California Partners in Flight Riparian Bird Conservation Plan
Tree Swallow (Tachycineta bicolor)
Photo by James Gallagher, Sea and Sage AudubonPrepared by: Amon Armstrong¹ (email@example.com), Elvis Cuevas Mendoza¹ (firstname.lastname@example.org)1, and David Winkler² (email@example.com)
¹ PRBO Conservation Science
4990 Shoreline Highway
Stinson Beach, CA 94970
² Cornell University
Department of Ecology and Evolutionary Biology
Ithaca, NY 14853
Armstrong, A., E.C. Mendoza, and D. Winkler. 2005. Tree Swallow (Tachycineta bicolor). In The Riparian Bird Conservation Plan: a strategy for reversing the decline of riparian-associated birds in California. California Partners in Flight. http://www.prbo.org/calpif/htmldocs/riparian_v-2.html
action plan summary
No subspecies taxonomy, although California breeders ("T. b. vespertina") average slightly smaller than birds from the rest of the range (Pyle 1997).
No special management status.
HISTORICAL BREEDING DISTRIBUTION:
Nearly entire length of California west of deserts according to references extending into the 1800's (Grinnell and Miller 1944) and in small pockets of riparian vegetation along the eastern slope of the Sierra Nevada. Extremes go beyond California as far north as Alaska and south into Mexico and as far south as Henshaw and Hodges Lakes, San Diego County within California (Grinnell and Miller 1944). Formerly a common breeder in lowland and foothill riparian groves of coastal Southern California in Santa Monica and El Monte (Los Angeles County), Riverside (Riverside County) and Lake Hodges (San Diego County), recovering where nest boxes have been provided (Ventura and Orange counties). Decline likely tied to destruction of riparian groves and competition with European Starlings for nest cavities (Garrett and Dunn 1981).
CURRENT BREEDING DISTRIBUTION:
Small (1994) describes range as the northern 1/3 of California, excluding most of the northeast Great Basin area. From the California Partners in Flight Focal Species Range Map (http://cain.nbii.gov/prbo/calpifmap/livemaps/index.php?spec=tres&plan =riparian) which includes 538 riparian study areas, the highest concentrations of nesting localities occur in the following bioregions: North Coast/Klamath, Modoc, Sacramento Valley, Bay Area/Delta, Sierra and San Joaquin Valley. Altitudes of nesting: near sea level in Santa Barbara (Upper Sonoran Life Zone) to about 9000 feet at lakes in the Sierra Nevada (Hudsonian Life Zone) (Grinnell and Miller 1944, Small 1994).
TIME OF OCCURRENCE AND SEASONAL MOVEMENTS
Arrival date on breeding grounds:
Arrive as early as the beginning of February and become common mid-February
in central California or even late January in southern California. Arrival continues
on breeding grounds into late April-early May (Small 1994). Tree Swallows arrive
earlier than other swallows due to more northerly wintering range. Arrival at
the breeding site is age and sex dependent. Generally, older birds arrive at
the breeding grounds before younger birds, and males arrive before females.
Females arrive up to a week later, and pair bonds form shortly thereafter. Pairs
begin to defend their sites immediately. (http://birds.cornell.edu/birdhouse/bird_bios/speciesaccounts/treswa.html
Departure date from breeding grounds:
Fall transients move south from late July to early October, but small numbers
may stay on non-wintering grounds as late as December in southern California.
Higher mountain nesters leave by mid-September (Small 1994).
Spring migration period:
Little information found, but see Arrival on- and Departure from Breeding Grounds.
Fall migration period:
Little information found, but see Arrival on- and Departure from Breeding Grounds.
Extent of wintering in CA:
Very rare from Mendocino County north. Uncommon from Sonoma County south to
San Diego County. In the Central Valley they winter in some years as far north
as Glenn and Butte counties south to northern Kern County. Fairly large winter
aggregations occur at the Salton Sea and along the lower Colorado River. Rare
spring and fall vagrant on all islands (Small 1994)
MIGRATION STOPOVER CHARACTERISTICS:Not limited by access to cavities as during the breeding season. Throughout the entire non-breeding season, birds gather in large nocturnal groups to roost every night, but forage largely solitarily during the day
No information found.
Prefer open foraging areas above water, as these presumably provide the highest densities of flying insect prey. Sites with substantial vegetation cover are avoided.
