Bluebird and Small Cavity Nester Conservation
Sialis - Bluebirds and other small cavity nesters

eggsAll About House Wrens

Contents: Species, Identification, Distribution, Diet, Nesting Behavior, Nestboxes, Monitoring, Nesting Timetable, More Info and Photos of Nests, Eggs and Young. Also see Deterring House Wrens and Video Clip of House Wren Attacking Eggs, and Other Brown Birds.

House wren - male? Photo by E Zimmerman

NOTE: my primary interest in House Wrens is deterring them from using nestboxes. While they are native birds, eat lots of insects and have a cheery song, they are aggressively territorial and prolific, and I choose not to encourage them to use nestboxes on my trail, especially since their populations are already on the increase.

Species: There are about 30 subspecies (previously treated as separate species) of the House Wren, or HOWR, divided into 5 different groups. This web page deals with the Northern House Wren (Troglodytes aedon - called Sylvia domestica prior to 1809). The Chippewa Indians called it O-du-na-mis-sug-ud-da-we-shi, meaning big noise for its size. Sometimes referred to as a "Jenny Wren." Also see Carolina Wren, and less common Bewick's Wren.

Identification: This little brown bird has a long pointy bill and up-cocked tail. Their short (stubby), round wings enable them to maneuver through dense vegetation. Sexes are hard to tell apart. It sports dusky brown "eyebrows" versus the distinctive white eyebrow-stripe of the bigger Carolina Wren (Thryothorus ludovianus). Juveniles have a reddish brown rump, and their underparts are a darker buff. The Winter Wren (Troglodytes troglodytes) is smaller, and darker, with a shorter tail that is more cocked upward. It is also more likely to bob its head and has distinctive tip-tip call. The Bewick's Wren (Thryomanes bewickii) is more slender, rather long necked, with a very long tail. It also has a long white stripe over its eye. Cactus Wrens are really big and look almost like thrashers. There are also Sedge, Marsh, Rock and Canyon Wrens.

Song: Both sexes sing (males 3-11 times per minute!). Loud singing bouts of 4-10 minutes might indicate an unmated male. They do continue to sing when mated. "The persistent repetition of its nervous energetic outbursts has after a time a tendency to tire the listener." (Gross, 1948) HOWR males do make a "Whisper Song" (without opening their bill) while copulating.

Nine day old House Wren chicks. Photo by LC Moore.Distribution: Ubiquitous in fragmented forest (edges, small woodlots), swamps, farmland, residential areas and parks near trees and shrubs. Breeds from across most of Canada down to the southernmost part of South America, and into the West Indies. See map. Generally not found in the desert, or in high elevations in mountains. Birds nesting in the southern US (e.g., Alabama) MAY be non-migratory, but most HOWRs migrate to southern U.S. or n. and central Mexico for the winter, arriving north in mid-March through mid-May, depending on the location. Older more experienced birds tend to arrive earlier.

Supposedly 90% of males stay within a 1.25 mile radius during nesting season (Fitzwater fact sheet reference)

Diet: 98% animal matter - generally less mobile critters like spiders, beetles, caterpillars and various larva, grasshoppers, bugs, ants, and grit.

Nesting Behavior: An unmated male does not sleep in nest cavity, but females may do so on most or all nights during nest-building until young fledge. Nobody knows exactly how much territory a male HOWR will defend, although some have speculated about one acre. Usually active nests are almost 100 to 230 feet apart.

HOWRs are very territorial within their species and with other species, and may peck and remove eggs and young from nests. Predation may be highest in areas where HOWR populations are highest. More nestboxes = more HOWR = more predation.

Polgynous. Pairs don't usually stay together after the end of the breeding season (only reuniting less than 4% of the time in one study), and they commonly switch mates for subsequent broods in the same year. A male was seen feeding babies of grosbeaks and then House Sparrows. Sometimes one male will serve two females.

Nestboxes: Tolerant of human activities. A typical nestbox recommended for HOWR has a 4 x 4", is 6-8" deep, hole 4-7" above the floor, 1-1.5" hole, mounted 6-10 feet off the ground. Nests naturally in existing tree cavities, especially old woodpecker holes, but readily uses nestboxes. Also nests in variety of other crevices or cavities—e.g., in a flower pot, tin can, boot, scarecrow, pocket of hanging laundry, hornet nest; deserted barn swallow, Baltimore House Wrens eggs. Notice gloss.  One in upper left corner is smaller than others. Photo by E Zimmerman.Oriole, osprey or phoebe nest; mailbox, farm machinery, soap dish, or cow skulls hung on walls. One pair of wrens built their nest on the rear axle of a car that was used daily. When the car was driven the wrens went along, and eggs actually hatched. (Northcutt, 1937).

HOWR may prefer cavities closer to ground (10.5-17.7 feet, up to 30 feet) and with smaller entrances (1.6-2.2 inches) than other cavity nesters in local area, and show no apparent preference for cavities that open in particular directions, for cavities in snags versus living trees, or for cavities in particular tree species. At one California site, they preferred nest sites that were farther above ground with a smaller floor area, and tended to open more southeasterly than northerly. Usually avoids nest sites ≥100 feet from any significant woody vegetation. However, also avoids sites in heavily vegetated locations where visibility is low.

