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Demography and Populations
Measures Of Breeding Activity
Age At First Breeding; Intervals Between Breeding
First nesting in third spring or summer, at age of 2 yr. Ibises in captivity may breed as early as second spring (9 – 10 mo of age). Nest yearly thereafter if conditions appropriate. No records of two successful nesting attempts by individuals in the wild in a single year.
2–5, averaging 2–3 depending on location; 6–7 eggs in single nests at a coastal S. Carolina site probably represent egg dumping. In s. Florida Everglades, mean clutch size ranges 2.20 (coastal habitat, SD = 0.64, n = 290, Kushlan 1977c) to 2.72 (interior marshes, SD = 0.534, n = 94, Frederick and Collopy 1989a); central Florida 2.49 (0.56, n = 202; Kushlan 1977c); n. Florida inland 2.16; coast 2.07 (Rudegeair 1975b). At a coastal N. Carolina colony site, 2.23 (n = 287) in 1981 (Allen-Grimes 1982), 2.65 (n = 262) in 1983, and 2.46 (n = 493) in 1984 (Shields 1985). Clutch size decreased over the course of the season in 1981 (2.32, Apr; 2.10, May; 1.95, Jun; Allen-Grimes 1982). In coastal Louisiana 1.97 (0.70, n = 100; Hammatt 1981). In coastal S. Carolina 2.55 (n = 380; Frederick 1987c).
Clutch sizes smaller on the coast than inland, at least at some locations in some years, suggesting a physiological or ecological constraint. In captivity with unlimited food, mean clutch size varied between years 2.66 (mostly first time breeders, n = 38, SE = 0.197), 3.28 (mostly second time breeders, n = 53, SE = 0.45, Frederick unpublished). A similar study of approximately 400 mixed-age Scarlet Ibises in captivity reported 2.4 ± 0.7 SD (Babbitt and Frederick 2008).
Annual Reproductive Success
In Florida, mean number of young/nest at 40 d of age: Everglades 1.05 (SE = 0.22, n = 42 nests), coast 1.03 (0.18, n = 93), central Florida 1.03 (0.16, n = 21 (Kushlan 1977c); at 20 d of age, 2.11 (SD = 0.65, n = 245) – 1.1 (SD = 1.11, n = 267) (Frederick and Collopy 1989a). In coastal N. Carolina 1.05 – 1.30 young fledged/nest over 2 yr (n = 755 nests). In coastal Louisiana only 10% of laid eggs produced 35 d-old young (n = 102 nests); in coastal S. Carolina over 2 yr at a single colony site, 39% (n = 390 eggs) and 3% (n = 347) produced 35 d-old young.
Considerable annual variation in per-pair productivity is typical at colony sites, depending upon local conditions. In coastal N. Carolina, 1.70 young per successful pair (n = 129, Allen-Grimes 1982). Nest success from all reports in all locations (% nests with eggs producing ≥1 young to 20 d of age, prorated for daily survival throughout the period) varied from <5% to 70%, No obvious geographic pattern in this variation, and this complete range of production was exhibited even within the Everglades. In N. Carolina, nest success 59% (Allen-Grimes 1982); Lake Okeechobee FL, 34 – 47.8%; Everglades, FL, 5 – 70% (Herring 2008, see also summary in Semones 2003).
Nesting success and productivity are determined both by predation on nest contents, and by availability and quality of available prey before and during nesting. Nest failure in s. Florida is often triggered by rising water resulting in dispersed prey and poor foraging (Frederick and Collopy 1989, Herring 2008), and abandonment of entire colonies or sections thereof is common during these conditions. In four years in Everglades (1986, 1987, 2001 and 2002), abandonment accounted for 47% (n = 168 nests), 68% (n = 90 nests), 96% (n = 422) and 97% (n = 324) of nest failures (Frederick and Collopy 1989a, Semones 2003). In years with high food availability in the Everglades, both adults and young were in better physiological condition and had lower stress levels than in years with poor food availability (Herring 2008). In years with high food availability, fledging success was 86.8%, compared with <20% in year with poor food availability. Clutch size and numbers of fledglings decreased by 21% and 26% in years of low food availability (Herring 2008).