No information found.
Tree Swallows eat mostly flying insects in open areas from 0 to 50 meters above
land or water (Robertson et al. 1992). They catch individual prey in the air
by turning abruptly, sometimes in large groups when insects are swarming (Tyler
1942). Tree Swallows are able to pick prey items off the surface of the water
(Hobson and Sealy 1987) and very rarely glean insects off vertical surfaces,
foliage, or ground (Robertson, et al. 1992). They spend over 80% of their time
foraging below 12 m and usually within 100-200 m of their nest when feeding
nestlings (McCarty and Winkler 1999a). Birds studied in Colorado are thought
to forage over plains 100 km or more from nesting sites in mountains (Robertson,
et al. 1992). Tree Swallows feed from dawn to dusk.
Beal (1918) found the following results by analyzing stomach contents of adults collected during different seasons at various locations: 80% of diet was animal, 20% vegetable. Animal matter: flies (41%), beetles (14%), and ants (6%); plus smaller amounts of grasshoppers, dragonflies, and spiders. Vegetable matter mostly bayberries; more common in winter, but found in summer-collected birds also. Elliot (1939) analyzed fecal matter of swallows wintering relatively far north, which showed remains of water-boatmen (Hemiptera, Corixidae), crustacean material, spiders, and bayberries, plus bulrushes, sedges, and other seeds. From 215 total diet samples obtained between 1989-1993, McCarty and Winkler (1999a) found a wide range of insect prey from at least 11 orders. Insect sizes ranged from less than 1 mm in length to large Anisopteran Odonates over 50 mm long. Insects in the 3-5 mm class made up the largest proportion of the diet.
Individuals can subsist on seeds and berries, especially bayberries (Myrica sp.), when insects are not available for extended periods of time, usually during harsh winters or early return to breeding grounds. These items may sometimes constitute approximately 20 percent of their diet in the non-breeding season (Robertson et al. 1992).
Tree Swallows drink from the water surface by briefly dipping in their beaks during flight. Individual raindrops can also be taken by perched birds in driving rain (Robertson et al. 1992).
The Tree Swallow prefers open areas, usually near water, including fields, marshes, shorelines, and wooded swamps with standing dead trees. Breeding sites are characterized by woodlands with a large amount of open area for foraging or entirely open country with nest-boxes. Breeding is never in dense woodland, and competition with wrens is lessened the farther that nests are from shrubby vegetation.
Usually males arrive at breeding grounds first and establish territory around nest site (Cohen 1984, 1985). Females arrive same day, or up to 7 days later. Selection process involves repeated entry and examination of cavity. Adding a few pieces of nesting material may have a role in claiming a cavity (Rustad 1972).
Tree Swallows nest in natural cavities, excavated woodpecker holes, or nest boxes. Also occasionally in cavities or crevices of building or wooden structures and signposts, pipes, and banks (Biacich and Harrison 1997). Entrance and cavity dimensions may vary with tree and woodpecker species. Cavities facing south may in some places be preferred over north-facing cavities (Lumsden 1986).
Table 1 (Robertson et al 1992). Characteristics of nest sites (mean, standard deviation, and sample size) in natural cavities. Data gathered in Ontario, Canada by Rendell and Robertson (1989). Some variation found in other studies.
Horizontal entrance diameter (cm)
|5.3||1.7||2.0 - 16.0||48|
|Vertical entrance diameter (cm)||7.6||4.0||3.7 - 26.0||48|
|Floor area (cm²)||86.5||41||25.0 - 143.0||48|
|Volume (cm³)||1252||1122||219 - 6370||48|
|Cavity depth (cm)||17.5||1.7||31|
|Height above substrate (m)||3.4||2.0||0.9 - 7.8||48|
|Snag height (m)||4.3||2.4||1.3 - 10.3||48|
|Snag diameter at breast height (cm)||83.4||24.2||36 - 138||48|
|Tree diameter at entrance (cm)||70.4||18.9||40-103||48|
HEIGHT OF NEST:
Natural nest cavities used are about 1 to 10 m above ground (Rendell and Robertson 1989). A study in Ontario, Canada, showed that cavities were located in snags that ranged from less than 2 m to more than 9 m in height, although most (75%) were in snags less than 6 m tall, and a substantial proportion of these (27%) were in stumps of fallen trees which were less than 2 m tall.
