Demography and Populations

Age At First Breeding; Intervals Between Breeding

No data; first breeding presumed to be during second spring (21–23 mo), on basis of molts.

Clutch

See Breeding: eggs, above. No information on age-specific clutch size. One clutch/yr unless breeding attempt fails; second clutches following successful breeding have been recorded, but are likely to be uncommon (Bancroft and Jewell 1987).

Annual And Lifetime Reproductive Success

See Appendix 2 . Typically, 1–2 young survive to fledging. Nest failure associated with interruptions in prey availability because prey become inaccessible during cold winter weather (Frederick and Loftus 1993, Frederick and Spalding 1994). Adults abandoned nests during a drought in Everglades when foraging distances from colony increased to >25 km (Bancroft et al. 1994). Smith and Collopy (1995) found that an average of 12% of entire colonies were aborted or were unsuccessful on Lake Okeechobee, FL.

Generally, 1.7–3 young 14 d of age per successful nest (see Appendix 2 .). One and sometimes 2 nestlings/brood (usually smallest individuals) died in nest, of emaciation or starvation (Rodgers 1978a). Broods in n.-central Florida were reduced from an initial 3.7 young/nest at hatching to 2.8 young/successful nest at 2 wk of age; this decrease was also attributed to starvation (Jenni 1969). In Georgia, Teal (1965) found that 32% of chicks were taken by predators, and 9% starved. Smith and Collopy (1995) found 1.6–2.2 young/successful nest by 21 d of age. Frederick et al. (1993) estimated 1.34 young/successful clutch survived to independence in coastal Everglades; 32.3% of chicks died between hatching and independence; major causes of mortality: predation (57%), malnutrition (29%), and disease (9%). Death of young often attributed to starvation (Jenni 1969, Rodgers 1978a), although necropsy results indicated that most of the apparent starvation cases in s. Florida were attributable to disease (Spalding and Forrester 1991). In Guyana, 70% of 10 nests had 2 young, and 30% 3 young, although stage of nestling growth was not given (Llowe-McConnell 1967).

Number Of Broods Normally Reared Per Season

Probably 1. Bancroft and Jewell (1987) found that some radio-tagged individuals switched colonies between breeding attempts in Everglades, but evidence in most cases suggested these were renesting attempts following failed nesting attempts. No evidence of a second cohort of nesting in Everglades colonies (PCF).

Proportion Of Total Females That Rear At Least One Brood To Nest-Leaving Or Independence

Of females that attempt to nest, success at rearing at least 1 young to independence varied from 5 to 67% in Florida Everglades (Frederick et al. 1992). Proportion of reproductive-aged females that attempt to breed is unknown, but for Everglades, average of 43% of estimated population of dark herons (Tricolored and Little Blue herons combined) does not breed during peak of incubation and hatching (Bancroft and Sawicki 1995, Frederick 1995).

Rate of survival from hatching to independence is 67.7% (Frederick et al. 1993); from 21 d to independence, 28.6% (Bancroft and Jewell 1987). From recoveries of chicks banded as nestlings in Texas between 1926 and 1976, 79% mortality in first year, and 31.6% annual mortality thereafter; life expectancy at fledging 1.16 yr; 2.69 yr after start of second year (Telfair 1979). Longevity record in the wild 17 yr 8 mo (Clapp et al. 1982).

Diseases

Verminous peritonitis from the nematode Eustrongylides ignotus is known to have effects at population level. A single parasite can cause nestlings to die within a day, usually from hemorrhage and secondary bacterial infection; 10% of wild nestlings examined in s. Florida were infected (Spalding and Forrester 1991, 1993). Two of 2 nestlings and none of 2 adults examined were infected during an epizootic involving several wading bird species in Delaware; 2–11 nematodes/infected bird, resulting in 82% reduction in fledgling success (Wiese et al. 1977).

Another nematode, Contracaecum microcephalum, reported from adults (Walton 1927). Among trematodes, both Ascocotyle sp. (Hutton 1964) and Posthodiplostomum sp. (Loftin 1962) reported. These parasites probably do not greatly impair hosts, but little information available.

