Editing Barn swallow (section)

== Behaviour ==
=== Habitat and range ===

The preferred habitat of the barn swallow is open country with low vegetation, such as pasture, meadows and farmland, preferably with nearby water. This swallow avoids heavily wooded or precipitous areas and densely built-up locations. The presence of accessible open structures such as barns, stables, or culverts to provide nesting sites, and exposed locations such as wires, roof ridges or bare branches for perching, are also important in the bird's selection of its breeding range.<ref name="BWP" />

Barn swallows are semi-colonial, settling in groups from a single pair to a few dozen pairs, particularly in larger wooden structures housing animals. The same individuals often breed at the same site year after year, although settlement choices have been experimentally shown to be predicted by nest availability rather than any characteristics of available mates.<ref>{{Cite journal|last=Safran|first=Rebecca J.|date=2007-06-13|title=Settlement patterns of female barn swallows Hirundo rustica across different group sizes: access to colorful males or favored nests?|url=http://link.springer.com/10.1007/s00265-007-0366-6|journal=Behavioral Ecology and Sociobiology|language=en|volume=61|issue=9|pages=1359–1368|doi=10.1007/s00265-007-0366-6|bibcode=2007BEcoS..61.1359S |s2cid=24784564|issn=0340-5443|access-date=10 June 2021|archive-date=14 September 2022|archive-url=https://web.archive.org/web/20220914222544/https://link.springer.com/article/10.1007/s00265-007-0366-6|url-status=live|url-access=subscription}}</ref> Because it takes around 2 weeks for a pair to build a nest from mud, hair, and other materials, old nests are highly prized.<ref>{{Cite journal|last=Safran|first=Rebecca Jo|date=November 2006|title=Nest-site selection in the barn swallow, Hirundo rustica: What predicts seasonal reproductive success?|url=http://dx.doi.org/10.1139/z06-176|journal=Canadian Journal of Zoology|volume=84|issue=11|pages=1533–1539|doi=10.1139/z06-176|bibcode=2006CaJZ...84.1533S |issn=0008-4301|url-access=subscription}}</ref>

[[File:Barn swallow (Hirundo rustica rustica) juveniles.jpg|thumb|right|''H. r. rustica'' juveniles|alt=A group of juvenile swallow resting in a tree.]]
[[File:BarnSwallowSlowMo960.ogv|thumb|The movement of two swallows in slow motion|alt=See caption]]

This species breeds across the Northern Hemisphere from sea level to {{convert|2700|m|ft|abbr=on}},<ref name=BirdLife>{{cite web |title = BirdLife International Species factsheet: ''Hirundo rustica'' |publisher = BirdLife International |access-date = 6 December 2007 |url = http://www.birdlife.org/datazone/species/index.html?action=SpcHTMDetails.asp&sid=7116&m=1 |archive-date = 15 December 2008 |archive-url = https://web.archive.org/web/20081215081352/http://www.birdlife.org/datazone/species/index.html?action=SpcHTMDetails.asp&sid=7116&m=1 |url-status = live }}</ref> but to {{convert|3000|m|ft|abbr=on}} in the [[Caucasus]]<ref name=BWP/> and North America,<ref name= umich/> and it is absent only from deserts and the cold northernmost parts of the continents. Over much of its range, it avoids towns, and in Europe is replaced in urban areas by the [[Common house martin|house martin]]. However, in [[Honshū]], Japan, the barn swallow is a more urban bird, with the [[Eastern red-rumped swallow|red-rumped swallow]] (''Cecropis daurica'') replacing it as the rural species.<ref name= Turner/>

