fact" is based on Murphy's personal observations, or on other reports, but Gochfeld and Burger (1994) regard this and similar reports of feeding while standing or by striking at the water to be "either rare or erroneous".