Breeding

Phenology

Pair Formation

Exact timing uncertain across range. Mid-Nov in British Columbia; prior to pair formation in Common and Red-breasted mergansers (Coupe and Cooke 1999). Pairs observed in Nov in Missouri (LHF) and Oregon (BDD), late Dec in New Brunswick (BDD), and mid-Jan in Florida (Barbour and DeGrange 1982).

First Brood

Figure 4 . Single brooded. Nest initiation dates vary with latitude; se. Missouri, 16 Mar (n = 313; range 12 Feb–6 May; LHF); Minnesota, estimated late Mar to early Apr for peak initiation (Zicus 1990); Oregon and coastal Washington, late Feb to mid-Apr (Fig. 4; Beall 1990, Morse et al. 1969); British Columbia, peak 4–27 May, (range 27 Mar–15 Jun; n = 38; Campbell et al. 1990). Two clutches in Maryland initiated on 14 and 23 Mar (McGilvrey 1966); 3 in Indiana initiated on 23 Mar and 1 Apr (Mumford 1952); 4 in S. Carolina on 24 and 28 Feb and 2 and 10 Mar (Kennamer et al. 1988). Evidence suggests experienced breeders initiate nests earlier than first time birds (Morse et al. 1969).

Nest Site

Selection

Cavity selection made by the female who may begin prospecting for suitable cavities the summer before a breeding attempt (Zicus and Hennes 1989, M. L. Mallory pers. comm., LHF). Nest boxes containing wood shavings (Lumsden et al. 1986) and boxes used the previous year are preferred (D. K. McNicol and M. L. Mallory unpubl. data).

Site Characteristics

Cavity nester; using live or dead trees (Fig. 5); 1 nest recorded 33 cm down in a burrow below the root wad of a fallen tree (Phillips 1926). Data from nest box studies indicate few characteristics seem preferred for nest site selection other than proximity to water (Morse et al. 1969, LHF). Studies in Oregon (Morse et al. 1969) showed box occupation began with those over or near water. Only when these boxes were occupied did females resort to boxes away from water. One record exists of a natural cavity nest 0.5 km from water. Records from nests boxes fixed directly to trees suggest any tree species with a suitable cavity is acceptable; even boxes mounted to poles are used readily (LHF).

In Ontario, 66% of nest attempts were in boxes on lakes containing fish, but fish present did not have any effect on mean hatching success, number of eggs laid or number hatched (McNichol et al. 1997).

Suitable cavities occur in live or dead tree trunks, large limbs, broken off limbs, and at the top of broken off tree trunks (“chimneys;” Bellrose 1976, Peck and James 1983, Western Foundation of Vertebrate Zoology [WFVZ] unpubl. data). Recorded cavity heights range from 2.7 m to extremes 24 and 27 m (G. Wallace pers. comm., WFVZ); 7 natural cavities in British Columbia ranged from 4–15 m. Nest boxes placed 1–4 m above the ground are used with equal frequency (Lumsden et al. 1986, LHF).

Nest

Construction

Only females construct and maintain nest; no nest material added. Shallow nest bowl made from material already present in the cavity. Down plucked from the female’s belly is added to line the completed nest. In Oregon, no deposition of down until 87% of the clutch laid (n = 27; Morse et al. 1969).

Dimensions

Nest boxes indicate cavity orientation not important. Birds prefer cavity openings measuring 10 x 13 cm over those 8 x 10, 6 x 7.5, or 21 x 13. Recorded cavity depths range from 25 cm to 1.7 m (WFVZ unpubl. data).

Microclimate

No information available.

Eggs

Eggs unusual in being almost spherical, particularly compared to other mergansers, with disproportionately thick shells. The adaptiveness of these unusual features is unknown (Mallory and Weatherhead 1990), but shell thickness does provide a reliable method of identifying Hooded Merganser eggs from those of sympatrically breeding Wood Ducks (Soulliere 1987).