In winter, the barn swallow is cosmopolitan in its choice of habitat, avoiding only dense forests and deserts.<ref name=SASOL>{{cite book |title=SASOL Birds of Southern Africa |last= Sinclair |first=Ian |author2= Hockey, Phil|author3= Tarboton, Warwick |location=Cape Town |publisher=Struik |year=2002 |isbn= 978-1-86872-721-6}} p294</ref> It is most common in open, low vegetation habitats, such as [[savanna]] and ranch land, and in Venezuela, South Africa and [[Trinidad and Tobago]] it is described as being particularly attracted to burnt or harvested [[sugarcane]] fields and the waste from the cane.<ref name = Hilty/><ref name = mercy/><ref name = ffrench>{{cite book |last = ffrench<!--this name should not be capitalized--> |first = Richard |title = A Guide to the Birds of Trinidad and Tobago |edition = 2nd |year = 1991 |location=Ithaca, New York |publisher = Comstock Publishing |isbn = 978-0-8014-9792-6}} p315–6</ref> In the absence of suitable roost sites, they may sometimes roost on wires where they are more exposed to predators.<ref>{{cite journal|author=George, PV|year=1965|title=Swallows ''Hirundo rustica'' Linnaeus roosting on wires|journal=Journal of the Bombay Natural History Society|volume=62|issue=1|page=160|url=https://biodiversitylibrary.org/page/47953533|access-date=23 December 2017|archive-date=26 July 2020|archive-url=https://web.archive.org/web/20200726145627/https://www.biodiversitylibrary.org/page/47953533|url-status=live}}</ref> Individual birds tend to return to the same wintering locality each year<ref name= burton/> and congregate from a large area to [[communal roosting|roost]] in reed beds.<ref name=mercy/> These roosts can be extremely large; one in Nigeria had an estimated 1.5 million birds.<ref name=Bijlsma/> These roosts are thought to be a protection from predators, and the arrival of roosting birds is synchronised in order to overwhelm predators like [[African hobby|African hobbies]]. The barn swallow has been recorded as breeding in the more temperate parts of its winter range, such as the mountains of Thailand and in central Argentina.<ref name= Turner/><ref name= Lekagul>{{cite book |last = Lekagul |first = Boonsong |author2=Round, Philip |title = A Guide to the Birds of Thailand |year = 1991 |location=Bangkok |publisher = Saha Karn Baet |isbn = 978-974-85673-6-5}} p234</ref>

Migration of barn swallows between Britain and South Africa was first established on 23 December 1912 when a bird that had been ringed by James Masefield at a nest in Staffordshire, was found in Natal.<ref name=MigrationAtlas>{{cite book |title=The Migration Atlas: Movements of the Birds of Britain and Ireland |publisher=[[T & AD Poyser]] | year=2002 |isbn= 978-0-7136-6514-7 |editor=Wernham, Chris |page = 462 }}</ref> As would be expected for a long-distance migrant, this bird has occurred as a vagrant to such distant areas as Hawaii, [[Bermuda]], Greenland, [[Tristan da Cunha]], the [[Falkland Islands]],<ref name= Turner/> and even Antarctica.<ref name="Korczak-Abshire">{{cite journal |last1=Korczak-Abshire |first1=Małgorzata |last2=Lees |first2=Alexander |last3=Jojczyk |first3=Agata |date=2001 |title=First documented record of barn swallow (''Hirundo rustica'') in the Antarctic |url=https://www.researchgate.net/publication/365362066 |journal=Polish Polar Research |volume=32 |issue=4 |pages=355–360 |doi=10.2478/v10183-011-0021-9 |doi-access=free}}</ref>

=== Feeding ===
[[File:Swallow August 2013-2.jpg|thumb|Chicks in the nest|alt=Three chicks in a nest with their beaks open.]]

The barn swallow is similar in its habits to other aerial [[insectivore]]s, including other swallow species and the unrelated [[Swift (bird)|swift]]s. It is not a particularly fast flier, with a speed estimated at {{convert|11|m/s|km/h|abbr=on}}, up to {{convert|20|m/s|km/h|abbr=on}} and a wing beat rate of approximately 5, up to 7–9 times each second.<ref name= flight>{{cite journal|last= Liechti|first= Felix|author2= Bruderer, Lukas|date= 15 August 2002|url= http://jeb.biologists.org/cgi/reprint/205/16/2461|title= Wingbeat frequency of barn swallows and house martins: a comparison between free flight and wind tunnel experiments|journal= The Journal of Experimental Biology|volume= 205|issue= 16|pages= 2461–2467|doi= 10.1242/jeb.205.16.2461|pmid= 12124369|access-date= 21 May 2009|archive-date= 15 August 2009|archive-url= https://web.archive.org/web/20090815022433/http://jeb.biologists.org/cgi/reprint/205/16/2461|url-status= live|url-access= subscription}}</ref><ref name= Park>{{cite journal |last=Park |first=Kirsty |author2=Rosén, Mikael|author3= Hedenström, Anders |year= 2001|title=Kinematics of the barn swallow (''Hirundo rustica'') over a wide range of speeds in a wind tunnel |journal=The Journal of Experimental Biology |volume=204 |issue=15 |pages=2741–2750 |doi=10.1242/jeb.204.15.2741 |url=http://jeb.biologists.org/cgi/content/full/204/15/2741|pmid=11533124 |hdl=1893/306 |issn=0022-0949| archive-url= https://web.archive.org/web/20071109084723/http://jeb.biologists.org/cgi/content/full/204/15/2741| archive-date= 9 November 2007 | url-status= live|hdl-access=free }}</ref>

