Demography and Populations

Large colonies tend to be traditional and are occupied each year, even after years of failure (Burger 1982a); small colonies likely to be abandoned (Erwin et al. 1981) after a failure due to predation, less likely if failure due to flooding (Burger 1982a). Banding recoveries of adults indicate strong tendency to return to same colony where they previously bred successfully (76% to same colony), but not necessarily to where they were hatched (35%). Some movement from year to year between beach and marsh habitats documented by banding (Burger and Gochfeld 1990).

Age At First Breeding; Intervals Between Breeding

One breeding-plumaged female found dead in a nesting colony at 11 mo of age. Several birds in nonbreeding plumage (white nuchal collar present) seen in colonies in most years. Youngest breeding adult trapped on nest was 2 yr old, but many do not breed before 3 yr; some males probably not until 4 yr. At present no reason to suspect that adults do not try to breed each year.

Clutch Size

From MG (unpubl. data). Varies from year to year, apparently dependent on food availability. In New York beach colonies, 1–5 eggs (rarely 6+; mode = 4), although in poor food years mode = 3. In most years, about 10% of earliest nests have 5 eggs; late or replacement nests only 2 or 3, and 3rd and 4th replacement clutches usually 1 or 2. In New Jersey marsh colonies, 1–6 eggs, mode 3–4 depending on year. Clutches of 7–10 almost certainly from 2 or more females. Clutch sizes may decrease southward with mean of 3.5 in Virginia (Erwin 1977a), 2.3–3.2 in Texas. The estimate of 2.9 in California (Grant and Hogg 1976) was based on a single visit and may be an underestimate (Clapp et al. 1983).

Annual And Lifetime Reproductive Success

In most years only about 50% of pairs raise a young to fledging. Two surviving chicks is not unusual; fledging of 3 chicks occurs < 1% of time (on Long Island, NY). No comparable data elsewhere in range.

Hatching success ranges from 0 to 85%. A comprehensive study of 14 New Jersey colonies in 1979 (Burger and Gochfeld 1990) showed from 0–54% of eggs hatched (weighted mean 43%), with 39% of eggs lost to flooding and 3% to predation; 16% of eggs infertile or addled, probably due to water damage rather than true infertility. In that sample, 137 of 297 nests hatched 0 eggs. Since chicks mobile at hatching, nest site often abandoned before last egg hatches (Burger and Gochfeld 1990). In Virginia, Erwin (1977a) found 84% hatching; in Texas, 50–60% of eggs hatched (Custer and Mitchell 1987).

Success usually higher in larger, well-established colonies. Salt marsh colonies often productive in years when beach colonies fail and vice versa. At Cedar Beach, NY, productivity was 0.91, 0.81, 0.25, and 0.54 young/pair for the years 1976–1979 (Burger and Gochfeld 1990), and ranged from 0.98 to 2.05 for different subcolonies in 1980 (Safina and Burger 1983). Considering each occupied colony in any year as an occupancy, 38% of occupancies in New Jersey from 1976–1988 had total reproductive failures and maximum average annual productivity was 1.41 young/pair, although one small colony yielded 2.77 young/pair in 1 year. Young fledged per pair reported as 0.4 in Virginia (Erwin 1977a), 1.2 in S. Carolina (Blus and Stafford 1980), and 0–2.0 (White et al. 1984) and 1.1 (Custer and Mitchell 1987) in Texas. Rooftop colonies in Florida have low success (Gore 1987).

Major causes of low productivity and even total colony failure are flooding and predation. A 1979 study of 297 nests in 14 New Jersey colonies showed that larger clutch size, earlier initiation date, greater synchrony, and smaller nearest neighbor distance explained 62% of variance in hatching success. These variables may be concomitants of successful colonies in general, rather than causes of improved hatching success (Burger and Gochfeld 1990). Species of nearest neighbor did not influence nest success (Erwin 1979).

In Virginia, Erwin (1977a) found that only 1st- or 2nd-hatched chicks fledged, but elsewhere 3rd chicks may fledge if older siblings die (MG, JB unpubl. data). Erwin (1977a) reported average chick survivorship of 16.9 d for 1st chicks, 9.3 d for 2nd chicks, but only 2.5 and 0.5 d for 3rd and 4th chicks, respectively.