In captivity, breeding and production of Scarlet Ibis young was related to body condition. Mean difference in mass between successful and non-successful breeding groups were 18.92 g (2.0%) for males and 31.42 g (4.2%) for females in 1998, and 16.81 g (1.8%) for males and 16.88 g (2.3%) for females in 1999 (Babbitt and Frederick 2008).
White Ibises require a succession of suitable feeding sites through the nesting season in order to maintain nesting (Kushlan 1989b, Herring 2008). Although successful nesting has often been reported on diet of crustaceans (Bildstein 1993, Frederick and Collopy 1989a), high food availability and productive nesting in Everglades is often associated with a high proportion of fish in the diet (Herring 2008, Dorn et al. 2008). In coastal/estuarine situations, chick survivorship depends on a diet with low salt content (Johnston and Bildstein 1990).
Parental care and nest success may also be negatively affected by contaminants such as mercury (Heath and Frederick 2005). Ibises raised in captivity on diets with mercury levels typical of those found in the Everglades food chain had significantly lower proportions of nest starts producing eggs than controls and high proportions of male-male pairing (P. Frederick unpublished).
Number Of Broods Per Season
Probably one, except when renesting occurs following failure of early nesting attempts. Two radio telemetry studies of breeding adults in Everglades suggest either no renesting following nest failure or successful nesting (Herring pers. comm.), or confirmed renesting in a small % of failures (Heath 2002). In captivity, up to 3 broods per season.
Proportion Of Total Females That Rear At Least One Brood To Nest-Leaving Stage
Not known. In captivity with unlimited food, 30 – 34% of mixed age females and 34% of males produced eggs or young (Babbitt and Frederick 2008). Some of these birds were old enough to be reproductively senile. In known-age birds, 100% of second-time breeders (age 3 yr) attempted to breed.
Life Span And Survivorship
No data on survivorship in the wild.
Mortality And Disease
Nestling mortality is greater during the first 20 d after hatching than thereafter. Although ibises typically lay 2-5 eggs, the number of young/nest in Florida 20 d after hatch declined considerably: Everglades 1.38 (0.49, n = 53), coast 1.14 (0.45, n = 64), central Florida 1.25 (0.43, n = 133) (Kushlan 1977c). Nestling survivorship to 10 d of age in coastal N. Carolina: 89% (n = 410 hatched eggs), 86% (n = 596 eggs), and 68% (n = 284 nests) during 3 yr at the same colony site (Allen-Grimes 1982). Nestling survivorship to 35 d in coastal Louisiana: 55% (n = 102 nests; Hammatt 1981). Nestling survivorship to 21 d in Everglades 37 – 83%.
Using radio telemetry, Semones (2003) measured survival in Florida: from 22 d to independence, 67 – 97%; to 60 d, 59 – 94%; and to 90 d, 44 – 78%. Young birds can be susceptible to inter-specific (e.g. Black-crowned Night Herons, Davis 1993) or rarely, intra-specific predation (Herring et al. 2005). Complete abandonment of nests happens when food resources become unavailable, and nestlings in entire colonies or large portions thereof can die during these events (Adams and Frederick 2008).
Causes of adult mortality poorly known. Many ibis bands recovered in Cuba during the 1950s and 1960s came from hunters (Frederick et al. 1996). At least 50 adult ibises were killed during a single fire in the Florida Everglades (Epanchin et al. 2002). Ibises may be susceptible to alligator predation because of feeding habits and habitat, but there are few quantitative studies of this (see Delaney 1986).
Diseases and parasites well studied in Florida, and summarized in detail by Forrester and Spalding (2003). 23 species of trematodes, 18 species of nematodes, 2 cestodes, 2 acanthocephalans, 3 species of mites and 5 species of chewing lice found in ibises, nearly all in Florida. Of tremadodes, some in high proportions of birds examined (eg, 61 – 75%). Birds from freshwater habitats harbored more nematode species, those from saltwater habitats more cestodes. Nestlings lacked four of the species found in adults in the same habitats, including the nematode (Skrjabinoclavia thapari), which uses fiddler crabs as an intermediate host.
