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Eastern Screech-owl Hatches Wood Duck Eggs



I describe an Eastern Screech-owl (Megascops asio) hatching three Wood Duck (Aix sponsa) eggs in a suburban nest box. Wood Duck(s) removed all five eggs of a completed screech-owl clutch, the earliest of which had already been incubated for at least 19 days, and laid three eggs in their place. The female screech-owl hatched the Wood Duck eggs, preened the ducklings, and attempted to feed them until they exited the nest box within 48 hrs of hatching.
Vol. 119, No. 1, March 2007
The Wilson Journal of Ornithology 119(1):110–112, 2007
Eastern Screech-owl Hatches Wood Duck Eggs
Christian Artuso
ABSTRACT.—I describe an Eastern Screech-owl
(Megascops asio) hatching three Wood Duck (Aix
sponsa) eggs in a suburban nest box. Wood Duck(s)
removed all five eggs of a completed screech-owl
clutch, the earliest of which had already been incubat-
ed for at least 19 days, and laid three eggs in their
place. The female screech-owl hatched the Wood Duck
eggs, preened the ducklings, and attempted to feed
them until they exited the nest box within 48 hrs of
hatching. Received 22 December 2005. Accepted 27
July 2006.
Wood Ducks (Aix sponsa) are well known
to lay parasitically with conspecifics (Hartman
1972, Semel and Sherman 1986) and other
cavity-nesting ducks (Bouvier 1974, Eadie et
al. 1998). Wood Ducks occasionally remove
conspecific eggs; however, such records usu-
ally involve damaged eggs (Semel and Sher-
man 1986). Wood Ducks are reported to
‘‘evict’’ other bird species including screech-
owls from nest boxes (Bellrose and Holm
1994). However, Semel and Sherman (2001)
report that when returning female Wood
Ducks found boxes in which they had previ-
ously nested occupied by heterospecifics, in-
cluding Eastern Screech-owls (Megascops
asio), they switched to another box (n
Here I record Wood Duck(s) removing an en-
tire clutch and laying in a nest of Eastern
While studying the reproductive ecology of
Eastern Screech-owls in suburban Winnipeg,
Manitoba, I installed a miniature video camera
in May 2004 inside a nest box in which a pair
had successfully reared broods in two previ-
ous years. Five chicks fledged from this box
in 2004. A female began laying on 3 April
2005 and by 10 April was incubating a clutch
of five eggs. There was no sign of any unusual
Department of Environment and Geography, Uni-
versity of Manitoba, Winnipeg, MB R3T 2N2, Canada;
activity at the nest until 22 April, when ob-
servers noted there were only three screech-
owl eggs and one much larger egg. On 23
April, there was a second large egg and the
three remaining owl eggs (Fig. 1A). On 24
April no change was noted; however, on the
morning of 25 April, two more owl eggs had
been removed and a third larger egg was pre-
sent (Fig. 1B). Video recordings were made
on a nightly basis but, unfortunately, did not
extend sufficiently into the morning to record
the removal of eggs. I monitored the nest box
from 0600 to 0730 hrs CST for the next 3
days and, on each morning, a pair of Wood
Ducks landed close to the box. The female
Wood Duck then flew to the roof of the box
and stepped repeatedly and heavily on it be-
fore making a short circular flight and landing
on the entrance hole. On each occasion I ob-
served this behavior, the incubating owl
jumped up to prevent the duck’s entrance. On
27 April, a female Wood Duck was recorded
gaining entrance to the nest box at 0638 hrs
but was expelled by the owl which tried to
bite the intruder on the back of the neck. On
the morning of 2 May, the remaining owl egg
was removed, although no new Wood Duck
eggs were added. Unfortunately egg removal
was not recorded due to a technical difficulty.
I concluded the larger eggs in the box were
Wood Duck eggs (confirmed upon hatching;
Helgeson Nelson 1993). There was no evi-
dence of any damaged eggs in the box and no
eggs had been buried in the nesting material.