Size

Length and breadth: S. Carolina 53.6 x 43.7 (n = 26; Kennamer et al. 1988); Missouri 53.7 x 43.8, (length range 51.3–56.7, breadth range 42.5–45.4; n = 24; LHF); Ontario, 53.6 ± 0.1 x 43.9 ± 0.1 (length range 42.5–62.4, breadth range 39.9–54.2, n = 511; H. Lumsden unpubl. data).

Mass

Fresh egg mass 59.0 ± 0.2 g in Ontario (n = 511; H. Lumsden unpubl. data), 59.0 ± 0.6 g in S. Carolina (n = 26; Kennamer et al. 1988), and 57.9 g (range 51.5–62.9) in Missouri (n = 24; LHF). Egg mass decreases approximately 17% during incubation (M. C. Zicus pers. comm.)

Eggshell Thickness

Shells disproportionately thick compared to other anatids but hatching success, shell porosity, and water conductance values similar to those of other ducks (Mallory and Weatherhead 1990). Mean at egg equator, including membranes, measured on eggs from 17 states 0.569 ± 0.007; low in New Hampshire 0.546 ± 0.004, high in Arkansas 0.630 ± 0.006 (see Table 1). Between 1900 and 1975, shell thickness has declined 6.9% (n = 96; White and Cromartie 1977). In Minnesota, shell thickness declined 9.6% (6% using Ratcliffe indices, n = 70 eggs from 21 broods; (Zicus 1998) through the mid-1980’s, but by 2003 thickness had increased 6.0% over values from 1981 (<em class=">n = 42 eggs); still 4.1% below pre-DDT use (Zicus and Rave 2003) but suggests increased shell thickness since DDT was banned. This is markedly less than declines of 17.7 and 23.5% recorded for Red-breasted and Common mergansers, respectively, during the same period (White and Cromartie 1977; Zicus et al. 1988). Regional comparisons found a significant negative correlation (r = -0.63, P < 0.05) between eggshell thickness and PCB concentration (White and Cromartie 1977). The authors stress caution when interpreting this correlation, which at least suggests further monitoring of contaminants is necessary.

Color

White.

Egg Laying

Begins with nest construction; reported to occur as late as 18:00 h (Morse et al. 1969). Laying rate slow compared with dabbling ducks, similar to that of other mergansers and Bucephala . Studies cited below did not consider the influence of conspecific nest parasitism (see Brood Parasitism), so rates may represent overestimates (BDD). Females generally believed to lay 1 egg/2 d, but laying rates calculated from 25 partial clutches in Missouri averaged 0.57 eggs/d (or 1.75 d/egg; Hansen 1971), and only 63% of females in Oregon laid on 2 d intervals (Morse et al. 1969). The remaining Oregon nests showed a series of 3 d where eggs were laid in succession.

Females may abandon nests if disturbed early in laying. Five of 6 females abandoned their nest when disturbed before any down was deposited, whereas only 2 of 8 females abandoned when some down was present (Morse et al. 1969). Little is known about the frequency of renesting in the wild, but one female in Missouri did renest after her first clutch failed (P. Blums pers. comm.) and captive females lay replacement clutches when first clutches are removed (Palmer 1976).

Incubation

Females enter the nest by flying at the cavity opening in an upward arc; extending their legs, then grasp the bottom edge of the cavity with their feet and pivot up and through the opening (M. Zicus pers. comm.). This species apparently more adept at this procedure than Common Goldeneyes, which have been observed to “miss” by bouncing off the nest box face 2–3 times before entering successfully (M. Zicus pers. comm.).

Incubation Period

Only females incubate; single brood patch. In Oregon, Missouri, and Ontario, period = 33 (n = 32), 32 (n = 4), and 29 d (n = 4), with a range in Oregon of 26–41 d (Morse et al. 1969, Peck and James 1983, LHF).