Both juvenile and adult ibises were, however, infested with a single ancanthocephalan species, Southwellina dimorpha, which uses crayfish as an intermediate host. Eustrongylides ignotus found in 5 of 162 nestling ibises, and was the probable cause of death in four of these individuals. Only two protozoans reported from ibises, and of these Haemoproteus plataleae were by far the most common. No bacterial related diseases reported for this species. No antibodies to Eastern equine encephalitis detected (n = 12), and antibodies to St. Louis encephalitis found in 1 of 12 birds examined from s. Florida.
Initial Dispersal From Natal Site
For juvenile birds banded at their natal site, 26% of returns found at the banding site at least 5 mo later (Melvin et al. 1999). Returns from outside the banding location ranged between 18.5 - 2,479 km (mean = 545 km, Melvin et al. 1999). These distances may represent post-fledging dispersal or migratory movements. Natal philopatry has not been demonstrated and, given enormous fluctuations in colony size and occupancy, is probably weak. Two nestlings individually marked at a coastal colony site in S. Carolina bred at a coastal colony site 100 km northeast in N. Carolina. Young birds more likely to disperse north or inland of colony than adults (Frederick et al. 1996).
Fidelity To Breeding Site And Winter Home Range
Strong propensity to change colony sites, so nesting numbers fluctuate annually (Frederick et al. 1996). Small numbers of individually marked breeding birds have returned in later years to a colony site in coastal S. Carolina (Frederick 1987c) and s. Florida (JH). One ibis returned to within 8 km of capture site in N. Everglades in next winter (Herring pers. comm.). Within S. Carolina, reciprocal shifts annually in numbers of pairs breeding at traditional colony sites suggest considerable movement, probably due to changing environmental conditions.
Dispersal From Colony
Among 10 marked adult birds breeding in s. Florida, dispersal between successive breeding colonies within the same year ranged from 29-54 km, average = 42 km (n = 5). The other 5 ibises wintered in s. Florida but dispersed > 150 km in the year following successful reproduction because of poor breeding conditions (JH). Following unsuccessful breeding attempts, radio-tagged adults dispersed to a site >30 km from colony (Herring pers. comm.)
At a coastal site in South Carolina, ibises traveled 4-32 km to freshwater wetlands to secure food for nestlings (De Santo et al. 1997). This is comparable to distances flown from freshwater colonies in s. Florida (<1 km – 33 km, Frederick and Collopy 1989a and JH), and a lake colony (0.1-33.3 km, mean = 2.7, n = 286, Smith 1995a). Ibises nesting in a North Carolina estuary flew shorter distances (1-6.2 km, Custer and Osborn 1978).
Estimates Or Counts Of Density
The most numerous ciconiiform wading bird in many areas where it occurs. Extreme inter-annual variation in local nesting numbers due in part to highly nomadic breeding habits. All measurements of population size are counts or estimates of breeding birds on nests. These likely to be underestimates because of visual occlusion by vegetation, asynchronous nesting, and incomplete or nonsystematic survey technique (Frederick et al. 2006, Williams 2007).See below:
Range has increased and consolidated over past decades. Because of propensity to change colony sites, nesting numbers fluctuate annually and among decades in any one area (Kushlan 1977c, 1979c). As a result, some local populations, such as that nesting in the Florida Everglades, have decreased markedly in the past few years. Even so, numbers as a whole continue to be relatively high for a large wading bird. The decrease in the Everglades is because water management has altered the natural hydrologic regime of the marsh (Kushlan 1987, 1989b). Similarly, in Trinidad, the cessation of breeding by Scarlet Ibises also appears to be due to an altered hydrologic regime (Bildstein 1990). Whether such populations have decreased primarily through mortality or population shifts remains unknown. In Trinidad, at least, the latter seems to have played a major role.