The edge of the Red River was only a few
meters from the base of the nest tree and, be-
cause there were no eggs or shells below the
box, the removed eggs may have been
dropped over water or consumed (Semel and
Sherman 1986).
Despite the absence of her own eggs, the
female owl incubated the three Wood Duck
eggs. The first egg hatched at 2240 hrs on 25
May, the other two hatched later that evening.
Shortly after hatching, the owl preened the
FIG. 1. (A) Three Eastern Screech-owl eggs and two Wood Duck eggs on 24 April 2005; (B) One Eastern
Screech-owl egg and three Wood Duck eggs on 25 April 2005, Winnipeg, Manitoba. The wing and tail of the
female screech-owl sitting at the box entrance are visible in the lower left corner in both images.
ducklings and ate pieces of eggshell. She also
brooded and attempted to feed the chicks.
When the ducklings attempted to exit the box,
the female owl gave whinny calls, which are
‘‘elicited particularly by dispersing juveniles’
(Gehlbach 1995:7). The first chick exited the
box just before 2200 hrs on 26 May and the
second shortly afterwards. The third chick de-
parted the box at 0730 hrs on 27 May. The
property owner took one duckling to a local
nature reserve, but the other two were not lo-
cated. Wood Duck chicks are highly precocial
and brood merging has been recorded (Kirby
1990), but it is not known whether the chicks
in question survived.
Raptorial birds occasionally incubate water-
fowl eggs. Dawson and Bortolotti (1997) re-
ported an American Kestrel (Falco sparverius)
incubating a Bufflehead (Bucephala albeola)
egg and four kestrel eggs (the Bufflehead and
two kestrel chicks fledged). Fannin (1894) re-
ported a mixed clutch of Canada Goose (Bran-
ta canadensis) and Osprey (Pandion haliae-
tus). The Black-headed Duck (Heteronetta atri-
capilla), an obligate brood parasite, at times
parasitizes diurnal raptors (Weller 1968, Ho¨hn
1975). Wood Duck eggs have occasionally
been found in nests of Western Screech-owls
(Megascops kennicottii) (J. M. Eadie, pers.
comm.). In Winnipeg, Manitoba there are sev-
eral records from volunteers of the Fort Whyte
Nature Centre of joint use of nest boxes by
Eastern Screech-owls and Wood Ducks (e.g.,
on 11 April 1997, 1 Eastern Screech-owl egg,
5 Wood Duck eggs, and 7 membranes were
found in one box). This does not imply syn-
chrony of use because some may have been
from the previous year or sequential nesting,
and it is possible that dumping or usurpation
may have occurred. Eastern Screech-owls have
been recorded incubating the eggs of other spe-
cies (Breen and Parrish 1996). Manlove (1998)
reported an Eastern Screech-owl apparently
evicting a nesting Wood Duck, laying on top
of the covered duck eggs, and subsequently
hatching at least one owlet and one duckling.
The average incubation periods for both
Eastern Screech-owl and Wood Duck are ap-
proximately 30 days (Gehlbach 1995, Hepp
and Bellrose 1995). In this case, the female
owl sat on the Wood Duck eggs for 31–34
days, within the normal range of incubation
for Wood Ducks (25–37 days) (Hepp and
Bellrose 1995). However, because the owl ini-
tiated egg laying much earlier than the Wood
Duck(s), her eggs would have hatched ap-
proximately 3 weeks before any of the duck’s
had they not been removed. By accepting the
Wood Duck eggs, the female owl’s total in-
cubation period was extended to 55 days.
Eastern Screech-owls have been recorded in-
cubating infertile eggs for as long as 78 days
(Gehlbach 1995).
