Demography and Populations

Measures Of Breeding Activity

No information available from birds breeding in natural cavities. Parameter values contained in this section come from studies conducted on populations breeding in nest boxes that may or may not have been enhanced with predator guards. The similarity between parameters measured in box populations and natural cavity nesting populations is unknown.

Age At First Breeding

First breed as 2-yr-olds, breeding every year afterwards.

Clutch Size

Maximum number laid by a single female unknown, probably not > 13 eggs; range reported in nests 5–44 (>1 female). Means (± SE) include: Minnesota, 13.0 ± 0.5 (n = 92, range 5–26; Zicus 1990); Missouri, 11.0 ± 0.22 (n = 102; LHF); N. Dakota, 9.0 (n = 11; Doty et a. 1984); Ontario, 10.3 (n = 228; Mallory et al. 2002); Oregon, 9.1 (n = 18; Morse et al. 1969). A mean of 13 eggs probably reflects high rates of undetected nest parasitism (Zicus 1990). First time breeders thought to have smaller clutches than experienced birds: Oregon, 9.4 vs. 10.8 (n = 8 and 10, respectively; Morse et al. 1969); Missouri, 10.2 vs. 11.9 (p < 0.05; LHF). Clutch size decreases as the nesting season advances (Morse et al. 1969).

Annual And Lifetime Reproductive Success

Nest success, calculated as the percent of incubated nests successfully hatching > 1 duckling, is high throughout the breeding range: Maine, 80% (n = 1,120; Allen et al. 1990); Minnesota, 78% (Zicus 1990); Missouri, 60%, n = 731 nests (1996–2006, LHF and P. Blums) and 56% (2001-2003, Lemons 2004); N. Dakota, 73% (n = 120; Doty et al. 1984); Ontario, 75% (Mallory et al. 2002); Oregon, 80.0% (range 66–100%, n = 55; Morse et al. 1969); Quebec, 82% (n 17; Bouvier 1974).

Mean hatching success of all eggs laid by a population varies: Minnesota, 65 (range 44–99%; Zicus 1990); Missouri, 49% (5,427 of 10,991 eggs laid 1996-2006, n = 731 nest attempts); N. Dakota, 63% (Doty et al. 1984); Ontario, 64.1% (range 46% - 90%) Mallory et al. 2002); Oregon, 71% (range 53–89%; Morse et al. 1969); Quebec, 72% (Bouvier 1974). Hatching success varies with nest type in Missouri (see parasitism section above). The percent of eggs hatching in successful nests shows much less yearly and geographic variation: Minnesota, 94%; Missouri, 75%; N. Dakota, 87%; Ontario, 92% (McNichol et al. 1997) and 87% (Mallory et al. 2002); Oregon, 92%; Quebec 90%.

Number Of Broods Reared Per Season

Probably only one.

Life Span And Survivorship

Annual apparent survival for adult females nesting in boxes in se Missouri based on live recapture data between 1986 and 1997 averaged (mean ± SE) 66 ± 4% (95% CI’s 59–73%; Dugger et al. 1999b). Survival estimates for this same population from 1994 to 2006 based on joint analysis of live recapture and dead recovery data, which allowed site fidelity to be estimated separately from survival, were higher and averaged 72% ± 5% (Pierce et al. in press). Annual variation in apparent survival between 1986 and 1997 was not correlated with annual harvest, winter temperature and rainfall, number of nest attempts or nest success (Dugger et al. 1999b).

Oldest known Hooded Merganser: > 11 yr old; 1 female banded as an incubating female in 1997, Missouri, recaptured on a nest in 2006 (P. Blums pers. comm.).

Disease And Body Parasites

Parasitic nematode (Streptocara crassicauda var. charadrii) found under gizzard cuticle, may cause lesions. Believed to be intermediate host for Hyelella knickerbockeri in Illinois. Nasal leeches and Sarcocystis rarely reported in mergansers, but common in other waterfowl species (Friend 1987). Nasal leeches can cause death through suffocation, but Sarcocystis is not fatal (Friend 1987). Helminth parasites recorded include Anatoecus dentatus, A. icterodes, Pseudomonopon sp., and Trinoton querquedulae (Threfall et al. 1979). One captive bird died from avian tuberculosis (Sato et al. 1996).

Causes Of Mortality

Only hunting and predation recorded. Predation recorded only on incubating females.

Because of this duck’s preferred food and feeding habits (see Food Habits: diet), lead poisoning is not thought to cause significant mortality in this species, unlike many species of Anas and Aythya (Friend 1987).

Range

Initial Dispersal From Natal Site

Females philopatric to general nesting habitat, but compared to Wood Ducks, less so to specific boxes (LHF). Assuming 50% sex ratios in newly hatched clutches, return rates of female ducklings using nest boxes at 2 sites in se. Missouri: 3.8% and 10.8% (LHF). Average distance returning females moved from their natal box: 4.7 km (n = 10, range 0.2–5.6; LHF).