Total Population. Frequent inter-year movements and low breeding site fidelity mean that opportunities for estimates of entire U.S. population are rare. During 1930s some evidence that vast majority of birds were breeding in south Florida (as below), up to 400,000 birds, more regularly 250,000. By 1970s, range-wide estimates suggested peak of 350,000 breeding birds, and by 1991, 102,000, an apparent declining trend. Since that time, years with simultaneous counts in Texas, S. Carolina, N. Carolina, Florida, and Louisiana indicate a minimum of 166,000 breeding birds in 2001, and 209,000 in 2004. This suggests U.S. population has held steady or increased somewhat since 1991. Note that sources of error in counts of ibis colony sizes are nearly all biased to result in undercounts, sometimes by very large amounts (Williams 2007).
South Florida. In 1930s and 1940s, 100,000 – 400,000 individuals breeding in several years, 50,000 – 250,000 more commonly (Crozier and Gawlik 2003b). The majority of ibises in the U.S. may have been concentrated by drought in s. Florida in the mid-1930s, suggesting a peak population of > 400,000 breeding adults at that time. By 1970s, maximum counts of at least 50,000 breeding individuals in each of 3 yr, including > 35,000 in one colony (Kushlan 1973b, Kushlan and White 1977). Breeding population averaged 9,200 pairs (range 500 – 26,000) 1986 – 1999. Approximately 50% decrease in nesting numbers between 1985 and 1994, coincided with large increases in Louisiana. Large increase in breeding numbers 2000 - 2007, average 44,511 birds (range 25,000 – 65,000).
S. Carolina. (W. Post, in litt., editing and consolidation by PCF): First statewide estimates from 1975 suggested about 80,000 breeding birds (40,000 on Drum Island, Charleston Co., 39,000 on Pumkinseed Is., Georgetown Co.). During normal rain cycles, total state population 28,000 – 40,000 birds usually found in 1-2 coastal and 1 – 2 inland colonies. Large inland colonies in Lake Marion (16,000 birds in 1999) and Boykin, in Kershaw Co. (6,000 birds in 2000, Post and Gauthreaux 1989). In non-drought years, breeding about equally divided between coastal and inland locations.
Population in S. Carolina has declined since 1975, with no birds nesting in large central-coast colonies. Large interior colonies likely abandoned because of persistent drought in late 1990s and early 2000’s. Since 2000, main colonies have been near the mouth of the Savannah River, in Jasper Co. (8,000 birds in 2000), and near Yemassee, in Colleton Co. (20,000 birds in 2000). No statewide surveys since 2005.
N. Carolina. Between 1977 and 2007, 3,878 - 34,086 nesting birds in coastal zone, with markedly increasing trend. Since 2000, average of 31,820 nesting birds annually. Important colonies at Battery I. (Brunswick Co., to 26,000 breeding birds 2007), Morgan I. (Carteret Co., to 2,880 birds, 2007), and Roanoke Sound Island “G” (Dare Co., to 2,920 birds, 2006).
Texas (R. Telfair): Between 1973 and 2000, annual breeding population fluctuated between 3,200 and 64,000 birds (Texas Col. Waterbird Soc., unpubl.). Notable colonies at N. Deer Island (Galveston Bay, Galveston Co., to 44,000 birds), Lavaca Bay Spoil (Cameron Co., to 6,600 birds), Dressing Pt. (Matagorda Co., to 12,000 birds), Demijohn Lake (Liberty Co., to 16,000 birds). In general, the wetter the winter-to-spring season, the larger the number of breeding birds (Telfair 2002). Increase over time, from average of 7,800 birds statewide early 1970s to average of 17,600 birds 1993 – 2006.
Louisiana. Spotty record of monitoring in this state, compared to other states in the species range, despite evidence of a large proportion of nesting in U.S. occurring there. Minimum of 40,000 breeding birds reported as early as 1950, though survey probably not systematic or statewide. More thorough surveys suggested 44,000 - 82,000 breeding birds between 1972 and 1976. A large increase in breeding and wintering numbers in mid – late 1980s was associated with a rapid increase in crayfish aquaculture (Fleury and Sherry 1995). Numbers of breeding birds during late 1980s (26,000 – 76,000) did not suggest an increase, but these surveys were not statewide.