Incidental egg dumping has been recorded in
many avian species (e.g., Sealy 1989). However,
the removal of host eggs over a 10-day period
suggests this was not a case of egg dumping. A
failed attempt at nest usurpation, however, can-
not be dismissed. Unusual interspecific interac-
tions of this nature have been attributed to com-
petition for nest sites (Eadie et al. 1988), but
Vol. 119, No. 1, March 2007
several studies suggest that nest parasitism in
Wood Ducks is not related to a lack of cavities
(Semel and Sherman 1986). Dawson and Bor-
tolotti (1997) argued that certain desirable qual-
ities of the nest site might be a factor. In this
case, there was a vacant nest box in the same
yard with no discernable structural differences
in which Wood Ducks had previously reared
several broods. Semel et al. (1988) demonstrat-
ed that conspicuous placement of nest boxes in-
creased intraspecific nest parasitism rates in
Wood Ducks and the box in question was highly
visible. This serves as a reminder, perhaps, of
the caution required when artificial nesting
structures are used as management tools (Eadie
et al. 1998).
I am grateful to David and Sigrid Warrenchuk for
assisting in the video recording and permitting obser-
vation on their property. S. E. Cascino and Barry Pom-
eroy produced the photographs. This manuscript was
greatly improved by the thoughtful comments of R. P.
Berger, J. R. Duncan, J. M. Eadie, F. R. Gehlbach, R.
W. Nero, S. G. Sealy, M. W. Shoesmith, and an anon-
ymous reviewer. Funding was provided by small grants
from the Special Conservation Fund of Manitoba Con-
servation, Manitoba Hydro, and the Gray Owl Fund
administered by R. W. Nero.
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... Predation at the nesting stage was minimal in the study area, damaged eggs being found on only one occasion, probably by Raccoon Procyon lotor. One clutch of five eggs was removed by a female Wood Duck (Artuso 2007). No evidence of chick mortality was found when cavities were inspected post fledging. ...
Random-stratified spring nocturnal surveys using tape playback were conducted for Eastern Screech Owls Megascops asio from 2004 through 2007 in Winnipeg, Manitoba, Canada, at the northern periphery of its range. Surveys were stratified by (1) human density, (2) riparian vs. non-riparian habitat, and 3) the presence or absence of suburban greenspace. A total of 120 transects (each 1.75 km long) were surveyed, with 55 Eastern Screech Owls detected. Eastern Screech Owl densities peaked in moderate to high-density suburban areas (>20 persons/ha), where 36 (66%) detections occurred, and were lowest in wildlands (<1 pair/ha), where no detections occurred. Fifty-one (93%) Eastern Screech Owls were detected in riparian areas. Only 20 (36%) Eastern Screech Owls were detected in suburban greenspaces. In 4 years I located a total of 46 successful Eastern Screech Owl nests, 6 failed nests, and 37 territories with non-breeding owls. Excluding repeat cavity uses, a total of 61 nest sites and territory centres were located, of which 38 (62.3%) were in natural cavities, 21 (34.4%) in nest boxes, and 2 (3.3%) at sites where cavity choice was undetermined. Fledging dates ranged from 28 May – 3 July (mean 15 June, SE 1.4) over the 4-year period. Eastern Screech Owls were closely tied to riparian habitat, and showed larger average brood sizes and earlier average fledging dates in moderate and high-density suburban areas than in low-density suburban and rural areas.
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Two forms of brood amalgamation occur frequently in several species of North American waterfowl: (i) pre-hatch brood amalgamation, whereby a female lays her eggs in the nest of another female and the recipient thereafter provides all further care of the eggs and resulting offspring, and (ii) post-hatch brood amalgamation, whereby a female abandons or loses her young to another female after hatch, and the recipient subsequently tends the foster young. Some authors have viewed these behaviours as accidental or aberrant and of little evolutionary significance. More recently, a number of alternative hypotheses have been suggested. However, few of these hypotheses have been contrasted as viable alternatives and tested in the field, largely because an appropriate theoretical framework is lacking. We analyze the frequency of occurrence of brood amalgamation in North American anatids. We also review the hypotheses that have been proposed to explain these behaviours and erect a theoretical framework which applies to the evolution of both pre-hatch and post-hatch brood amalgamation, and which may apply to species other than those of the Anatidae. Finally, we show that the occurrence of brood amalgamation in North American waterfowl may be associated with low relative resource availability and K-type life-history traits.