House Wren habitat often overlaps with chickadee and titmouse habitat, resulting in competition which House Wrens typically win. I wonder if forest fragmentation is resulting in increased HOWR populations.

Monitoring: Generally quite tolerant of occasional disturbances at nests. Female may desert nest if trapped in nest before midpoint of incubation stage, but usually does not desert later. Young are likely to fledge prematurely if disturbed late in the nesting cycle.

At fledging, nests may contain >50,000 mites (ugh). Egg sacs of predatory jumping spiders are often stuccoed to the sticks used in nests, and the baby spiders eat (in the lab at least) the mites. One researcher developed psittacosis apparently after contracting the bacterium Chlamydia psittaci from a nest in Minnesota (probably after breathing dust-borne, dried fecal matter.)

Nesting Timetable (typical): Breeding generally occurs from March through August in the U.S.

  • Arrival: breeding season begins in late April for birds living in south; early May in the north. Breeding is later at higher latitudes and altitudes. Male arrives first, establishing territory with dummy nests. Male begins constructing nests in empty cavities within a few days if not hours of arriving back from migration. Female selects actual nesting site.
  • Excavation: HOWR do not excavate their own holes.
  • Nest construction:
    • The male generally removes old nesting material from a site (but often re-uses the same sticks.) The male puts sticks in nest site(s), and then the female selects the cavity where she will lay her eggs.
    • Dummy nests: HOWR males often build loose dummy nests in other nearby cavities, of <10 to >400 sticks. If sticks are removed repeatedly, and the pair intends to actually use the nest site, the nest may eventually consist of a typical nest cup material only.
    • Once the female has selected a site, she starts over (but may reuse sticks from the "dummy nest" made by the male.) Nests are finished rapidly in 3-14 days by the female. See nest and egg ID information. Hundreds of small dry sticks are placed in bottom of cavity to form a tall base 4-6" deep. One nest in Iowa was made up of"52 hairpins, 68 nails (large), 120 small nails, 4 tacks, 13 staples,10 pins, 4 pieces of pencil lead, 11 safety pins, 6 paper fasteners, 52 wires, 1 buckle, 2 hooks, 3 garter fasteners, and 2 odds and ends." The narrow nest cup is eventually lined with grass, inner bark, hair, and finally feathers. It may be located in the rear of the cavity for protection from predators. Total trips to nests/day for female before egg-laying may exceed 300 (average about 170). Late in season, female may begin laying eggs before cup lining is complete. In some cavities, birds have to crawl up and over mound of sticks to reach nest cups deep in rear of cavity, which probably protects egg and young from predation.
  • Egg laying: 1 egg per day, laid very early in the morning, with a typical clutch having 5-8 eggs. Three and 9-10 egg clutches are rare - up to 12 reported.
  • Incubation: by female only, for 12–13 days, depending on the temperature - the period shortens later in the season. Up to 15 days after the penultimate (next-to-last) egg is laid. The hotter it is, the less time the female spends on the eggs. The female may actually apply some heat to even the first egg that is laid, but does not start full incubation through the night until laying the antepenultimate or penultimate egg.
  • Hatching: Since incubation begins on the ante/penultimate egg, hatching may be asynchronous, over a 1-4 day period. Babies are fully altricial (practically naked). Eyes are closed.
  • Development: Both parents feed babies. At day 3, parents may begin feeding grit and bits of shell. The ability to thermoregulate depends on the number of siblings in the nest, ranging from 4 days to 9 (more siblings = more insulation = earlier themoregulation.) Brooding stops in the latter 1/3 of the nestling stage. Whole broods of healthy young can die of apparent hyperthermia in poorly ventilated, unshaded nestboxes when temperatures exceed about 90 °F. Parents often remove both damaged eggs and dead nestlings (even nestlings almost full-size) from the nest. Parents eat or later remove fecal sacs. Nests may contain blow flies (Protocalliphora sp.)
  • Fledging: 12 to 18 days, usually 16-18 days. The earliest “normal” fledging could be 24 days, up to 33 days after next to last egg is laid. Very small runts can be left behind to die in the nest. Young are fully independent about 12-13 days after fledging. Female may re-nest at this point. Prone to premature fledging if box is opened late in the cycle. It is difficult to visually sex juveniles, as they all look like females, until the black bib and chestnut colored feathers on the wings start to develop several weeks after fledging.
  • Number of broods: Usually 2, sometimes 3 broods per season. One brood at high altitudes or latitudes. For second brood, production may begin 50–60 days after first egg of season is laid. Maybe 1/4 to 1/3 of males may return to the same breeding territory each year, but results of different studies differ. Females with failed nestings in the last attempt of the previous season moved farther away than successful females (same not true of males).
  • Longevity: 7 years and 1 month recorded (Valentine 1971). Fitzwater says an individual lived in captivity for 23 years, however no reference is provided.

References and More Information:

One cannot watch a pair of wrens in their repeated attempts to get long unwieldy sticks through a narrow box entrance scarcely large enough to admit their tiny bodies without being greatly impressed by their dogged persistence, energy, and skill.
- Alfred Otto Gross, Life Histories of Familiar North American Birds, 1948

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