Demography and Populations

Age At First Breeding; Intervals Between Breeding

Although in Alaska, may not breed until age 2 or 3 yr (Safriel 1971, Safriel inMyers 1981), some yearlings do breed farther south, in Manitoba (Gratto et al. 1983, Gratto 1988). In Manitoba, 25% of females bred first as yearlings, 60% at age 2, and 13% at age 3. Equal numbers of males bred first at ages 1 to 4. Most yearling breeders (70%) were female.

Normally breed every year after first attempt; some males unable to obtain females each year, although they hold territories. Occasionally some females do not lay eggs, even though paired, presumably due to poor feeding conditions. In Manitoba, approximately 50% of females present in 2 of 8 yr did not lay eggs (Gratto-Trevor 1991).

Clutch Size

Almost invariably four eggs, assumed ancestral sandpiper clutch size (Maclean 1972). Many nests with fewer than four eggs have lost egg(s) through predation (Gratto 1983). No evidence that yearling breeders lay smaller clutches (Gratto et al. 1983), although in most years females tend to lay only 3 eggs in renests (CLG-T).

Annual/Lifetime Reproductive Success

In Manitoba, mean number of young fledged per laying pair approximately 0.3 to 1.2 in different years. Mean number of young fledged per successful pair (those hatching chicks) about 1.5 to 1.7. Never more than one brood per season. Proportion of pairs (females) that successfully hatched at least one chick varied from 13%–74% (average approximately 42%); that successfully fledged (to independence) at least one young, from 9%–52% (average approximately 27%, CLG-T).

Survival of eggs to hatching varies greatly among years (see Predation). In one area in Manitoba, survival ranged from 25% to 59% of those laid (0.5–2.4 eggs hatched per female). Survival of chicks from hatching to fledging was about 50%, probably lower in high predation years (CLG-T). Survivorship from fledging to first summer is probably lower than that of adults, but data are lacking. Juveniles often arrive at wintering areas with much lower fat reserves than adults (see Migration). Survival of yearling breeders may be lower than that of adults, but survivorship is difficult to separate from philopatry. Spaans (1984), however, found no difference in the survival of yearlings who did not migrate from that of adults who did. Gratto et al. (1985) estimated average adult survival at 70%. Survival of females might be slightly lower, as mortality appears greater for females on the breeding grounds (Gratto-Trevor 1991), and females seem to have an easier entrance into the breeding population (Gratto 1988). The oldest known individual was 12 yr old (Holroyd and Brown 1970).

Diseases And Body Parasites

No data.

Natal Philopatry

Return rates of chicks (most banded 1–2 d after hatching) to their natal area vary from 4%–12%, averaging 7% (La Pérouse Bay, Man.). Sexes do not differ in return rates, or in actual distance moved between natal nest and first known breeding site. Birds breeding as yearlings nest closer to natal site than those breeding first at two or three years (Gratto 1988).

Fidelity To Breeding Site And Winter Home Range

No information available on fidelity to wintering areas, but evidence of high fidelity to migratory staging areas (R. Morrison pers. comm., L. White unpubl. data). Return rate to previous breeding area high, especially if successful in hatching young the previous year. Females tend to return at slightly lower rates than males (perhaps due to higher mortality), but difference not significant. Males normally return to same territory as previous year, whether or not they retain same mate. Females who change mates nest, on average, farther from their previous nest site. Nest success in previous year affects return rates in both sexes, but not actual distance between old and new nests, or percent divorce for those that return. Returning pairs nest on average less than 50 m from their previous nest, sometimes in the same nest cup (Gratto et al. 1985).

Dispersal From Breeding Site Or Colony

Since only 33% of all yearling breeders in one area had hatched there, > 50% of all breeders must have immigrated from elsewhere (Gratto 1988). No information on dispersal distances, except as discussed under Natal philopatry.

Home Range

No information.

Estimates Or Counts Of Density

Breeding density estimates range from 0.11 pairs/ha (Barrow, AK: J. Myers and F. Pitelka unpubl. data), to 1 pair/ha (La Pérouse Bay, Man., Gratto and Cooke 1987). Distribution appears patchy, even in appropriate habitat.

Population Numbers

Aerial surveys in South America yielded estimate of 2 million for a wintering population (Morrison and Ross 1989). Flocks of over 100,000 may gather during migration (see Migration). Trend analysis of International Shorebird Survey data taken during migration did not indicate significant declines in numbers of Semipalmated Sandpipers in the past 12 years (Howe et al. 1989).

Populations seem primarily regulated through egg and chick predation, and climatic factors influencing breeding and mortality (CLG-T).