The barn swallow typically feeds in open areas<ref>{{cite book|author=Angela Turner|title=The Barn Swallow|url=https://books.google.com/books?id=bpfGYOqRDW0C&pg=PA41|date=29 January 2010|publisher=Bloomsbury Publishing|isbn=978-1-4081-2821-3|page=41|access-date=14 July 2017|archive-date=26 July 2020|archive-url=https://web.archive.org/web/20200726150505/https://books.google.com/books?id=bpfGYOqRDW0C&pg=PA41|url-status=live}}</ref> {{convert|7|–|8|m|ft|abbr=on}} above shallow water or the ground often following animals, humans or farm machinery to catch disturbed insects, but it will occasionally pick prey items from the water surface, walls and plants.<ref name=BWP/> Swallows have been observed feeding on insects that fly around active [[white stork]] nests as well.<ref>{{Cite journal |last1=Tryjanowski |first1=Piotr |last2=Jankowiak |first2=Łukasz |last3=Myczko |first3=Łukasz |last4=Mikula |first4=Peter |last5=Łuczak |first5=Andrzej |date=2024-01-25 |title=White stork Ciconia ciconia nests as an attractant to birds and bats |journal=Journal of Ornithology |volume=165 |issue=2 |pages=551–555 |language=en |doi=10.1007/s10336-023-02143-y |issn=2193-7192|doi-access=free |bibcode=2024JOrni.165..551T }}</ref> In the breeding areas, large [[fly|flies]] make up around 70% of the diet, with [[aphid]]s also a significant component. However, in Europe, the barn swallow consumes fewer aphids than the [[Common house martin|house]] or [[sand martin]]s.<ref name=BWP/> On the wintering grounds, [[Hymenoptera]], especially flying [[ant]]s, are important food items.<ref name=Turner/> [[Grasshopper]]s, [[Cricket (insect)|cricket]]s, [[dragonflies]], [[beetle]]s and [[moth]]s are also preyed upon.<ref>{{cite web | url=https://animaldiversity.org/accounts/Hirundo_rustica/ | title=Hirundo rustica (Barn swallow) | website=[[Animal Diversity Web]] }}</ref> When egg-laying, barn swallows hunt in pairs, but otherwise will form often [[Group size measures|large flocks]].<ref name=Turner/>

The amount of food a clutch will get depends on the size of the clutch, with larger clutches getting more food on average. The timing of a clutch also determines the food given; later broods get food that is smaller in size compared to earlier broods. This is because larger insects are too far away from the nest to be profitable in terms of energy expenditure.<ref name="Waugh1978">{{cite thesis|type=PhD|last=Waugh|first=David R.|date=1978|title=Predation strategies in aerial feeding birds|publisher=University of Stirling}}</ref>