Longevity up to 20 yr (Clapp et al. 1982), but most breeding adults of known age are 5 to 9 yr old. High annual survivorship after 2nd yr inferred from banding studies, but no quantitative data. Needs study.

Little information. Chicks with subcutaneous emphysema found in some years (Gochfeld 1974b). Other conditions include myiasis (maggot infestation), premature feather loss (Gochfeld 1971), and oiling (Gochfeld 1979b). In some years, diarrheal epidemics have killed 10–15% of fledglings and some adults in New York and New Jersey, but no successful identification of pathogen. Occasionally succumbs to botulism, with Type C enterotoxin identified in Florida (Kinsella 1972). In early 1970s, some Long Island, NY, skimmer chicks developed feather loss (Gochfeld 1971), and other developmental defects occurred at levels up to 1% of all chicks; causes not documented but possibly due to malnutrition, viral infection, or toxic chemicals (Burger and Gochfeld 1990).

Kinsella (1972) reported 1 species of cestode, 6 trematode, and 3 nematode enteroparasites among 5 skimmers that died from botulism; Parvitaenia ibis (Cestoda) and Stephanoprora denticulata (Trematoda) were present in 100% of the birds. Although not specifically studied, external parasites are rarely seen, although 1 chick with a damaged bill was grossly contaminated with feather lice (Burger and Gochfeld 1990).

Skimmers are exposed to metals, pesticides, and other toxicants primarily through their food; also to oil by direct contact. King and Cromartie (1986) found levels of mercury in skimmer livers at 1–2 µg/g (ppm wet wt). We have seen an increase in skimmer eggshell thickness in New York and New Jersey since the early 1980s (see Breeding: eggs/eggshell thickness). DDE concentrations in Texas birds declined from 1979–1981 (mean 9.8 ppm) to 1984 (mean 3.2 ppm; max = 28.4 ppm), and were higher in eggs from nests where none hatched (mean = 5.9 ppm) than in eggs from nests where some eggs hatched (mean = 1.9 ppm). Custer and Mitchell (1987), however, concluded that organochlorines are not currently responsible for population declines in Texas.

At Lavaca Bay, TX, skimmer eggs averaged 460 µg/g (ppm) of mercury, 750 µg/g of selenium, 3,400 µg/g DDE and 1,300 µg/g of PCBs. These levels were below levels known to be toxic to embryos, and there was no decrease in clutch size or shell thickness (King et al. 1991). PCB levels averaged 800 µg/g (max = 9,100) (Custer and Mitchell 1987). In Galveston Bay, TX, PCB levels averaged 0.31 ppm in fish, 5.5 ppm in adult carcass, and 4.2 ppm in eggs, with a maximum of 10 ppm; thus fish to bird concentration factor was about 18 (King 1989). Near agricultural areas, DDE:PCB ratio >1; near industrial areas, ratio <1 (King 1989).

Hatching failure may be related to adult behavior rather than to toxicity; i.e., poor incubation, low attentiveness (MG). Breast feathers of 18 adult females and 13 males from Cedar Beach, NY, (1991) were analyzed for metals. Females had significantly higher levels of lead (geometric mean 1,480 vs. 1,130 µg/g) and cadmium (48 vs. 26 µg/g). Other elements did not differ with gender (mercury about 1,200 µg/g; chromium about 9,000 µg/g; manganese about 2,400 µg/g; copper about 26,000 µg/g; selenium about 1,200 µg/g) (Burger and Gochfeld 1992b). In 1989, lead and cadmium levels were measured in 8–15 skimmer eggs and in feathers from 8–13 fledglings from 2 Long Island, NY, and 2 Barnegat Bay, NJ, colonies. Geometric means for lead ranged from 297 to 411 µg/g (ppb dry wt) in eggs and 982–1,820 in feathers; cadmium ranged from 3–4 µg/g in eggs and 33–105 in feathers. Thus adults did not have significantly higher levels than young. Skimmers had lower levels of cadmium than Common or Roseate (Sterna dougallii) terns or Herring Gulls in New York and New Jersey (Burger and Gochfeld 1993). Melanin-rich black back feathers had higher cadmium (171 µg/g vs. 83 µg/g) but equal lead (about 1,800 µg/g) than white breast feathers (Gochfeld et al. 1991).