Fidelity To Breeding Site

Females show strong fidelity to breeding area. In Minnesota, 29% of 13 returning adult females used the same nest box, with the remaining birds moving from 0.2 to 7.3 km (Zicus 1990). Movement not related to nest failure. In se. Missouri, mean annual fidelity estimated from joint analysis of live recapture and dead recovery data was 92% (Pearce et al. 2008). No information available on fidelity to winter range.

Dispersal From Breeding Site

Recoveries of birds banded from May through Aug in Maine, Vermont, Pennsylvania, and New York suggest a significant west and north post-breeding dispersal into Ontario and Quebec (BDD). However, only 4 of 64 recoveries from these states occur within Mississippi Flyway states, indicating most birds probably return to coastal Atlantic states to winter. Similarly, adults nesting in Missouri have been recovered as far north as 49°N in fall (700km north), with data suggesting such northward movements may be the norm (Lemons 2004; Pearce et al. 2008). Increased numbers of adults in Northwest Territories in fall and summer, suggest post-breeding dispersal into more northern sites may be increasing (Fournier and Hines 1996). Records of sightings in Sep and Oct as far north as the MacKenzie River, Pribilof Is., and Wrangle I. (Phillips 1926) probably reflect atypical dispersal.

Recent genetic research suggests higher rates of dispersal and emigration in Hoodeds compared to wood ducks which have a similar distribution (Pearce et al. 2008). Site fidelity and dispersal likely vary annually and geographically in relation to foraging habitat and availability of nesting cavities and this variation may contribute to recent observations of population and range expansions (Pearce et al. 2008).

Home Range

No information available.

Population Status

Estimates Of Density

Density estimates of breeding pairs in Ontario and New Brunswick average 11.1 ± 1.36 and 1.4 pairs/100 km², respectively (McNicol et al. 1987b, Parker et al. 1989); much lower than the median density of 40 pairs/100 km² (thought to be an underestimate) in Minnesota (Zicus 1990). On wintering grounds, Christmas Bird Count data indicate a peak density of 2.8 birds/h of observation (Root 1988) and aerial surveys in Florida (1989–1992) ranged from 6,600 to 21,000 birds (Fla. Game and Fresh Water Fish Comm. unpubl. data).

Population Numbers And Trends

Estimating population size is difficult because of the Hooded Merganser’s retiring nature and preference for wooded habitats. May and mid-winter surveys conducted by U.S. Fish and Wildlife and Canadian Wildlife service personnel are of little value because all merganser species are grouped together. Bellrose (1976) estimated continental populations at 76,000 individuals, suggesting Hooded Mergansers are one of the rarest Anseriformes in North America but this estimate is much too low given hunter harvest estimates (see Conservation and Management: Effects of Human Activity). Using harvest figures, an estimate of 72% for annual survival (see life span and survivorship), and assuming that harvest accounts for 100% of annual mortality, the number of birds harvested each year represents 28% of the population. Based on these numbers, fall population estimates range between 270,000–385,000 birds from 1970-1985 and 329,000-414,000 from 1999 to 2006 (BDD). Population trends difficult to quantify, but limited BBS data (n = 64-90 routes) suggest populations are stable or slightly increasing in the eastern breeding range (Sauer et al. 2007). Even this estimate, however, probably underestimates true population size, because hunter harvest is not the only source of annual mortality.

Little known about long-term population trends nationwide. Increased nest box use and higher numbers counted on Christmas Bird Counts in Massachusetts (Heusmann et al. 2000) suggests at least localized population increases. Nest attempts in nest boxes increased in Ontario between 1987 and 1996 (McNichol et al. 1997) while populations in central Ontario increased significantly from 1985–1989 (M. Mallory pers. comm.). Populations in Minnesota and locally in se. Missouri and w. Tennessee also seem to be increasing (M. Zicus pers. comm., LHF). These observations, plus the increased hunter harvest throughout the continent from 1961–1988 (USDI 1988), suggest populations are currently stable and possibly increasing over much of the range.

Population Regulation

The current impact of hunting on populations of this species is unknown (for historical impact see Distribution: historical changes). In many areas, lack of nest cavities due to logging probably limits breeding in once suitable areas. Increasing local populations associated with nest box programs in Missouri, Maine, Iowa, and Oregon support this conclusion (Morse et al. 1969, Allen et al. 1990, Fleskes et al. 1990, LHF). Non-human related factors limiting populations remain largely unknown, but probably vary across the species range. Cavity densities probably play a role in population regulation, but our understanding of this resource is limited. More localized factors may include nest site competition with Common Goldeneyes in the north and Wood Ducks in the south. Interspecifically territorial goldeneyes may also exclude mergansers from suitable habitat, as suggested in Ontario (M. Mallory pers. comm.), although this is not believed to happen in Minnesota (M. Zicus pers. comm.). Competition with fish for invertebrate resources also may play a role in more oligotrophic habitats. In the south, annual variation in river floodplain hydrology creates apparent high variability among years in habitat availability and food abundance, which may regulate densities in these systems.