Editing Tree swallow (section)

===Breeding===
[[File:Tachycineta bicolor -California, USA-8.jpg|thumb|left|A natural nest hole]]
The tree swallow nests in structures with pre-existing holes, both natural and artificial. These were once found only in forested regions, but the building of nest boxes has allowed this bird to expand into open habitats.<ref name="Turner"/> This swallow usually nests in the area it bred the year before; only about 14% of females and 4% of males disperse to breed at a new site per year. Most do not go far, usually breeding at sites less than {{convert|10|km|mi}} away from their original grounds. Dispersal is influenced by breeding success; of the adult females that fail to fledge a chick, about 28% disperse, compared to 5% of successful breeders.<ref name="WinklerWrege2004"/> Natal dispersal (when a bird does not return to the site it was born at to breed) is common in the tree swallow and occurs more frequently than breeding dispersal.<ref name="ShutlerClark2003">{{cite journal|last1=Shutler|first1=Dave|last2=Clark|first2=Robert G.|title=Causes and consequences of tree swallow (''Tachycineta bicolor'') dispersal in Saskatchewan|journal=The Auk|volume=120|issue=3|year=2003|pages=619–631|issn=0004-8038|doi=10.1642/0004-8038(2003)120[0619:CACOTS]2.0.CO;2|s2cid=28251342 }}</ref> It nests both in loose groups and isolated pairs. When nesting in loose groups, nests are usually spaced at least {{convert|10|to|15|m|ft}} apart,<ref name="hbw"/> and those that are closer in distance are usually further apart in terms of laying date.<ref name="MuldalGibbs1985">{{cite journal|last1=Muldal|first1=Alison|last2=Gibbs|first2=H. Lisle|last3=Robertson|first3=Raleigh J.|title=Preferred nest spacing of an obligate cavity-nesting bird, the tree swallow|journal=The Condor|volume=87|issue=3|year=1985|pages=356–363|issn=0010-5422|doi=10.2307/1367216|jstor=1367216}}</ref> In natural cavities, the tree swallow nests about {{convert|27|m|ft}} apart from its neighbor. The nest hole in these situations is, on average, {{convert|3.4|m|ft}} above ground level, although about 45% of them are less than {{convert|2|m|ft}} above the ground. Higher cavities are likely favored because they reduce predation, while lower nest holes may be chosen to avoid competition. Entrance widths are often between {{convert|4|and|5|cm|in}}, whereas entrance heights are more variable: a 1989 study found openings ranging from {{convert|3.5|to|26|cm|in}}. Cavity volume is generally below {{convert|1000|cm3|cuin|abbr=on}}.<ref name="RendellRobertson1989">{{cite journal|last1=Rendell|first1=Wallace B.|last2=Robertson|first2=Raleigh J.|title=Nest-site characteristics, reproductive success and cavity availability for tree swallows breeding in natural cavities|journal=The Condor|volume=91|issue=4|year=1989|pages=875|issn=0010-5422|doi=10.2307/1368072|jstor=1368072}}</ref> After finding a suitable place to nest, the male perches near it and calls frequently. A lack of sites can cause fights between birds, sometimes resulting in deaths. This swallow usually defends an area around the nest with a radius of about {{convert|15|ft|m|order=flip}},<ref name="Turner"/> as well as extra nests inside of that {{birdgloss|territory}},<ref name="hbw"/> by blocking the entrance to the nest and chasing intruders.<ref name="Turner"/> The nest cup itself is made from grass, moss, pine needles, and aquatic plants collected mostly by the female,<ref name="hbw">{{cite journal |last1=Turner|first1=A.|editor1-last=del Hoyo |editor1-first=Josep |editor2-last=Elliott |editor2-first=Andrew |editor3-last=Sargatal |editor3-first=Jordi |editor4-last=Christie |editor4-first=David A. |editor5-last=de Juana |editor5-first=Eduardo |year=2020 |title=Tree Swallow (''Tachycineta bicolor'') |journal=Handbook of the Birds of the World Alive |url=https://www.hbw.com/species/tree-swallow-tachycineta-bicolor |publisher=Lynx Edicions |location=Barcelona, Spain |doi=10.2173/bow.treswa.01|s2cid=216382529|access-date=10 December 2017 |url-access=subscription }}</ref> and is lined with feathers gathered primarily by the male in fights.