[[Isotope]] studies have shown that wintering populations may utilise different feeding habitats, with British breeders feeding mostly over grassland, whereas Swiss birds utilised woodland more.<ref name= Evans>{{cite journal|last= Evans|first= K. L. |author2= Wadron, S.|author3= Bradbury, R. B. |year= 2003|title= Segregation in the African wintering grounds of English and Swiss Barn Swallows ''Hirundo rustica'': a stable isotope study |journal= Bird Study|volume= 50|issue= 3|pages= 294–299|doi= 10.1080/00063650309461322 |s2cid= 82263008 |doi-access= free}}</ref> Another study showed that a single population breeding in [[Denmark]] actually wintered in two separate areas.<ref name= Mollerhobson>{{cite journal|year= 2004|title= Heterogeneity in stable isotope profiles predicts coexistence of populations of barn swallows ''Hirundo rustica'' differing in morphology and reproductive performance |journal= Proceedings of the Royal Society of London. Series B: Biological Sciences|volume= 271|issue= 1546|pages= 1355–1362 | doi = 10.1098/rspb.2003.2565|pmid= 15306333|last1= Møller|first1= AP|last2= Hobson|first2= K A|issn= 0962-8452|pmc= 1691733}}</ref>

The barn swallow drinks by skimming low over lakes or rivers and scooping up water with its open mouth.<ref name = umich/> This bird bathes in a similar fashion, dipping into the water for an instant while in flight.<ref name= burton>{{cite book |last = Burton |first = Robert |title = Bird behaviour |year = 1985 |location = London |publisher = Granada |isbn = 978-0-246-12440-1 |url-access = registration |url = https://archive.org/details/birdbehaviour0000burt }}</ref>

Swallows gather in communal roosts after breeding, sometimes thousands strong. Reed beds are regularly favoured, with the birds swirling ''en masse'' before swooping low over the reeds.<ref name= Mullarney >{{cite book |last = Mullarney |first = Killian |author2= Svensson, Lars|author3= Zetterstrom, Dan|author4= Grant, Peter |title = Collins Bird Guide |year = 1999 |location=London |publisher = HarperCollins |isbn = 978-0-00-219728-1}} p242</ref> Reed beds are an important source of food prior to and whilst on migration; although the barn swallow is a diurnal migrant that can feed on the wing whilst it travels low over ground or water, the reed beds enable fat deposits to be established or replenished.<ref name=Euring>{{cite web|last=Pilastro |first=Andrea |title=The EURING Swallow Project in Italy |url=http://www.euring.org/about_euring/newsletter2/euring_swallows_italy.htm |date=December 1998 |work=Euring Newsletter, Volume 2 |access-date=1 December 2007 |archive-url=https://web.archive.org/web/20071203210458/http://www.euring.org/about_euring/newsletter2/euring_swallows_italy.htm |archive-date=3 December 2007 |url-status=dead }}</ref>

=== Song ===
Males sing to defend small territories (when living in colonies, less so in solitary pairs) and to attract mates. Males sing throughout the breeding season, from late April into August in many parts of the range. Their song is made up of a "twitter warble," followed by a rising "P-syllable" in European ''H. r. rustica'' and the North American ''H. r. erythrogaster.''<ref name="Wilkins2018">{{Cite journal|last1=Wilkins|first1=Matthew R|last2=Scordato|first2=Elizabeth S C|last3=Semenov|first3=Georgy A|last4=Karaardiç|first4=Hakan|last5=Shizuka|first5=Daizaburo|last6=Rubtsov|first6=Alexander|last7=Pap|first7=Peter L|last8=Shen|first8=Sheng-Feng|last9=Safran|first9=Rebecca J|date=2018-03-30|title=Global song divergence in barn swallows (Hirundo rustica): exploring the roles of genetic, geographical and climatic distance in sympatry and allopatry|url=https://academic.oup.com/biolinnean/article/123/4/825/4943524|journal=Biological Journal of the Linnean Society|language=en|volume=123|issue=4|pages=825–849|doi=10.1093/biolinnean/bly012|issn=0024-4066|access-date=10 June 2021|archive-date=10 June 2021|archive-url=https://web.archive.org/web/20210610023112/https://academic.oup.com/biolinnean/article/123/4/825/4943524|url-status=live|url-access=subscription}}</ref> In all subspecies, this is followed by a short "Q-syllable" and a trilled series of pulses, termed the "rattle."<ref name="Galeotti1996">{{Cite journal|last1=GALEOTTI|first1=PAOLO|last2=SAINO|first2=NICOLA|last3=SACCHI|first3=ROBERTO|last4=MØLLER|first4=ANDERS PAPE|date=April 1997|title=Song correlates with social context, testosterone and body condition in male barn swallows|url=http://dx.doi.org/10.1006/anbe.1996.0304|journal=Animal Behaviour|volume=53|issue=4|pages=687–700|doi=10.1006/anbe.1996.0304|s2cid=54358836|issn=0003-3472|access-date=10 June 2021|archive-date=14 September 2022|archive-url=https://web.archive.org/web/20220914222545/https://www.sciencedirect.com/science/article/abs/pii/S0003347296903048?via%3Dihub|url-status=live|url-access=subscription}}</ref> The rattle is sometimes followed by a terminal "Ω-Note" in some subspecies' populations, and always at the end of ''H. r. tytleri'' song.<ref name="Wilkins2018"/>