Two systematic surveys in 2001 (48,000) and 2004 (68,000 breeding birds) suggest Louisiana remains an important place for breeding ibises in the southeast. Large colonies in 2001 survey include northwest of Lake Pontchartrain (19,000 birds, Tangipahoa Parish), northwest of Ville Platte (8,200 birds, Evangeline Parish), northwest of Raceland (7,300 birds, LaFourche Parish) and west of Houma (4,000 birds, Terrebonne Parish).
In winter, in the continental U.S., small populations found in coastal S. Carolina, Louisiana, and Texas, but greatest concentrations in peninsular Florida (> 1 individual/CBC party hour), especially along the Gulf Coast near Tampa Bay (Root 1988; http://www.mbr-pwrc.usgs.gov/bbs/cbc.html). From 1999-2003 Christmas Bird Count counts, 41,000 – 48,000 individuals in Florida (http://www.mbr-pwrc.usgs.gov/bbs/cbc.html).
See above. Range increased and consolidated 1930s – 1990s. The Everglades/s. Florida population declined by > 95% from the 1930s to 1970s, and by an additional 80% from the highs of the 70s to the highs of the 80s (Rodgers et al. 1996). During late 1990s to 2006, population apparently increased sharply.
Decreases in the Everglades are likely the result of water management that has altered the natural hydrologic regime of the marsh (Kushlan 1987, 1989b). In addition, exposure to mercury may have hindered reproductive success.
In Trinidad, the cessation of breeding by Scarlet Ibises also appears to be due to an altered hydrologic regime (Bildstein 1990). Whether such populations have decreased primarily through mortality or population shifts remains unknown. In Trinidad, at least, the latter seems to have played a major role.
McFarlane (2002) found the Texas breeding population between 1973-2000 had an annual increasing trend of 3.0%. Breeding Bird Survey data for Texas (Sauer et al. 2005) give annual trends of +8.4% (1966-1979), +8.9% (1980-2005), and +18.3% (1966-2005).
Over the entire range, Breeding Bird Survey (BBS) data suggest a slight positive trend in the U.S., 1966-2006 (Sauer et al. 2007). This echoes the analysis (above) of colony counts throughout the range. For 1980-2006: Florida (+2.2%/yr); Georgia (-1.5%); S. Carolina (-7.7%); N. Carolina (+3.9% [few data]); Texas (+8.0%) (Sauer et al. 2007).
Although local annual reproductive success may be low at a particular colony, the southeastern population appears to be stable or increasing. Apparently, dispersal patterns and an ability to postpone nesting mean that some reproduction occurs each year within the North America range. However, at a local scale populations and reproductive effort can fluctuate dramatically. For example, at a well-studied S. Carolina colony (cf., Bildstein et al. 1990), the number of breeding ibises plummeted from over 10,000 pairs in 1989 to zero in 1990, following the passage of a category 4 hurricane. Although the site itself suffered little habitat modification as a result of the storm, nearby freshwater feeding sites, which have been shown to be essential for successful breeding at the site (Johnston and Bildstein 1990), underwent substantial saltwater inundation as the result of the storm surge that accompanied the hurricane (Shepherd et al. 1991). See Food Habits: nutrition, for a discussion of the importance of freshwater prey to nestling ibises. Similarly, several years with over 200,000 birds breeding in the Everglades in the 1930s were interspersed with years when fewer than 1,000 bred (Ogden 1994).
Nesting failures (Semones 2003) and predation of eggs and nestlings, especially when exacerbated by reduced prey availability (Frederick and Collopy 1989a) and competition for nest sites, have the potential of exerting a strong effect on reproductive rates. High mortality among young, disease or exposure to contaminants, and habitat loss are probably important components of mortality rates. High availability of food throughout the reproductive season seems to be a driver of adult physiological condition and fecundity.
Heath, Julie A., Peter Frederick, James A. Kushlan and Keith L. Bildstein. 2009. White Ibis (Eudocimus albus), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/009