<ref name="Winkler1993"/> The feathers may function to insulate the nest, decreasing incubation time<ref name="Lombardo1995">{{cite journal|last1=Lombardo|first1=Michael P.|last2=Bosman|first2=Ruth M.|last3=Faro|first3=Christine A.|last4=Houtteman|first4=Stephen G.|last5=Kluisza|first5=Timothy S.|title=Effect of feathers as nest insulation on incubation behavior and reproductive performance of tree swallows (''Tachycineta bicolor'')|journal=The Auk|volume=112|issue=4|year=1995|pages=973–981|issn=0004-8038|doi=10.2307/4089028|jstor=4089028|s2cid=55119375 |url=https://scholarworks.gvsu.edu/biopeerpubs/10|url-access=subscription}}</ref> and likely preventing [[hypothermia]] in chicks. In addition to faster growth for chicks,<ref name="Winkler1993">{{cite journal|last=Winkler|first=David W.|year=1993|title=Use and importance of feathers as nest lining in tree swallows (''Tachycineta bicolor'')|journal=The Auk|volume=110|issue=1|pages=29–36}}</ref> eggs cool slower in nests with feathers than those without.<ref name="WindsorFegely2013">{{cite journal|last1=Windsor|first1=Rebecca L.|last2=Fegely|first2=Jessica L.|last3=Ardia|first3=Daniel R.|title=The effects of nest size and insulation on thermal properties of tree swallow nests|journal=Journal of Avian Biology|volume=44|issue=4|year=2013|pages=305–310|issn=0908-8857|doi=10.1111/j.1600-048X.2013.05768.x}}</ref> However, a study published in 2018 did not find a significant correlation between the number of feathers in nests that were artificially warmed versus those that were not. Additionally, it found that nests in [[St. Denis, Saskatchewan]] used significantly less feathers than those in [[Annapolis Valley]], despite the former being further north. However, temperatures in Nova Scotia (where Annapolis Valley is) are generally lower than those in Saskatchewan, possibly explaining the unexpected result.<ref name="HollandShutler2018">{{cite journal|last1=Holland|first1=Erika R.|last2=Shutler|first2=Dave|title=Nest feathering responses by tree swallows (''Tachycineta bicolor'') to experimental warming|journal=Journal of Ornithology|volume=159|issue=4|year=2018|pages=991–998|issn=2193-7192|doi=10.1007/s10336-018-1568-6|bibcode=2018JOrni.159..991H |s2cid=46971093}}</ref>

[[File:Tree Swallow-27527-4c.jpg|thumb|right|A pair mating]]
During courtship, a male tree swallow attacks an unknown female. This can be stimulated through wing-fluttering flight by the female, which may be an invitation to court. The male may then take a vertical posture, with a raised and slightly spread tail and wings flicked and slightly drooped. This prompts the female to try to land on the male's back, but he flies to prevent this; this is repeated. After courting the female, the male flies to his chosen nest site, which the female inspects. During copulation, the male hovers over the female, and then mounts her, giving ticking calls. He then makes {{birdgloss|cloacal contact}} with the female while holding her neck feathers in his bill and standing on her slightly outstretched wings. Copulation occurs multiple times.<ref name="Turner"/>