Female songs are much shorter than male songs, and are only produced during the early part of the breeding season.<ref name="Wilkins2020"/> Females sing spontaneously, though infrequently, and will also countersing in response to each other.<ref name="Wilkins2020"/>

=== Breeding ===

[[File:Hirundo rustica MHNT.jpg |Swallow eggs, hatched|thumb|alt=See caption]]

The male barn swallow returns to the breeding grounds before the females and selects a nest site, which is then advertised to females with a circling flight and song.<ref name=BWP/> Plumage may be used to advertise: in some populations, like in the subspecies ''H. r. gutturalis'', darker ventral plumage in males is associated with higher breeding success. In other populations,<ref name="LiuScordato2018">{{cite journal|last1=Liu|first1=Yu|last2=Scordato|first2=Elizabeth S. C.|last3=Safran|first3=Rebecca|last4=Evans|first4=Matthew|s2cid=19235964|title=Ventral colour, not tail streamer length, is associated with seasonal reproductive performance in a Chinese population of barn swallows (''Hirundo rustica gutturalis'')|journal=Journal of Ornithology|volume=159|issue=3|year=2018|pages=675–685|issn=2193-7192|doi=10.1007/s10336-018-1555-y|bibcode=2018JOrni.159..675L }}</ref> the breeding success of the male is related to the length of the tail streamers, with longer streamers being more attractive to the female.<ref name=BWP/><ref name = Saino>{{cite journal |last=Saino |first=Nicola|author2=Romano, Maria|author3=Sacchi|author4=Roberto|author5=Ninni, Paola|author6=Galeotti, Paolo|author7= Møller, Anders Pape |s2cid=35334066|date=September 2003 |title=Do male barn swallows (''Hirundo rustica'') experience a trade-off between the expression of multiple sexual signals? |journal=Behavioral Ecology and Sociobiology |volume=54 |issue=5 |pages=465–471 |doi=10.1007/s00265-003-0642-z|bibcode=2003BEcoS..54..465S }}</ref> Males with longer tail feathers are generally longer-lived and more disease resistant, females thus gaining an indirect fitness benefit from this form of selection, since longer tail feathers indicate a genetically stronger individual which will produce offspring with enhanced vitality.<ref name= Moller/> Males in northern Europe have longer tails than those further south, whereas in Spain the male's tail streamers are only 5% longer than the female's; in Finland, the difference is 20%. In Denmark, the average male tail length increased by 9% between 1984 and 2004, but it is possible that climatic changes may lead in the future to shorter tails if summers become hot and dry.<ref name= BB102/>

[[File:Hirundo rustica 14105.JPG|Older chicks in nest|alt=see caption|thumb|left]]

Males with long streamers also have larger white tail spots, and since feather-eating [[chewing louse|bird lice]] prefer white feathers, large white tail spots without [[Parasitism|parasite]] damage again demonstrate breeding quality; a positive association exists between spot size and the number of offspring produced each season.<ref name= Kose>{{cite journal |last=Kose |first=Mati |author2=Mänd, Raivo|author3=Møller, Anders Pape |s2cid=24583746 |date=December 1999 |title=Sexual selection for white tail spots in the barn swallow in relation to habitat choice by feather lice |journal=Animal Behaviour |volume=58 |issue=6 |pages=1201–1205 |doi=10.1006/anbe.1999.1249 |pmid=10600140 |issn=0003-3472}}</ref>