Parental Behavior

Males abandon females shortly after incubation begins. Female nest attentiveness high, 85.3% ± 3.2% (n = 7), increasing as incubation progresses (r = 0.28, n = 89, P < 0.01; Mallory et al. 1993). Females incubate throughout the night, but take an average of 4.7 (± 0.2) incubation breaks/day, the highest number recorded for any species in the tribe Mergini. Number of recesses increases as incubation progresses (r = 0.39, n = 89, P < 0.001), but recess duration decreases (r = -0.34, n = 89, P < 0.01, mean 60 ± 6 min.). In the nest, females move eggs within the clutch, presumably to maximize incubation efficiency and hatching synchrony.

Females easily caught on nest; sometimes hissing during capture (Bouvier 1974). Often possible to place female back into nest box upon completion of nest check or simply remove eggs from underneath her without interrupting incubation (Mallory et al. 1993, KMD). When females flush from nest, they may vocalize in flight and perform Broken Wing Distraction Displays in the water around the nest. Flushing distance does not change significantly as incubation increases as females generally do not flush until observer is on tree (Mallory et al. 1998). However, as incubation progresses, females exhibit increased intensity of other nest defense behaviors including decreased landing distance after flushing, increased vocalization, and increased occurrence of broken wing displays (Mallory et al. 1998). Females in Missouri and Maryland both incubated a dead clutch until the eggs were removed after 70 d (LHF, McGilvrey 1966). Anecdotal observations indicate females may remove damaged eggs from the nest cavity (M. L. Mallory pers. comm., KMD), and do not as a rule defecate in the nest unless disturbed. After hatching, eggshells and membranes are not removed.

Females lose mass as incubation progresses; estimates include 16% in Ontario (r = -0.73, n = 9, P = 0.03; Mallory et al. 1993), 8% (LHF) and 11% (Lemons 2004) in se. Missouri, 5.7% – 6% in Minnesota (n = 100; Zicus 1998) and 8.7% and 13% for two females in S. Carolina (Kennamer et al. 1988). In Minnesota, heavier females did not initiate nests earlier, but did incubate the largest clutches, hatch the most young, and incubate nests with more parasitic eggs (Zicus 1998).

Hardiness Of Eggs

Females commonly known to desert uncompleted clutches and some eggs have cracked in se. Missouri after freezing temperatures (LHF).

Hatching

Preliminary Events And Vocalizations

Tapping noises and “peep” calls heard 72 h before hatch (KMD, LHF).

Shell Breaking And Emergence

The first star-shaped crack in shell appears 30–48 h before hatch; first hole 12–24 h before hatch. For some species this phase is accompanied by increased nest defense by females, but such a response not noted among Hoodeds in Ontario (Mallory et al. 1993). Hatching highly synchronous, 4 h lapsing between hatching of the first and last duckling in a non-parasitized clutch (KMD, LHF). Shell membranes remain intact, providing a conservative method of estimating clutch size if nests are found after the brood has departed.

Young Birds

Condition At Hatching

Mean: mass = 31 g, culmen = 17 mm, femur = 16 mm, tibiotarsus = 31 mm, total leg length = 74 mm, middle toe length = 25 mm, claw length = 4.0 mm; claw curvature = 107 degrees of arc, mean length caudal down shaft = 17.4 and width = 18.0 epu (n = 11; Siegfried 1974). In Missouri, mean body mass at hatch: 35.4 g ± 0.04 SD (range 26 - 45 g; n=4,694; years = 1996-2005).

Ducklings precocial, down-covered at hatch. Brown on back and upper breast; cheeks buffy or rufous; throat, lower breast, and belly white. A grayish spot on each side of back caudal and medial to trailing edge of wing, and at base of each side of tail. Trailing edge of inner wing grayish white or buffy.

Departure From The Nest

Ducklings leave the nest within 24 h of hatching. Just before departure, females check the area around the nest, either from the cavity opening or from the water (Fig. 5). Females then call to the ducklings from the water below the cavity with a repetitive soft guttural note (LHF).