Eggs are laid from early May to mid-June (although this is happening earlier due to [[climate change]]<ref name="DunnWinkler1999">{{cite journal|last1=Dunn|first1=P. O.|last2=Winkler|first2=D. W.|title=Climate change has affected the breeding date of tree swallows throughout North America|journal=Proceedings of the Royal Society B: Biological Sciences|volume=266|issue=1437|year=1999|pages=2487–2490|issn=0962-8452|doi=10.1098/rspb.1999.0950|pmid=10693819|pmc=1690485}}</ref>) and chicks fledge between mid-June and July.<ref name="hbw"/> Latitude is positively correlated with laying date,<ref name="hbw"/> while female age and wing length (longer wings allow more efficient foraging<ref name="NookerDunn2005"/>) are negatively correlated.<ref name="WinklerAllen1996">{{cite journal|last1=Winkler|first1=David W.|last2=Allen|first2=Paul E.|title=The seasonal decline in tree swallow clutch size: physiological constraint or strategic adjustment?|journal=Ecology|volume=77|issue=3|year=1996|pages=922–932|issn=0012-9658|doi=10.2307/2265512|jstor=2265512|bibcode=1996Ecol...77..922W |s2cid=84919928}}</ref> The tree swallow is likely an [[income breeder]], as it breeds based on food abundance and temperatures during the laying season.<ref name="NookerDunn2005">{{cite journal|last1=Nooker|first1=Jacqueline K.|last2=Dunn|first2=Peter O.|last3=Whittingham|first3=Linda A.|title=Effects of food abundance, weather, and female condition on reproduction in tree swallows (''Tachycineta bicolor'')|journal=The Auk|volume=122|issue=4|year=2005|pages=1225|issn=0004-8038|doi=10.1642/0004-8038(2005)122[1225:EOFAWA]2.0.CO;2|s2cid=4995233 |doi-access=free}}</ref> This species is generally [[Monogamy in animals|socially monogamous]], but up to 8% of breeding males are [[Polygyny in animals|polygynous]].<ref name="hbw"/> Polygyny is influenced by territory: males having territories with nest boxes at least {{convert|5|m|ft}} apart are more likely to be polygynous.<ref name="DunnHannon1991">{{cite journal|last1=Dunn|first1=Peter O.|last2=Hannon|first2=Susan J.|title=Intraspecific competition and the maintenance of monogamy in tree swallows|journal=Behavioral Ecology|volume=2|issue=3|year=1991|pages=258–266|issn=1045-2249|doi=10.1093/beheco/2.3.258}}</ref> It is suggested that this polygyny depends on the conditions during the laying season: better conditions, such as an abundance of food, allow females in polygyny who do not receive help foraging to lay more eggs.<ref name="DunnHannon1992">{{Cite journal| issn = 0004-8038| volume = 109| issue = 3| pages = 488–499| last1 = Dunn| first1 = Peter O.| last2 = Hannon| first2 = Susan J.| title = Effects of food abundance and male parental care on reproductive success and monogamy in tree swallows| journal = The Auk| year = 1992}}</ref>
[[File:Tree swallow nest.jpg|thumb|left|The inside of a tree swallow nest]]
[[File:20240501 tree swallow gathering nesting material wm.webm|thumb|left|A male gathering nesting material]]
The tree swallow has high rates of extra-pair paternity, 38% to 69% of nestlings being a product of extra-pair paternity, and 50% to 87% of broods containing at least one nestling that was the result of an extra-pair copulation.<ref name="hbw"/> One factor that might contribute to this is that females have control over copulation, making paternity guards ineffective.<ref name="LifjeldDunn1993">{{cite journal|last1=Lifjeld|first1=Jan T.|last2=Dunn|first2=Peter O.|last3=Robertson|first3=Raleigh J.|last4=Boag|first4=Peter T.|title=Extra-pair paternity in monogamous tree swallows|journal=Animal Behaviour|volume=45|issue=2|year=1993|pages=213–229|issn=0003-3472|doi=10.1006/anbe.1993.1028|s2cid=53195131}}</ref> This may be mitigated by more frequent copulations just before egg laying, according to a 2009 study which found that within-pair copulation attempts peaked three to one days before the first egg was laid and that more successful attempts during this period increased the share of within-pair young males had. This latter finding contradicts those of a 1993 and a 1994 study.<ref name="CroweKleven2009">{{cite journal|last1=Crowe|first1=Susan A.|last2=Kleven|first2=Oddmund|last3=Delmore|first3=Kira E.|last4=Laskemoen|first4=Terje|last5=Nocera|first5=Joseph J.