In Florida, 8.7% of nestlings infected with Haemoproteus spp., and 4% with Trypanosoma avium, though the effects of these parasites are unknown (Telford et al. 1992). Generally low levels of blood parasites in Florida by comparison with other locations (Spalding and Forrester 1993). Avian cholera (Pasteurella multocida) possible in western U.S.

Body Parasites

Mallophaga (Ardeolica nigra) found in Florida (Spalding and Forrester 1991). Dermestid beetles rarely inhabit nests. Lesions from dermestid beetle larvae (Dermestes nidum) found on 1 of approximately 80 nestlings regularly examined in central Florida (Black et al. 1984), and on 2 of 1,483 nestlings examined in Florida Everglades (Frederick and Collopy 1988, Spalding and Forrester 1991).

Exposure

In n.-central Florida, young died and nests blew down during windstorms and periods of heavy rain; temporary abandonment of nests by adults usually led to theft of nest material by neighbors and eventual destruction of nest (Jenni 1969).

Predation

Predation of eggs and young (see Behavior: predation, above) can be major source of mortality. In coastal S. Carolina, predation of nest contents by mammals and birds caused decline in nest success across years from 90.4% (incubation) and 80.5% (nestling) to 31.7% (incubation) and 45.9% (nestling; Post 1990). In n.-central Florida, predation resulted in survival of less than half of nest initiations to clutch completion; however, once clutches were completed, nest success was fairly certain (Jenni 1969). Nest predators are rare in freshwater Everglades, because of high water levels, presence of alligators, and few other food sources acting as attractants. Of 144 nests, <2% failed as result of predation (Frederick and Collopy 1989a). In coastal Georgia, estimated 19% of 47 eggs taken by predators (Teal 1965).

Competition With Other Species

Niche partitioning among Tricolored Herons, Snowy Egrets and Little Blue Herons in marine habitat may result from differences in prey type and size, though these species show significant differences in use of foraging habitat; Tricolored Heron is more common in open water and pools (Kent 1986a, 1986b). In coastal New Jersey, however, almost complete prey size overlap between Tricolored Heron and Great and Snowy egrets, and use of foraging habitat did not differ among Tricolored Heron, Snowy Egret, and Little Blue Heron (Willard 1977). Direct interspecific aggression in foraging flocks is generally rare.

Initial Dispersal From Natal Site

In coastal Everglades, Bancroft and Jewell (1987) found 1 radio-tagged fledgling 95 km from natal colony almost immediately after fledging. Another juvenile remained for 3 mo in same cordgrass marsh 4.2 km from natal colony. Young banded in Texas colonies tended to disperse to northeast and southwest along coast rather than inland (Telfair 1979). No information on distances moved between fledging and first breeding.

Fidelity To Breeding Site And Winter Home Range

May be site-faithful to breeding areas between years as long as there is no history of nesting failure (Bancroft et al. 1994). May switch colonies in local areas within and between nesting seasons (Bancroft and Jewell 1987). At Drum I., S. Carolina, eventually abandoned colony after several years of frequent predation by Fish Crows and mammals (Post 1990). In subtropics and tropics, adults may remain within 100 km of breeding colony during nonbreeding season (Bancroft and Jewell 1987). At temperate breeding sites, seasonal migration to warmer climates is the rule.

Dispersal From Breeding Site Or Colony

Postbreeding dispersal much less pronounced in this species than in other ciconiiforms (Palmer 1962, Recher 1972). Postbreeding dispersal seems to be concentrated in coastal habitat, and may result in mid- and late-summer influxes of adults and juveniles (Willard 1977); see Distribution, above.

Home Range

Home range not defended; described only from breeding season. In Florida Everglades, mean distance from colony to food source is 5.6 km ± 6.0 SD (n = 265; Bancroft et al. 1990).

