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The tree swallow has an average lifespan of 2.7 years<ref name="VleckVleck2011">{{cite journal|last1=Vleck|first1=Carol M.|last2=Vleck|first2=David|last3=Palacios|first3=Maria G.|title=Evolutionary ecology of senescence: a case study using tree swallows, ''Tachycineta bicolor''|journal=Journal of Ornithology|volume=152|issue=S1|year=2011|pages=203–211|issn=0021-8375|doi=10.1007/s10336-010-0629-2}}</ref> and a maximum of 12 years. About 79% of individuals do not survive their first year, and those that do face an annual mortality rate of 40% to 60%.<ref name="hbw"/> Most deaths are likely the result of cold weather, which reduces insect availability, leading to starvation.<ref name="Turner"/> Lifespan is associated with telomere length: a 2005 study that used return rates (to the breeding site of the previous year) as a proxy for survival found that those with the longest telomeres at one year of age had a predicted lifespan of 3.5 years, compared to the 1.2 years for those with the shortest telomeres.<ref name="HaussmanWinkler2005">{{Cite journal| doi = 10.1098/rsbl.2005.0301| issn = 1744-9561| volume = 1| issue = 2| pages = 212–214| last1 = Haussmann| first1 = Mark F.| last2 = Winkler| first2 = David W.| last3 = Vleck| first3 = Carol M.| title = Longer telomeres associated with higher survival in birds| journal = Biology Letters| date = 2005-06-22| pmid = 17148169| pmc = 1626238}}</ref> Whether short telomeres cause a reduction in fitness or are simply an indicator of it is unknown. Regardless, a 2016 thesis found that measures of condition were positively correlated with telomere length. Males also generally had longer telomeres than females, as did smaller-winged birds. Individuals with shorter telomeres may compensate for potential losses in fitness by increasing reproductive effort, whereas those with longer telomeres may decrease their investment, as evidenced by the smaller proportion of chicks females with longer telomeres fledged.<ref name="Belmaker2016">{{cite thesis|last=Belmaker|first=Amos|year=2016|title=The Role of Telomere Length in Tree Swallow Behavior and Life History|chapter=Patterns of co-variation of telomere length, condition, life history and fitness in a short-lived bird species|publisher=Cornell University|type=PhD}}</ref> Telomere length is highly heritable, and is especially dependent on that of the mother.<ref name="BelmakerHallinger2019"> {{Cite journal| doi = 10.1002/ece3.5386| issn = 2045-7758| volume = 9| issue = 14| pages = 8175–8186| last1 = Belmaker| first1 = Amos| last2 = Hallinger| first2 = Kelly K.| last3 = Glynn| first3 = Rebbeca A.| last4 = Winkler| first4 = David W.| last5 = Haussmann| first5 = Mark F.| title = The environmental and genetic determinants of chick telomere length in tree swallows (''Tachycineta bicolor'')| journal = Ecology and Evolution| date = 2019}}</ref>
===Predation===
The tree swallow is susceptible to a wide range of predators. Eggs, nestlings, and adults in the nest fall victim to [[Pantherophis obsoletus|black rat snakes]], [[American crow]]s, [[American kestrel]]s, [[common grackle]]s, [[northern flicker]]s, [[chipmunk]]s, [[Peromyscus|deermice]], [[domestic cat]]s, [[weasel]]s,<ref name= Winkler>Winkler, D. W.; Hallinger, K. K.; Ardia, D. R.; Robertson, R. J.; Stutchbury, B. J.; Chohen, R. R. (2011). Poole, A. F., ed. "Tree Swallow (''Tachycineta bicolor'')". ''The Birds of North America''. [[Ithaca, New York]]: Cornell Lab of Ornithology.</ref> [[American black bear]]s,<ref name="ZachMayoh1984">{{cite journal|last1=Zach|first1=Reto|last2=Mayoh|first2=Keith R.|title=Gamma radiation effects on nestling tree swallows|journal=Ecology|volume=65|issue=5|year=1984|pages=1641–1647|issn=0012-9658|doi=10.2307/1939142|jstor=1939142}}</ref> and [[raccoon]]s,<ref>Chapman, L. B. (1955). "Studies of a tree swallow colony". ''Bird-Banding''. '''6''' (2): 45–70.</ref> While flying or perched, predators to the tree swallow include American kestrels, [[black-billed magpie]]s,<ref name= Winkler/> [[barred owl]]s,<ref>Errington, P. L. (1932). "Food habits of southern Wisconsin raptors. Part I. Owls". ''The Condor''. '''34''' (4): 176–186.</ref> [[great horned owl]]s, [[Merlin (bird)|merlins]], [[peregrine falcon]]s, and [[sharp-shinned hawk]]s. Whilst evasive flight is the usual response to predators in free-flying swallows, mobbing behavior is common around the nest,<ref name= Winkler/> and is directed not just towards predators, but also towards nest site competitors, who might be scared off by it.<ref name="Winkler1992"/> This behaviour involves the swallow swarming and diving towards (but not actually striking) the intruder<ref name= Winkler/> from around {{convert|5|to|20|m|ft}} above the ground, usually giving soft ticking calls near the end and coming within about {{convert|.5|to|2|m|ft}} of the predator.<ref name="Winkler1992"/> It seems to alter the intensity of its attacks based on which predator approaches;<ref name= Winkler/> a 1992 study found that ferrets elicited a more vigorous defence than black rat snakes,<ref name="Winkler1992"/> and a 2019 thesis similarly discovered that black rat snake models were dived at the least and [[eastern chipmunk]] models the most.<ref name="Maass2019">{{cite thesis|last=Maass|first=Natalia May|title=Perceived predation risk and the responses of adult and nestling tree swallows (''Tachycineta bicolor'')|year=2019|type=[[Master of Science|MS]]|publisher=[[Eastern Kentucky University]]}}</ref> It is suggested that the snake prompted a weaker response because defence behaviours may be less effective<ref name="Winkler1992"/> and more dangerous against it.<ref name="Maass2019"/>
===Parasites===
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