|last6=Lifjeld|first6=Jan T.|last7=Robertson|first7=Raleigh J.|title=Paternity assurance through frequent copulations in a wild passerine with intense sperm competition|journal=Animal Behaviour|volume=77|issue=1|year=2009|pages=183–187|issn=0003-3472|doi=10.1016/j.anbehav.2008.09.024|s2cid=13722818}}</ref> Extra-pair paternity does not change the level of parental care the male contributes in the tree swallow.<ref name="KempenaersLanctot1998">{{cite journal|last1=Kempenaers|first1=Bart|last2=Lanctot|first2=Richard B.|last3=Robertson|first3=Raleigh J.|title=Certainty of paternity and paternal investment in eastern bluebirds and tree swallows|journal=Animal Behaviour|volume=55|issue=4|year=1998|pages=845–860|issn=0003-3472|doi=10.1006/anbe.1997.0667|pmid=9632472|bibcode=1998AnBeh..55..845K |s2cid=24760456}}</ref> A significant number of extra-pair fathers may be floaters (those present at breeding grounds that presumably do not breed). A 2001 study found that out of 35 extra-pair nestlings, 25 were sired by local residents, three by residents of nearby sites, and seven by male floaters. In the tree swallow, floating thus helps males in good condition produce more chicks, while allowing males in bad condition to be successful by investing in parental care.<ref name="KempenaersEverding2001">{{cite journal|last1=Kempenaers|first1=Bart|last2=Everding|first2=Susie|last3=Bishop|first3=Cheryl|last4=Boag|first4=Peter|last5=Robertson|first5=Raleigh J.|title=Extra-pair paternity and the reproductive role of male floaters in the tree swallow (''Tachycineta bicolor'')|journal=Behavioral Ecology and Sociobiology|volume=49|issue=4|year=2001|pages=251–259|issn=0340-5443|doi=10.1007/s002650000305|bibcode=2001BEcoS..49..251K |s2cid=25483760}}</ref> There is also a significant population of female floaters; a 1985 study estimated that around 23% to 27% of females were floaters, of which about 47% to 79% were subadults.<ref name="StutchburyRobertson1985">{{Cite journal| doi = 10.1093/auk/102.3.651| issn = 0004-8038| volume = 102| issue = 3| pages = 651–654| last1 = Stutchbury| first1 = Bridget J.| last2 = Robertson| first2 = Raleigh J.| title = Floating populations of female tree swallows| journal = The Auk| year=1985}}</ref>

Why females engage in extra-pair copulation and how they choose extra-pair mates is controversial. One theory, called the genetic compatibility hypothesis, states that increased offspring fitness results from increased [[heterozygosity]], and thus that female tree swallows would prefer to mate with males that are less genetically similar to them. Females may also choose sperm after copulation to ensure a compatible mate. In support of this theory, a 2007 study found that extra-pair offspring were more heterozygous than within-pair offspring.<ref name="StapletonKleven2007">{{cite journal|last1=Stapleton|first1=Mary K.|last2=Kleven|first2=Oddmund|last3=Lifjeld|first3=Jan T.|last4=Robertson|first4=Raleigh J.|title=Female tree swallows (''Tachycineta bicolor'') increase offspring heterozygosity through extrapair mating|journal=Behavioral Ecology and Sociobiology|volume=61|issue=11|year=2007|pages=1725–1733|issn=0340-5443|doi=10.1007/s00265-007-0404-4|bibcode=2007BEcoS..61.1725S |s2cid=22513713}}</ref> However, a 2005 paper discovered a slight negative correlation between a pair's genetic similarity and the proportion of extra-pair young in their nest.<ref name="BarberEdwards2005">{{cite journal|last1=Barber|first1=Colleen A.|last2=Edwards|first2=Mandy J.|last3=Robertson|first3=Raleigh J.|title=A test of the genetic compatibility hypothesis with tree swallows, ''Tachycineta bicolor''|journal=Canadian Journal of Zoology|volume=83|issue=7|year=2005|pages=955–961|issn=0008-4301|doi=10.1139/z05-091|bibcode=2005CaJZ...83..955B }}</ref> The good genes theory says that females choose extra-pair males based on the quality of their genes. This would explain why some tree swallows do not have any extra-pair young, whereas others do.