Nests concealed by structure supporting the cavity. Holes may be slightly concealed by branches or leaves of the nest tree.
VEGETATION SURROUNDING THE NEST:
In SW Manitoba, preferred long grass instead of wooded or grass pastures, apparently due to competition with Mountain Bluebirds (Munro and Rounds 1985). No preference for nest boxes near wooded edges of fields (Rendell and Robertson 1990). Indeed, wood edges expose the birds to nest-competition with House Wrens, at least in eastern North America.
Nest in cavities, either natural or man-made. Nests often made exclusively of grass (Kuerzi 1941), especially when located over fields. Also may contain mosses, rootlets, aquatic vegetation, and other plant materials (Dring 1981). A wider variety of materials are used in nests located over water. The variety of materials used in different nests within the same habitat indicate individual preferences (Robertson et al. 1992).
Feathers may be added to nest. Well-feathered nests have lower cooling rate than unfeathered nests (H. Gibbs and R.J. Robertson unpub. data [from Robertson et al. 1992]), although primary function of feathers may be moisture barrier (Mertens 1977). Winkler (1993) found that nests with more feathers had higher nestling growth rate and less parasitic infestations. Nesting material covers floor of cavity, and ranges in depth from about 2 to 8 cm (Dring 1981b).
BREEDING TERRITORY SIZE AND DENSITY:
Since breeding pairs are patchily distributed, density counts are difficult to obtain. Furthermore, size of non-breeding floating population is not reflected in counts of breeding pairs. Mean of 4 birds per route reported in breeding bird survey in western region, 1 in central region, and 6 in eastern region of the United States (Robbins et al. 1986). When nesting cavities are plentiful, semi-communal breeding can occur, although Tree Swallows are less communal than other species of swallows. Colonies of ten to fifty pairs may be seen (Forbush and May 1955).
Male may give "vertical posture" courtship display to unfamiliar females along with a nest site advertising call or day song. If female is responsive, she will come down on male in a "courtship pounce" similar to a male's copulation approach. Often followed by nest site showing by male, landing at hole and giving intense, high-pitched nest site advertising call, and often enters cavity. Female may respond by flying to hole, and then, after perhaps a few false entries, she typically enters to investigate cavity (Cohen 1984). Just prior to attempts to copulate the adult birds may face each other, assume an erect stance, and loudly chattering, move their heads up and down, bills opening and closing rapidly (Shaeffer 1970).
Tree Swallows are behaviorally monogamous (Leffelaar and Robertson 1986). In
two populations, less than 5% of breeding males were polygynous (Robertson et
al. 1992). In contrast to these behavioral associations, Tree Swallows have
one of the highest rates of extra-pair fertilizations of any passerine (Lifjeld
et al. 1993).
Amount of available food is probably linked to frequency of polygyny (Quinney 1983). Smaller food supply requires more male parental care (Leffelaar and Robertson 1986).
Copulation usually occurs on perch near nest site (Leffelaar and Robertson 1984), rarely in cavity (Robertson et al. 1992). Copulations involve one to several (1-11) vent contacts. Male hovers over female giving a distinctive rattle call, while female crouches in a horizontal position. While still hovering, male lightly lands on back of female, often holding feathers on back of her head with his beak, and makes vent contact (Robertson 1990). Most copulatory behavior begins about 5 days before egg laying, although copulations can begin about 2 weeks before the first egg is laid (Robertson et al. 1992). Pairs occasionally breed in the same area for several consecutive years (Cohen 1985), though far fewer pairs stay together than the pair members survive (Llambías & Winkler in prep.).
Range from 2 to 8 eggs; most commonly lay 4 to 7 (Stutchbury and Robertson 1988). Sites with higher food abundance have larger clutch sizes (Hussell and Quinney 1987) and there are generally higher clutches in the north and in the interior of North America (Dunn et al. 2000).
Female usually begins incubation on day penultimate egg is laid, although delayed by up to a week in rare cases (Zach 1982). Only females incubate. Kuerzi (1941) reported an average of 11 minutes on nest, and 9 minutes off, but incubation rhythms depend a great deal on environmental temperatures. Male does not feed incubating female, but will often perch near or at nest site when female is absent. (Kuerzi 1941). Females often remain inside until hearing male's contact call (Kuerzi 1941).
Range from 11-19 days, but more commonly 14-15 days (Kuerzi 1941).