Departure from the nest described by Siegfried (1974). Ducklings exit the nest using a series of vertical leaps to ascend the nest cavity wall. Physical adaptations for this process include lengthened and widened caudal down shaft and lengthened claws, relative to ground-nesting species like Mallard. The tail remains in contact with the cavity wall throughout most of the jump to maintain a vertical aspect, releasing only as the feet regain contact, toes fully spread, and wings swing upward and outward. Experimental trials show ducklings orient their jumping only to the wall with the cavity opening, compared to ground-nesting mallard ducklings which jumped at walls randomly (Siegfried 1974). Although sample sizes were small, the same study suggested Hooded Merganser ducklings were not as efficient at climbing as Wood Ducks. One account of a brood leaving a nest box found 10 ducklings took less than 2 min to exit.

Brood Behavior

Ducklings very active, feeding themselves from the first day. Can dive upon leaving the nest, although during their first week of life dives are shallow and of short duration (Beard 1964). In addition to diving, ducklings feed by swimming with just their heads under water or frequently with their entire bodies submersed (Beard 1964). Foraging bouts recorded for 4 broods averaged 76 min (range 30–150), interspersed with loafing bouts averaging 45 min (range 30–90; Beard 1964). Limited observations indicate broods respond to potential predators by either swimming away when the predator is at a distance (Beard 1964) or remaining motionless in vegetation (BDD, KMD), but if surprised at close range the ducklings dive (Beard 1964).

FOod Of Downy Young

In Ontario, ducklings (n = 25) forage primarily on invertebrates living in the water column, including back swimmers (Notonectidae), water boatmen (Corixidae), and diving beetles (Dytiscidae; McNicol et al. 1987a). In acid lakes with pH < 5.5, ducklings show a large increase in the consumption of 1 species of acid tolerant dragon fly larvae (Leucorrhinia glacialis; McNicol et al. 1987a). In another study in Ontario, aggregate percent volume from esophagi of class I and II ducklings (n = 6) found Odonata (36.1%), Osteichthyes (24.4%), Arachnida (20.4%), Trichoptera (10.6%) dominated; as above limited data suggests diet varies with lake acidity and fish presence (Bendell and McNicol 1995).

Growth And Development

No information available.

Brood Survival

No information available.

Parental Care

Only females provide care for ducklings (Beard 1964, McGilvrey 1966, Kitchen and Hunt 1969).

Brooding

Young remain in the nest approximately 24 h after hatching, presumably brooded by female during that time. Brooding by the female after leaving the nest has not been reported.

Duration Of Parental Care

One female abandoned her brood 5 wk after hatch (Beard 1964).

Cooperative Breeding

Never observed.

Brood Parasitism

Nest parasitism, defined as laying eggs in another bird’s nest, is a common part of Hooded Merganser breeding biology (see references this section), although rates reported below from nest box studies may overestimate its occurrence in natural populations (Semel and Sherman 1986).

Frequency Of Occurrence

Conspecific nest parasitism (CNP). Defined as laying eggs in the nest of females of the same species, CNP is difficult to detect because host and parasite eggs look the same; conservative estimates based on maximum clutch size estimates (> 13 eggs/nest considered CNP) in Minnesota, as well as maximum clutch size and egg accretion rates in Missouri, are 45% (n = 91; Zicus 1990) and 34% (n = 429; LHF), respectively. CNP rates have increased with increased population densities in Missouri (LHF).