<ref name="LifjeldDunn1993"/> However, most studies have not found [[phenotypic]] differences between extra-pair and within-pair males (although a 2007 study did find that older males with brighter plumage were more likely to mate outside of the pair bond).<ref name="BittonO'Brien2007">{{cite journal|last1=Bitton|first1=Pierre-Paul|last2=O'Brien|first2=Erin L.|last3=Dawson|first3=Russell D.|title=Plumage brightness and age predict extrapair fertilization success of male tree swallows, ''Tachycineta bicolor''|journal=Animal Behaviour|volume=74|issue=6|year=2007|pages=1777–1784|issn=0003-3472|doi=10.1016/j.anbehav.2007.03.018|s2cid=53170228}}</ref> Additionally, according to a 2017 thesis, extra-pair offspring are no more likely to join their natal population than within-pair offspring. Another theory suggests that extra-pair paternity is context dependent, with extra-pair young outperforming within-pair young in certain situations, and underperforming in other environments. A 2017 dissertation, for example, found that extra-pair young were larger, heavier, and longer-winged than within-pair young when both were exposed to predator mounts, while within-pair young were heavier than extra-pair young when they were shown non-predator mounts. This thesis also found that within-pair young outperformed extra-pair young in terms of life-time fitness when they were raised in less-variable environments, suggesting that extra-pair offspring have less developmental plasticity than within-pair offspring.<ref name="Hallinger2017">{{cite thesis|last=Hallinger|first=Kelly Kristen|title=Context-dependent fitness consequences of extra-pair paternity in tree swallows (''Tachycineta bicolor'')|year=2017|type=PhD|publisher=[[Cornell University]]}}</ref> A 2018 study weakly supported this context dependent hypothesis, finding that extra-pair offspring were more likely to fledge than within-pair offspring in experimentally enlarged broods; however, neither [[telomere]] length (a correlate of survival and reproductive success) nor size 12 days after hatching were significantly different among these young, and no significant differences between the two types were found in non-enlarged broods.<ref name="BelmakerHallinger2018">{{cite journal|last1=Belmaker|first1=Amos|last2=Hallinger|first2=Kelly K.|last3=Glynn|first3=Rebecca A.|last4=Haussmann|first4=Mark F.|last5=Winkler|first5=David W.|title=Is there a context-dependent advantage of extra-pair mating in tree swallows?|journal=The Auk|volume=135|issue=4|year=2018|pages=998–1008|issn=0004-8038|doi=10.1642/AUK-18-3.1|hdl=10150/631782|s2cid=91253157|hdl-access=free}}</ref>

Studies attempting to prove the adaptability of extra-pair paternity for females have been criticized for the lack of positive effect that increased offspring fitness would have when compared with the potential cost of decreased fitness for the female,<ref name="ArnqvistKirkpatrick2005">{{cite journal|last1=Arnqvist|first1=Göran|last2=Kirkpatrick|first2=Mark|title=The evolution of infidelity in socially monogamous passerines: The strength of direct and indirect selection on extrapair copulation behavior in females|journal=The American Naturalist|volume=165|issue=S5|year=2005|pages=S26–S37|issn=0003-0147|doi=10.1086/429350|pmid=15795859|bibcode=2005ANat..165S..26A |s2cid=32810854}}</ref> such as increased predation from searching for mates.<ref name="DunnWhittingham2006">{{cite journal|last1=Dunn|first1=Peter O.|last2=Whittingham|first2=Linda A.