Demography and Populations

The below pertains to the Hawai'i 'Akepa, unless otherwise noted. Presumably, the Maui 'Akepa is similar.

Age At First Breeding

One female seen to successfully breed in her second year (first spring after hatching); other females first seen to successfully breed in their third year, on basis of observations of birds banded as juveniles (JKL). Other females believed to breed successfully in their second year, on basis of plumage characters. No successful breeding by second-year males (female-like subadult plumage), and only 2 attempts recorded in 7 breeding seasons (Lepson and Freed 1995 ). Third-year males (male-like subadult plumage) breed with success comparable to that of adult males, on basis of percentage of nests that fledged young: 83% (n = 6) versus 79% (n = 47), respectively, of nests of known fate (Lepson and Freed 1995 ).

Clutch

Clutch size 1–2 eggs; 1 report of 3 eggs (2 of which hatched; Sincock and Scott 1980 ). Geographic variation not known, but likely to be small, given small clutch size. No data from Maui or O‘ahu akepas.

Annual And Lifetime Reproductive Success

Annual reproductive output remarkably low for a small passerine. Maximum number of young fledged in 1 yr by a pair is only 2; frequently only 1 young is fledged. Overall, 42 of 53 (79%) nests for which fates were known over 7 yr at Hakalau Forest NWR fledged young (Lepson and Freed 1995 ).

Few data exist on lifetime reproductive success, but some females are in the breeding population at least 7 yr (JKL). Only 1 brood normally raised per season, although 1 pair apparently renested once after successfully fledging 1 young; second brood failed (Lepson and Freed 1995 ).

Two color-banded females at Hakalau Forest NWR in 1995 were at least 10 yr old (JKL). Breeding individuals are resident year-round with little documented emigration; often reuse same nest cavity in successive years, so annual survival is probably accurately estimated by proportion of birds that remain each year. Adult males at Hakalau Forest NWR had minimum annual adult survival of 0.83 (n = 36), and adult females 0.81 (n = 46). Hatch-year birds (sexes combined) had estimated survival of 0.42 (n = 57), second-year males 0.79 (n = 18), and third-year males 0.57 (n = 23; Lepson and Freed 1995 ). Recaptures and resightings were used to calculate unbiased survival estimates with the demographic computer programs JOLLY and SURVIV (Brownie et al. 1986 , Pollock et al. 1990 , White and Garrett 1990 ). Similar estimates were obtained using enumeration methods (Lepson and Freed 1995 ). Survival estimate for birds at Ki¯lauea-Keauhou (east slope of Mauna Loa, 30 km south of Hakalau Forest NWR) was 0.70 for adults (n = 61), calculated with program JOLLY, and 0.23 for juveniles (n = 30), using enumeration (Ralph and Fancy 1994b ). Survival estimate for adults in Ka‘u¯ Forest Reserve (south slope of Mauna Loa, 60 km southwest of Hakalau Forest NWR) was 0.77, on basis of recapturing 3 of 5 birds after 2 yr (Freed 1988 ).

Native Hawaiian forest birds are extremely susceptible to introduced diseases and suffer high mortality from them (Warner 1968 , van Riper et al. 1986 , Atkinson et al. 1995 ). Avian diseases, particularly avian malaria (Plasmodium relictum ) and avian pox (Poxvirus avium ), are considered to be the most important factor in the decline and extinction of many Hawaiian land birds; 9 of 10 ‘I‘iwi died after experimental exposure to single bite of malaria-infected mosquito (Culex quinquefasciatus ; Atkinson et al. 1995 ). Several native forest birds, including ‘Akepa, were noted with pox sores in 1890s, during period of major population declines (Henshaw 1902 , Perkins 1903 ). The almost total absence of most native forest birds below 1,000 m is believed to be largely due to presence of disease vectors, such as introduced mosquitoes, in lowland areas; and low incidence of mosquitoes at higher elevations is considered a major factor in the higher densities of native birds in these areas (Scott et al. 1986 ).

Blood samples from 47 ‘Akepa from Hakalau Forest NWR (where malaria occurs in small numbers of native forest birds) revealed no instances of infection with malarial parasites when examined for unique molecular markers (R. Cann pers. comm.). Also at Hakalau Forest NWR, 2 of 89 (2.25%) mist-netted after-hatch-year ‘Akepa had small pox sores, but no pox sores seen on 69 hatch-year individuals (JKL). No pox noted on ≥30 ‘Akepa mist-netted from e. Mauna Loa (C. Atkinson pers. comm.). Ectoparasites (feather lice and mites) noted on flight-feathers or body on 4 of 89 (4.5%) after-hatch-year ‘Akepa and 2 of 69 (2.9%) hatch-year ‘Akepa (JKL). No data for nestlings.