Interspecific nest parasitism. Hooded Mergansers parasitize, and are parasitized by, cavity nesting Common Goldeneye, Common Mergansers, and Wood Ducks (Bouvier 1974, Doty et al. 1984, Allen et al. 1990, Beall 1990, Zicus 1990, Mallory et al. 1993, LHF). Variation among species in nesting chronology, nest densities in an area, and female body size and physiological condition all probably impact the frequency of parasitism and identity of parasite. Wood Duck parasitism of merganser nests ranges from 0% in Ontario (Mallory et al. 1993) and Quebec (Bouvier 1974) to 42% in se. Missouri (n = 429; LHF), an area of high Wood Duck density. Merganser parasitism of Wood Duck nests in Missouri is significantly less because of relatively low merganser densities and the later nesting of Wood Ducks there (LHF). In Minnesota, Hoodeds parasitized 38% and 33% of Wood Duck (n = 13) and Common Goldeneye (n = 3) nests, respectively, comprising 12% and 20% of all eggs incubated by those species, but only 2% of all merganser eggs laid by the population (Zicus 1990). Common Goldeneyes parasitized 15% and 35% of all merganser nests in Ontario (n = 141) and Quebec (n = 17); while mergansers parasitized 7% and 29% of goldeneye nests (Bouvier 1974, Mallory et al. 1993). Limited data suggest parasitism of Hoodeds by Common Mergansers is rare (< 1%; n = 141) compared with Hooded Merganser parasitism of Common Mergansers (22%; n = 9; Mallory et al. 1993), possibly because the larger Common Merganser females are unable to enter the smaller species nest. In a high density nest box population in Missouri, 13% (range 6 -19%) of all Hooded Merganser ducklings produced hatched in Wood Duck nests (LHF and P. Blums). Band recovery (indirect + direct) rates similar (6.2 v. 6.0%) for Hooded Merganser ducklings hatched in Wood Duck nests (n = 257) and Hooded Merganser nests (n = 2,010), respectively.

Seasonal Trends

Little known. Relative to the mean nest initiation date for all merganser nests in Missouri (26 Mar ± 1.1 d), mean nest initiation dates for non-parasitized nests (defined here as nests with clutch size < 14, and no sign of CNP), CNP nests, and nests parasitized by Wood Ducks was 20 Mar ± 1.9 d, 22 Mar ± 2.3 d, and 27 Mar ± 1.2; LHF), respectively.

Timing Of Laying In Relation To Host Clutch

No information available.

Response To Parasitic Female, Eggs, Nestlings

Hooded Merganser females moved eggs laid by Common Goldeneye to the clutch periphery (Mallory and Weatherhead 1993), but wood duck eggs in se Missouri were not (Dugger et al. 1999a); limited evidence suggests females may remove parasitic eggs from nests (Mallory and Weatherhead 1993); in se Missouri Wood Duck eggs were removed from nests more often than expected; removal may have reflected higher tendency for wood duck eggs to break in nests because of thinner shells compared to Hooded eggs (Dugger et al. 1999a).

Effects Of Parasitism On Host

Largely unknown. In nests parasitized by conspecifics, no study has specifically focused on host egg or duckling success. For incubated nests in Missouri (LHF): mean merganser clutch size in nests parasitized by Wood Ducks averaged 1.1 eggs less than non-parasitized nests (defined above), although this may be misleading because early merganser clutches are larger and Wood Duck parasitism occurs most frequently in late merganser clutches. Nest success for non-parasitized and CNP nests was similar (70.6% vs. 70.0%), both were higher than nests parasitized by Wood Ducks (61.4%); hatching success declined from 75 ± 5% in non-parasitized nests to 59 ± 7.2% in nests parasitized by conspecifics and 50 ± 5.2% for nests parasitized by Wood Ducks (LHF). In the same study, Wood Duck clutch size in nests parasitized by mergansers averaged 1.6 eggs less than non-parasitized nests (LHF). On the population level, parasitism was thought to cause yearly differences (91.4% to 44.3%) in hatching efficiency of all merganser eggs laid over a 5-yr period (Zicus 1990). In se. Missouri, hatching efficiency was higher for merganser eggs incubated by wood ducks (56%) compared to merganser eggs incubated by mergansers (46%; Lemons 2004).

Fledgling Stage

Fledging occurs at approximately 70 d (McGilvrey 1966). Direct band returns from birds banded at hatch in Missouri suggest some fledglings move up to 700km north (recoveries between ND and MI) before returning south in winter (Lemons 2004).

Immature Stage

No information available.