|title=Search costs influence the spatial distribution, but not the level, of extra-pair mating in tree swallows|journal=Behavioral Ecology and Sociobiology|volume=61|issue=3|year=2006|pages=449–454|issn=0340-5443|doi=10.1007/s00265-006-0272-3|s2cid=4975256}}</ref> Thus, theories based on the non-adaptivity of extra-pair paternity for females have been postulated. These theories are based on [[Biological constraints|genetic constraint]], where an allele resulting in a maladaptive behavior is maintained because it also contributes to a beneficial phenotype. The theory of intersexual [[Antagonistic pleiotropy hypothesis|antagonistic pleiotropy]] says that strong selection for extra-pair paternity in males (as seen in this bird<ref name="ArnqvistKirkpatrick2005"/>) overrides the [[weak selection]] against extra-pair paternity in females. The hypothesis of intrasexual antagonistic pleiotropy, meanwhile, argues that extra-pair paternity is present because the genes regulating it have pleiotropic effects on aspects of female fitness,<ref name="ForstmeierNakagawa2014">{{cite journal|last1=Forstmeier|first1=Wolfgang|last2=Nakagawa|first2=Shinichi|last3=Griffith|first3=Simon C.|last4=Kempenaers|first4=Bart|title=Female extra-pair mating: adaptation or genetic constraint?|journal=Trends in Ecology & Evolution|volume=29|issue=8|year=2014|pages=456–464|issn=0169-5347|doi=10.1016/j.tree.2014.05.005|pmid=24909948|bibcode=2014TEcoE..29..456F }}</ref> like within-pair copulation rate.<ref name="ArnqvistKirkpatrick2005"/>

[[File:Tachycineta bicolor MWNH 1895.JPG|thumb|right|A tree swallow egg]]
The tree swallow lays a clutch of two to eight, although usually four to seven,<ref name="hbw"/> pure white, and translucent at laying, eggs that measure about {{convert|19|by|14|mm|in|abbr=on}}.<ref name="Hauber2014">{{cite book|last=Hauber|first=Mark E.|title=The Book of Eggs: A Life-Size Guide to the Eggs of Six Hundred of the World's Bird Species|url=https://books.google.com/books?id=evQvBAAAQBAJ|date=1 August 2014|publisher=University of Chicago Press|location=Chicago|isbn=978-0-226-05781-1|page=448}}</ref> These eggs are incubated by the female,<ref name="hbw"/> usually after the second-to-last egg is laid,<ref name="ClotfelterWhittingham2000"/> for 11 to 20 days,<ref name="Hauber2014"/> although most hatch after 14 to 15 days. About 88% of nests produce at least one nestling, but this can be lowered by poor weather and a younger breeding female.<ref name="hbw"/> The eggs generally hatch in the order they were laid. They also hatch slightly asynchronously, with an average of 28 hours between when the first and final nestling emerges. This can result in a weight hierarchy where earlier-hatched chicks weigh more (especially early in the nestling period) than those hatched later, allowing the female to prioritize which chick to give food to during food shortages. This likely has its greatest effect early in the nestling period, as by 12 days after hatching, there are generally no significant weight differences.<ref name="ClotfelterWhittingham2000">{{cite journal|last1=Clotfelter|first1=Ethan D.|last2=Whittingham|first2=Linda A.|last3=Dunn|first3=Peter O.|title=Laying order, hatching asynchrony and nestling body mass in tree swallows ''Tachycineta bicolor''|journal=Journal of Avian Biology|volume=31|issue=3|year=2000|pages=329–334|issn=0908-8857|doi=10.1034/j.1600-048X.2000.310308.x}}</ref> Infanticide of the chicks and eggs sometimes occurs when a male is replaced by another male. Infanticide usually does not occur when the clutch is not complete, as replacement males then have a chance to fertilize at least one egg. When the male arrives during incubation, it sometimes commits infanticide, but other times adopts the eggs, as there is a chance that some eggs were sired from the replacement male. If the replacement male arrives after the chicks hatch, infanticide is usually committed, though the female will sometimes prevent this.<ref name="Robertson1990">{{Cite book|last1=Robertson|first1=Raleigh J.|year=1990|pages=381–390|doi=10.1007/978-3-642-75110-3_32|title=Population Biology of Passerine Birds: An Integrated Approach|series=NATO ASI Series|publisher=Springer |location=Berlin, Heidelberg |article=Tactics and Counter-Tactics of Sexually Selected Infanticide in Tree Swallows|isbn=978-3-540-51759-7}}</ref>