DEVELOPMENT AT HATCHING:
Embryos may vocalize pips in response to adult contact calls on the day before hatching (Robertson et al. 1992). Young hatch throughout day. Pipping usually indicates beginning of hatching and egg hatches within 1-2 hours. Most clutches hatch over a 1-2 day period (Kuerzi 1941), but may take 3 days (Zach 1982). Asynchrony is weather dependent (Robertson et al. 1992). Females brood their nestlings for three days, after which they only brood at night and sometimes during uncommonly cold weather (Kuerzi 1941). The young are fed soon after they hatch, and both parents share this duty.
Nestlings fledge after 15 to 30 days (most often 18-22 days) (Kuerzi 1941, Winkler 1993). Fecal sacs are removed from the nest until the young are about 14 days old. Disturbance after day 12 can cause nestlings to fledge prematurely and unsuccessfully.
Parents can sometimes feed fledglings for about 3 days after departure from nest (Kuerzi 1941), but most young appear to be independent soon after departure. Fledglings weigh as much as or more than adults and feather development is complete.
POST FLEDGING BIOLOGY OF OFFSPRING:
Hatch-year birds sometimes visit nest sites of other pairs (Lombardo 1986), possibly exploring for nest sites for the following year. They mix with foraging flocks during the winter, and then are assumed to migrate with adults. Banded nestlings that return to natal sites the next year ranges from 0.8% to 12% (Butler 1988), and an Ithaca study revealed an average return rate to a single study site of 3% (Winkler unpubl. data). Females from food-poor areas had a lower return rate as yearling breeders: 4.3% compared to 11.5% in areas with more abundant food (Robertson et al. 1992). A 5-year study in Colorado showed no strong indication of sex-biased natal philopatry, though females disperse farther than males in upstate New York (Winkler et al. in press). In Colorado, 23% of yearlings bred in their natal area whereas 77% bred in a different area at least 5 km away (Robertson et al. 1992).
POST FLEDGING BIOLOGY OF OFFSPRING:
POST BREEDING SOCIAL BEHAVIOR:
Generally solitary diurnal foragers that aggregate every night in large roosts. Roosting behavior is spectacular, with the aggregation of a large assemblage of birds circling densely over a reed bed in water at an altitude of about 100-200 m after sunset. Just before dark, the flock whirls and condenses until one brave pioneer descends into the reeds. That bird is followed immediately by hundreds of thousands, all of which spiral into the reeds within seconds (Winkler unpubl. data, Burney 2002).
Delayed breeding has not been demonstrated with the Tree Swallow. Birds begin breeding as second-year birds.
NUMBER OF BROODS:
Tree Swallows have one brood per breeding season except in southern California (Wasserman 2003). Pairs will nest again if the first attempt fails, but true second broods are extremely rare throughout most of the range. The same nest sites are often used for more than one breeding season, and pairs will often nest on top of old nests.
Brown-headed Cowbird parasitism is very rare (Mills 1988).
See Current Breeding Distribution.
Breaking up of intact continuous blocks of suitable foraging habitat surely alters migration routes and times, but fragmentation for Tree Swallows is opposite to that in other tree-nesting passerines: their habitat is fragmented by the development of dense woods.
Home range tends to change with breeding phase and differ with geographic area, food abundance, weather etc. Prior to incubation, birds are not strongly tied to breeding area and frequently forage up to 60 km or more from nesting site if food is scarce early in season. During incubation and nestling phases, patch size used is much smaller, especially for females; usually good foraging within 4 km is necessary for breeding success (Robertson et al. 1992).
DISTURBANCE (natural or managed):
Increase in European Starling and House Sparrow populations associated with human habitats results in displacement of Tree Swallows from potential nest sites (Weitzel 1988). Nest usurpation occurs by predatory species such as Common Grackles (Quiscalus quiscula) and Northern Flickers (Colaptes auratus) (Rendell and Robertson 1991). Other possible competitors in California are Violet-green Swallows (Tachycineta thalassina), Western Bluebirds (Sialia mexicana), Pygmy Nuthatches (Sitta pygmaea), White-breasted Nuthatches (Sitta carolinensis), Ash-throated Flycatchers (Myiarchus cinerascens), Oak Titmice (Baeolophus inornatus), Chestnut-backed Chickadees (Poecile rufescens), Purple Martins (Progne subis) and various woodpeckers. A high management priority is maintaining dead stands of trees, which provide nest sites for Tree Swallows, along with most other cavity-dwellers. Loss of forest due to cutting is counterbalanced approximately 2% by nest box construction, according to a study in Canada (Erskine 1979).