An Eastern Screech-Owl (Otus asio) was found incubating her own egg and that of a Southeastern American Kestrel (Falco sparverius paulus) in late March 1995. Three owlets and the kestrel successfully hatched in early May; all appeared normal. The kestrel was not present in the nest box with the three owlets when the box was checked two days later, and its fate is not known.
Intraspecific brood parasitism occurs frequently among Wood Duck (Aix sponsa) populations nesting in artificial structures. To assess the effects of such parasitism on population ecology, we analyzed 12 years of Wood Duck nesting records (1976-1987) from a study site in northeastern Illinois. Hatchability of eggs (ducklings produced/total eggs laid) was inversely correlated with population density, with the frequency of parasitism, and with the number of eggs laid per nest. The hatchability of all eggs laid in nests that had been parasitized (16-44 eggs) was 57.5% vs. 67.3% for eggs laid in "normal" nests (7-15 eggs). The negative consequences of parasitism were due mainly to nest abandonment, damaged eggs, and eggs laid after the start of incubation, and occurred despite the consistent availability of suitable unused boxes. The frequency of brood parasitism was strongly affected by box placement. During 1976-1987, parasitism occurred in 49.5% of boxes erected singly in highly visible locations, in 49.5% of boxes erected in highly visible groups, but in only 29.8% of boxes that had been erected singly in visually occluded habitat. Mean clutch sizes for the visible-isolated (15.7 eggs) and visible-clumped (16.3 eggs) boxes were significantly higher (F = 4.49, P = 0.012) than for the well-hidden boxes (12.4 eggs). Hatchability in successful well-hidden nests was 82.0% vs. ca. 74.0% in successful visible boxes. The data suggest that reduced parasitism and increased hatchability occur when artificial nesting structures are placed in habitats and at densities resembling the natural circumstances in which Wood Ducks evolved. These results have implications not only for the study and management of A. sponsa populations, but also for the placement of nest boxes in behavioral and ecological studies of other cavity-nesting birds.
We investigated the behavioural mechanisms and adaptive significance of intraspecific brood parasitism in wood ducks, Aix sponsa, by observing a colour-marked population in northeastern Illinois for seven breeding seasons (1989–1995). The birds nested in boxes that were dispersed widely and mounted high on tree trunks, mimicking the distribution and locations of natural nesting cavities. During 158 mornings of observation, 103 parasitic eggs were laid: 44 (43%) by females that had nested in the population previously and 59 (57%) by new recruits. Parasitism was a facultative behaviour, and its occurrence varied with a female's age and contents of the box in which she had previously nested. Returning females (2–7 years old) usually laid again in last year's box, even if another female already was laying there. Each female behaved as if the box were hers. Indeed, jointly nesting females behaved so similarly that it was impossible to differentiate the ‘host’ from the ‘parasite’. Eventually one of these females was forced to depart, often after a fight. In 66% of cases the earlier-arriving female prevailed. Displaced females left behind eggs that were ‘parasitic’ in the definitional sense (i.e. incubated by another female), but parasitism actually was a consequence of being evicted from a favoured nestbox. Displaced females usually laid and incubated a complete clutch in a nearby box; the following year, they again attempted to nest in the box they had used initially. New recruits to the population were younger (1–2 years old), and they often did not nest. However, they did search widely for nesting cavities. When a new recruit found a suitable site that contained an active nest, she sometimes laid a few parasitic eggs there and, in subsequent seasons, attempted to nest in the box she had parasitized. Natal females often returned to and laid in the box from which they hatched, unless it was occupied by their mother. Adult females also actively avoided parasitizing close kin. In general, a nest cavity that was previously used successfully may be worth returning to, and fighting for, because its location is known and its quality has been proven. Scarcity of preferred nesting sites is probably the key ecological factor underlying all four unusual reproductive behaviours that characterize female wood ducks and other cavity-nesting waterfowl: natal philopatry, nest-site fidelity, aggressive competition for nest sites and high levels of intraspecific parasitism.