Numbers

During first half of twentieth century, probably the most abundant heron in Florida and se. U.S. (Bent 1926, Howell 1932). During late 1970s, 1 of 3 most common wetland birds in Louisiana and probably the second most abundant heron in North America after Cattle Egrets (Portnoy 1977). In comprehensive breeding surveys of waterbirds in Louisiana, Florida, Yucatán Peninsula, and Atlantic Coast of U.S., Tricolored Herons represented 6–22% of breeding ciconiiform birds (Osborn and Custer 1977, Portnoy 1977, Nesbitt et al. 1982, Runde 1991, Correa 1992).

Comprehensive surveys in nearly entire U.S. breeding range in 1976 suggested minimum breeding population of 96,807 pairs, or about 193,600 adults (Custer and Osborn 1977, Portnoy 1977, Nesbitt et al. 1982, Texas Colonial Waterbird Society 1982)—probably an underestimate, however, given the low detectability of this dark-colored species by aerial surveys. These surveys suggest that the minimum measured population size of this species was larger in 1976 than for any other North American heron except the Cattle Egret. While this certainly was true for the areas surveyed, it may also be true in general—no heron other than these 2 have been found with a total population of ≥100,000 in the U.S. Earlier in this century, the Tricolored Heron was noted as “the most abun-dant heron” in Florida (Sprunt 1954: 32). In 1976, 72% of U.S. breeding population was found in Louisiana, 12% in Texas, 6.3% in Florida, and 9.7% in Atlantic coastal states north of Florida. In Suriname, numerous; colonies totaling 10,000–15,000 pairs of mixed small herons, “of which Tricolored and Little Blue Herons were the most numerous” (Haverschmidt and Mees 1994: 64). In Guyana, total population estimated at 5,000 birds (Dejonghe et al. 1992).

Trends

Probably declining as a breeding bird, perhaps rapidly, throughout North American range. In Everglades, declined from 10,000–15,000 breeding individuals in 1930s to 2,000 in 1980s (Ogden 1994). In Florida, 54% decrease in breeding pairs between 1976–1978 and 1986–1989 (Nesbitt et al. 1982, Runde 1991). In Louisiana and Texas, marked increases between 1950s and 1970s (Ogden 1978, but see Oberholser 1974), followed by probable decline thereafter.

During late 1980s, only 11,450 pairs nested in Louisiana (Martin and Lester 1990), a decline of 84% from Portnoy’s (1977) survey. Although there were differences in survey methods, a close look at the studies in Louisiana suggests that the magnitude of the decline was unlikely to have been due to differing survey techniques. Marked increases in Christmas Bird Counts of wintering birds in Louisiana during 1990s, likely influenced by increases in food made available through burgeoning aquaculture industry (Fleury and Sherry 1994).

On East Coast, rapid increase in population in many states during 1940s–1970s, coinciding with rapid range expansion, including Virginia, Maryland, Delaware, New Jersey, New York, Connecticut, and Massachusetts (Kain 1987, Andrle and Carroll 1988, Sibley 1993, Veit and Petersen 1993, Bevier 1994, Robbins and Blom 1996). Rapid increases in population in Maryland; rare before 1960s, but during 1970s breeding population tripled, from about 300 to about 900. In s. New Jersey, breeding population has been declining rapidly since early 1980s. In New York, Massachusetts, and Delaware, population has been stable since mid-1970s.

On Yucatán Peninsula during 1980s, stable population of about 250 pairs nesting in Sian Ka’an Biosphere Reserve wetlands (Lopez-Ornat and Ramo 1992).

Consistent reduction of broods because of starvation (Rodgers 1978a), as well as nesting failure in response to interruptions in food supply (Frederick and Collopy 1988, Frederick and Loftus 1993, Smith and Collopy 1995), suggest that food availability often limits reproduction. Little information on factors that affect postfledging and adult survival, however, which could be demographically more important.

Populations also likely limited by habitat. Significant correlations between coastal breeding abundance and amount of wetland area by state along Atlantic Coast (Custer et al. 1980). Majority of population outside of Florida seems restricted to estuarine and coastal nesting habitat (Ogden 1978). Estuarine productivity may therefore be important in regulating distribution and abundance of this species. This species has shown ability to colonize new nesting habitat when created (see Distribution, above).