ADJACENT LAND USE:
Main land uses affecting Tree Swallows are logging, reforestation, agriculture, grazing, recreation, and transportation. Swallows are fairly tolerant of human interference, however predators are often introduced and breeding patch size is constricted. Proximity to edge of an open area, where shrubs or trees are close to the nest-box, may have an effect, since House Wrens (Troglodytes aedon), which occupy shrub habitat, may interfere with Tree Swallow nesting (Rendell and Robertson 1990).
Reproductive success of Tree Swallows is greatly impacted by weather patterns that cause short-term changes in environmental conditions (McCarty and Winkler 1999b). There are varying reports of how long-term climate change affects breeding dates. In North America, Dunn and Winkler (1999) reported a continent-wide advance in egg-laying dates of up to 9 days between 1959 and 1991. Increasing surface air temperatures at the time of breeding are associated with this advance in phenology. Also David J.T. Hussell (2003) found that timing of laying in Long Point, Ontario, was strongly correlated with spring temperatures, but he argues that spring temperatures have not increased and timing of the breeding season has not advanced from 1969 - 2001. Factors related to temperature such as plant growth and insect abundance could be the cues used by swallows to trigger laying (Hussell 2003). Hussell also indicates that warming trends over the past 50 years are stronger in the south and west (therefore affecting California populations).
Tree Swallows readily use nesting boxes, a controllable and accessible environment, making them a good study species for the effect of pollutants on birds. No detrimental affects found on eggshell thickness. PCB's and DDE have been found in high levels in adults, eggs, and nestlings of western populations (Shaw 1984). High PCB levels in Tree Swallows nesting along the Hudson River are associated with altered nest-building behavior (McCarty and Secord 2000). Increasing mortality rates during egg laying, in females older than 2 years, suggests PCB and DDE residues are harming this species, a possible long-term problem for the Tree Swallow (Robertson et al. 1992).
On Eggs and Nestlings:
Terrestrial: Raccoons (Yunick 1971), black bears (Zach and Mayoh 1984), snakes, chipmunks, weasels, deer mice, and feral cats (Robertson et al. 1992).
Avian: American Kestrels (Weydemeyer 1935), American Crows (Stocek 1970), and Northern Flickers (Rendell and Robertson 1991).
On Flying or Perched Birds:
Sharp-shinned Hawks, American Kestrels, Merlins, Peregrine Falcons, Great Horned Owls, and Magpies. Mobbing of terrestrial predators is common on breeding grounds. Birds swarm and dive-bomb potential predator while giving alarm and aggression calls (Winkler 1991), though the reaction to most threatening aerial predators (especially Accipiters) is to alarm call and fly high up above the predator (Winkler unpubl. data).
DEMOGRAPHY AND POPULATION TRENDS:
AGE AND SEX RATIOS:
Both sexes breed as yearlings if possible. Apparently, a wide range of proportions of second year females breed; from 10% - 80% (Stutchbury and Robertson 1985, Rendell and Robertson 1990). Higher side of range is from newly established populations where nest-site competition is not as significant (Robertson et al. 1992). In a study of 40 broods, Whittingham and Dunn (2000) found the mean sex ratio per brood (+ SD) to be biased significantly towards males (57 + 2% male). This bias was associated with females in better body condition, who were more likely to produce male offspring in better condition. Variance in male reproductive success is greater than that of females due to high levels of extra-pair paternity (38-76% extra-pair young). Therefore, more sons per brood will return higher reproductive yields than daughters, if sons in better condition have greater success (Whittingham and Dunn 2000).
Information highly variable. A summary of many North American studies in Robertson et al. (1992) found 78.8% (+17.2% SD) nesting success (i.e., at least one nestling fledges). Reproductive success seems to vary with time of season and age and breeding experience of female (Stutchbury and Robertson 1988). Most populations studied are breeding in nest boxes, where predation is artificially reduced. Success may be much lower in natural nest sites (Robertson and Rendell 1990).