Little known. Juvenile mortality higher than that of adults (see Life span and survivorship, above); predation by ‘Io observed on juveniles (JKL). Before human settlement 1,000–1,500 yr ago ‘Akepa would have been subject to predation by specialized bird-eating owls (Grallistrix orion on O‘ahu I. and G. erdmani on Maui I.; another species of this genus probably lived on Hawai‘i I., which has a much poorer fossil record) and an accipiter-like harrier (Circus dossenus on O‘ahu and possibly the other islands), all of which are now extinct (Olson and James 1991 ). Rats appear to prey on nestlings (P. Hart pers. comm.). Nestling mortality also likely from starvation, particularly when prolonged inclement weather interferes with foraging by parents. Disease likely causes mortality, particularly at lower edges of range, where mosquito vectors become common (see Disease and body parasites, above). A more detailed discussion of limiting factors for Hawaiian forest birds is found in Scott et al. (1986 : 351–371).

Initial Dispersal From Natal Site

Juveniles range >300 m from nest while part of mixed-species feeding flocks. Dispersal may begin when flocks dissipate in autumn, but males may not disperse for >1 yr after fledging (JKL). Of 58 juveniles banded at Hakalau Forest NWR from 1987 to 1992, 12 males (41%, assuming 1:1 sex ratio of juveniles at banding) and 10 females (34%) were found in study area ≥1–2 yr later, the time needed to accurately sex young birds in the field. Both males and females known to breed within 250 m of natal nest. Maximum distance seen to travel is approximately 5 km for an adult female and, separately, a juvenile (J. Jeffrey pers. comm., P. Hart pers. comm.).

Fidelity To Breeding Site And Home Range

Both sexes strongly philopatric to breeding area. Pairs frequently reuse nest cavities, and have not been seen to move >200 m when they do change nest sites (JKL, LAF).

Dispersal From Breeding Site

Little known. Probably disperse most frequently Oct–Feb after mixed-species flocks dissipate and before breeding season starts. Most known dispersal at Hakalau Forest NWR by juveniles or subadults. One juvenile moved 5 km within 3 mo (P. Hart pers. comm.). Only other known long-distance movement was by an adult female, which moved 5 km from study site within 5 mo of banding, then was recaptured and resighted at banding site 1 yr after banding, and was seen again near original dispersal location 5 yr after banding.

Numbers

The only population estimates are from Hawaiian Forest Bird Survey (Scott et al. 1986 ), which estimated 14,000 ± 2,500 (95% CI) for Maui in mid-1970s and early 1980s (using variable circular plot method during late breeding and early postbreeding seasons, when populations should be at or near maximum). Maui estimate appears overly optimistic, given paucity of recent sightings in comparison with other endangered species on that island (e.g., Maui Parrotbill). This apparent overestimate may reflect limitations of survey method when applied to extremely rare species (only 8 individuals recorded during survey, plus 2 incidental records; Scott et al. 1986 ), or survey may have caught the Maui 'Akepa during a final population collapse. Maui 'Akepa may already be extinct (T. K. Pratt pers. comm.).

Trends

Maui 'Akepa apparently declining, if it even still exists: 10 were detected in 1980 on Hawaiian Forest Bird Survey (Scott et al. 1986 ), 1 in 1988 (Engilis 1990 ), and none confirmed since, despite extensive fieldwork since 1992 (T. K. Pratt pers. comm., JKL).

Little known. Current population probably limited mostly by habitat abundance and quality—particularly availability of suitable large trees for nest cavities, and disease.

Predation known to occur, but significance as a limiting factor not known. Black rats (Rattus rattus ) occur at Hakalau Forest NWR in some of the highest densities ever documented in the wild (G. Lindsey pers. comm.), and probably harm ‘Akepa populations either directly through predation or indirectly through competition for cavities (where rats roost during day), yet ‘Akepa nesting success and survival are both high (see Measures of breeding activity and Life span and survivorship, above). Low reproductive output of ‘Akepa (see Measures of breeding activity, above) means that even low levels of predation may have serious negative consequences.

Years with periodic outbreaks of introduced western yellow jackets (Vespula pensylvanica ), which prey extensively on exposed insects (Vitousek et al. 1987 ), coincided with years of poor reproductive success in ‘Akepa and other insectivorous Hawaiian honeycreepers at Hakalau Forest NWR, indicating possible competition for food resources (P. Hart pers. comm., JKL, LAF). Nesting success (based on number and proportion of nests where young survived to departure) during 1 yellow-jacket outbreak (1992) was comparable to years with few yellow jackets, but many fewer juveniles were captured or observed in flocks, suggesting an increase in postfledging mortality (JKL). In another yellow-jacket outbreak (1995), ‘Akepa and other honeycreepers appeared to forgo breeding nearly entirely or to experience brood failure before fledging (P. Hart pers. comm., LAF). Food resources likely to be limited also at lower elevations from effects of introduced ants (Pheidole megacephala , Iridomyrmex humilis , and Anoplolepis longipes ), which decimate populations of native arthropods where they co-occur (Zimmerman 1979 , Vitousek et al. 1987 ).

Disease is also likely to have an effect (see Disease and body parasites, above), particularly at lower edge of species' range, where mosquito vectors are more common, but may also affect higher-elevation populations because of vertical movements by birds or localized incursions of vectors into higher elevations (Scott et al. 1986 ).