Nests produced by females of better condition often have sex ratios skewed towards high quality males. A 2000 study hypothesized this to be because males have more variable reproductive success, and therefore that a high quality male produces more offspring than a female of similar quality.<ref name="WhittinghamDunn2000">{{cite journal|last1=Whittingham|first1=Linda A.|last2=Dunn|first2=Peter O.|title=Offspring sex ratios in tree swallows: females in better condition produce more sons|journal=Molecular Ecology|volume=9|issue=8|year=2000|pages=1123–1129|issn=0962-1083|doi=10.1046/j.1365-294x.2000.00980.x|pmid=10964231|bibcode=2000MolEc...9.1123W |s2cid=3720108}}</ref>

The growth and survival of nestling tree swallows is influenced by their environment. In both younger and older nestlings (those between two and four days old and between nine and eleven days, respectively) growth is positively influenced by a higher maximum temperature, particularly in the former. A later hatching date negatively impacts growth, especially for younger nestlings. Older chicks grow somewhat faster when insects are abundant. Growth in younger nestlings increases with age, while in old nestlings, it decreases as they get older.<ref name="McCartyWinkler2008">{{cite journal|last1=McCarty|first1=John P.|last2=Winkler|first2=David W.|title=Relative importance of environmental variables in determining the growth of nestling tree swallows ''Tachycineta bicolor''|journal=Ibis|volume=141|issue=2|year=2008|pages=286–296|issn=0019-1019|doi=10.1111/j.1474-919X.1999.tb07551.x}}</ref> Young tree swallows are able to thermoregulate at least 75% as effectively as the adult at an average age of 9.5 days when out of the nest, and from four to eight days old when in the nest (depending on the size of the brood).<ref name="Dunn1979">{{Cite journal| issn = 0043-5643| volume = 91| issue = 3| pages = 455–457| last = Dunn| first = Erica H.| title = Age of effective homeothermy in nestling tree swallows according to brood size| journal = The Wilson Bulletin| year = 1979}}</ref> The nestlings fledge after about 18 to 22 days, with about 80% fledging success. Like hatching success, this is negatively affected by unfavourable weather and a younger female.<ref name="hbw"/> Chicks may be preyed on by snakes<ref name="De Steven1980">{{cite journal|last1=De Steven|first1=Diane|title=Clutch size, breeding success, and parental survival in the tree swallow (''Iridoprocne bicolor'')|journal=Evolution|volume=34|issue=2|year=1980|pages=278–291|issn=0014-3820|doi=10.1111/j.1558-5646.1980.tb04816.x|pmid=28563429|hdl=2027.42/137562|s2cid=19454000|hdl-access=free}}</ref> and raccoons.<ref name="RobertsonRendell1990">{{cite journal|last1=Robertson|first1=Raleigh J.|last2=Rendell|first2=Wallace B.|title=A comparison of the breeding ecology of a secondary cavity nesting bird, the tree swallow (''Tachycineta bicolor''), in nest boxes and natural cavities|journal=Canadian Journal of Zoology|volume=68|issue=5|year=1990|pages=1046–1052|issn=0008-4301|doi=10.1139/z90-152|bibcode=1990CaJZ...68.1046R }}</ref> This predation can be exacerbated by begging calls.<ref name="LeechLeonard1997">{{cite journal|last1=Leech|first1=Susan M.|last2=Leonard|first2=Marty L.|title=Begging and the risk of predation in nestling birds|journal=Behavioral Ecology|volume=8|issue=6|year=1997|pages=644–646|issn=1045-2249|doi=10.1093/beheco/8.6.644|doi-access=free}}</ref>