Mortality in the first year of life estimated at 79%, based on recoveries of birds banded as nestlings. Thus, about 20% survive first year and 40% to 60% annual survival in subsequent years (Houston and Houston 1987). There is little evidence of age or sex bias. Causes of death at nest are predation, cold temperature leading to hypothermia, and starvation due to scarcity of food late in season or during cold, rainy periods (Robertson et al 1992). Maximum lifespan records range from 8 to 11 years (Houston and Houston 1987). Butler's (1988) life table shows average life span of 2.7 years; maximum of 8 years.
Natal dispersal distance for males and females in upstate New York averages 2.44 and 8.38 km, respectively (Winkler et al. in press). Proportion of banded nestlings returning to same study area the next year ranges from 0.8% to 12% (Butler 1988). Percentage range relates to food richness and availability of suitable nesting sites. Between 13% to 60% of breeding adults return to the same study area to breed next year (Butler 1988). Nest site fidelity in general is lower in females, especially after nest failure. (Robertson et al 1992); in upstate New York, 4% of males and 14% of females move to new nesting areas at least 1 km distant per year on average (Winkler et al. 2004).
Over the past 25 years, populations have increased (Robbins et al 1986). Breeding
populations are limited by number of nest sites, which in turn limits total
population size (Stutchbury and Robertson 1985). Short-term reductions occur
due to extreme weather conditions (Kuerzi 1941). There is little evidence that
competition for food is a population-limiting variable (Robertson, et al. 1992).
Although fairly common throughout North America, Tree Swallows have been almost
completely extirpated from southern California, especially in Ventura County.
To make way for development and agriculture, most trees with nesting potential
have been cut down (www.treeswallows.org/history).
EXOTIC SPECIES INVASION/ENCROACHMENT:
Human introduction of exotic/invasive bird species has created competition for nesting sites. Many of these species, such as European Starlings and House Sparrows, thrive at a high cost to native birds.
Destruction of marshes for housing, agricultural and industrial land use.Water pollution also caused by these developments. Loss of open forests with dead snags necessary for nesting.
Woodpeckers that excavate holes usable as nest cavities: Downy Woodpecker, Hairy Woodpecker, Northern Flicker, Nuttall's Woodpecker, Red-breasted Sapsucker (Grinnell and Miller 1944). Other native species that use similar nesting habitat: Violet-Green Swallows, Western Bluebird, Chestnut-backed Chickadee, Pygmy Nuthatch, House Wren and potentially other cavity nesters.
MONITORING METHODS AND RESEARCH NEEDS:
1. Methods of monitoring Tree Swallow activity include nest box monitoring, natural cavity monitoring, point counts and mist-netting/banding.
2. Little information was found on these subjects, necessitating more research: migration periods; stop-over needs/characteristics; historical distribution and differences between nest box and natural cavity breeders.
3. Coordinated efforts of box monitoring are done through networks such as Golondrinas de Las Americas (http://golondrinas.cornell.edu), The Birdhouse Network (birds.cornell.edu/birdhouse/), and Tree Swallows.org (www.treeswallows.org).
ACTION PLAN SUMMARY
SPECIES: Tree Swallow, Tachycineta bicolor
STATUS: Tree swallows are under no special threat, but their extirpation from areas of southern California and the removal of snags of appropriate size from riparian areas throughout the West indicate a necessity for continuing research.
Necessary to have fresh water, marshlands, or open areas, usually near water, including fields, marshes, shorelines, and wooded swamps for aerial foraging.
Also need nest cavities in standing dead trees in open areas that have been excavated by woodpeckers, or artificial nest boxes.
Pollution of feeding and nesting areas.
Loss of necessary riparian habitat.
Loss of dead and dying trees with nest cavities.
Invasion by non-native bird species such as European Starlings and House Sparrows.
Lack of information about migration periods and routes
Increase habitat for breeding and migration needs.
Increase awareness of relationship between wildlife health and human health.
1. Educate land managers and land owners about necessity of maintaining stands of dead trees and encourage nest box building.
2. Educate agriculture and industry leaders about effects of land use, pesticides, and pollution.
3. Research subjects with missing information, such as migration periods and routes.
4. Encourage public participation in information gathering by publicizing events and activities such as bird censuses and nest-box building.
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Martens, J. A. L. 1977. Thermal conditions for successful breeding in Great Tits (Parus major). II. Thermal properties of nest and their importance for range of temperature tolerance in Great Tit broods. Oecologia 28: 31-56
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