Because skimmers do not dive into water, and rarely land on it, they should experience little oiling. On w. Long Island, NY, however, these birds had higher rates of oil spotting (up to 2%) than terns, perhaps due to contamination while resting on beaches (Gochfeld 1979b). Clapp et al. (1983) found no evidence of oil-related mortality other than 2 reports of band recoveries from oiled birds in Florida.

Adults and young sometimes tangle in (and are killed by) kite string, fishing line, or plastic six-pack holders (Gochfeld 1973).

Most data are from the late 1970s, with irregular censusing since then. In the 1970s, the Black Skimmer was considered a vulnerable and declining species (Downing 1973), but recent evidence suggests its population has stabilized in many states, albeit at lower levels than in the early 1960s. Only a few pairs breed in Massachusetts (Veit and Petersen 1993), but about 500 pairs breed on Long Island, NY (population appears stable over the past 20 yr with 339 to 562 pairs), over 900 in New Jersey (a decline), and about 2,800 pairs on the Delaware-Maryland-Virginia (Delmarva) peninsula (Erwin et al. 1981). Clapp et al. (1983) estimate about 30,000 breeding pairs from N. Carolina to Texas in 1976, and the North Gulf Coast (Alabama to Texas) had about 15,000 pairs of skimmers in 37 colonies, most in Louisiana (Portnoy 1978a, b).

New York had 339 to 495 pairs (mean = 418) in 10 to 13 colonies from 1974–1978 (Buckley and Buckley 1980) and 359 to 562 pairs (mean = 450) in 7 to 11 colonies from 1984–1989 (Downer and Liebelt 1990). New Jersey had about 1,130 pairs in 1976 and 828 in 1992, with 17% drop between 1979 and 1980; its population now stable with 8 to 12 colonies (JB). N. Carolina had 1,525–1,925 breeding pairs from 1973–1977 (Parnell and Soots 1980); S. Carolina 790 pairs in 10 colonies in 1975 (Blus and Stafford 1980) and 1,600 pairs in late 1980s (Post and Gauthreaux 1989); Georgia about 380 pairs in 3 colonies in 1976 (Portnoy et al. 1981). In the mid-1970s at least 1,450 pairs bred in ne. Florida (Clapp et al. 1983); nests scarcer in s. Florida, but at least 800 pairs bred on Gulf Coast in mid-1970s (Clapp et al. 1983). In Alabama, breeds mainly at Dauphin Is. (about 250 pairs), and in Mississippi about 360 pairs in 4 colonies (Portnoy 1977). A key breeding concentration in Louisiana, with 14,380 pairs estimated in 1976 (Portnoy 1977); the 2,425 pairs on East Timbalier Is. is now the largest in the e. U.S. (Clapp et al. 1983). In Texas, 9,810 pairs estimated in 1976 (Clapp et al. 1983), 8,300 in 1980 (Texas Colonial Waterbird Society 1982).

Colony Sizes

Highly variable, from single pairs to many thousands on the Gulf Coast (for summary of 1976 U.S. population, see detailed map in Clapp et al. 1983: 732). Along the Atlantic Coast, average size increases from New England southward. New York and New Jersey beach colonies range up to 400 pairs. Salt marsh colonies much smaller than dredge island or barrier beach colonies in New Jersey and Delmarva peninsula (Erwin et al. 1981, Burger and Gochfeld 1990); largest marsh colony 82 pairs. Colonies of 1–2 nests almost always eliminated by predators. On the n. Gulf Coast in 1976, 11 of 37 colonies exceeded 1,000 pairs (Portnoy 1977, 1978a); colonies averaged 810 pairs (median 457). Comparable data on the distribution of colony sizes are available for several states from 1977 (see Appendix 1).

No evidence of density-dependent regulation; indeed, larger and denser colonies tend to be more successful than smaller ones. Flooding, storms, predation, and human disturbance are major causes of colony failure, and contribute to low reproductive success in some areas. Food limitation is important in some years (Erwin 1977a). Large colonies on favorable sites remain stable, while colonies on unfavorable sites shift, so that in most years at least some colonies are successful. Major mortality occurs with prolonged summer storms early in Jul, when many vulnerable 2-wk-old young are killed. Young breeding for the first time and re-nesting adults have low success, and late-hatching young probably do not survive, although there has been no adequate study of recruitment to breeding populations.