The breeding season of the barn swallow is variable: in the southern part of the range, the breeding season usually is from February or March to early to mid September, although some late second and third broods finish in October. In the northern part of the range, it usually starts late May to early June and ends the same time as the breeding season of the southernmost birds.<ref name="Turner2010">{{cite book|author=Angela Turner|title=The Barn Swallow|url=https://books.google.com/books?id=bpfGYOqRDW0C&pg=PA113|date=29 January 2010|publisher=Bloomsbury Publishing|isbn=978-1-4081-2821-3|pages=113–114|access-date=14 July 2017|archive-date=26 July 2020|archive-url=https://web.archive.org/web/20200726150441/https://books.google.com/books?id=bpfGYOqRDW0C&pg=PA113|url-status=live}}</ref>

Both sexes defend the nest, but the male is particularly aggressive and territorial.<ref name=Turner/> Once established, pairs stay together to breed for life, but extra-pair [[Mating|copulation]] is common, making this species [[gene]]tically [[Animal sexual behaviour#Polygamy|polygamous]], despite being socially [[monogamy in animals|monogamous]].<ref>
{{cite journal |last=Møller |first=Anders Pape |author2=Tegelstrom, Håkan |s2cid=33591800 |date=November 1997 |title=Extra-pair paternity and tail ornamentation in the barn swallow |journal=Behavioral Ecology and Sociobiology |volume=41 |issue=5 |pages=353–360 |doi=10.1007/s002650050395}}</ref> Males guard females actively to avoid being [[cuckold]]ed.<ref>
{{cite journal |last=Møller |first=Anders Pape |s2cid=24056440 |date=October 1985 |title=Mixed reproductive strategy and mate guarding in a semi-colonial passerine, the swallow ''Hirundo rustica'' |journal=Behavioral Ecology and Sociobiology |volume=17 |issue=4 |pages=401–408 |doi=10.1007/BF00293220|bibcode=1985BEcoS..17..401M }}</ref> Males may use deceptive alarm calls to disrupt extrapair copulation attempts toward their mates.<ref>{{cite journal |last=Møller |first=Anders Pape |year=1990 |title=Deceptive use of alarm calls by male swallows, ''Hirundo rustica'': a new paternity guard |journal=Behavioral Ecology |volume=1 |issue=1 |pages=1–6 |doi=10.1093/beheco/1.1.1}}</ref>

As its name implies, the barn swallow typically nests inside accessible buildings such as barns and stables, or under bridges and wharves.<ref name="Duffin"/> Before man-made sites became common, it nested on cliff faces or in caves, but this is now rare.<ref name=Turner/> The neat cup-shaped nest is placed on a beam or against a suitable vertical projection. It is constructed by both sexes, although more often by the female, with mud pellets collected in their beaks and lined with grasses, feathers, algae<ref name="Duffin">{{cite journal|last=Duffin|first=K.|year=1973|title=Barn Swallows use freshwater and marine algae in nest construction|journal=Wilson Bull.|volume=85|pages=237–238}}</ref> or other soft materials.<ref name=Turner/> The nest building ability of the male is also sexually selected; females will lay more eggs and at an earlier date with males who are better at nest construction, with the opposite being true with males that are not.<ref name="SolerCuervo1998">{{cite journal|last1=Soler|first1=Juan José|last2=Cuervo|first2=José Javier|last3=Møller|first3=Anders Pape|last4=De Lope|first4=Florentino|s2cid=12957069|title=Nest building is a sexually selected behaviour in the barn swallow|journal=Animal Behaviour|volume=56|issue=6|year=1998|pages=1435–1442|issn=0003-3472|doi=10.1006/anbe.1998.0938|pmid=9933540}}</ref> After building the nest, barn swallows may nest colonially where sufficient high-quality nest sites are available, and within a colony, each pair defends a territory around the nest which, for the European subspecies, is {{convert|4|to|8|m2|sqft|sigfig=1|abbr=on}} in size. [[Group size measures|Colony size]] tends to be larger in North America.<ref name= umich/>

[[File:Hirundo rustica -West Sussex, England -chick-8.jpg|right|thumb|A juvenile swallow on a red brick in Sussex]]

In North America at least, barn swallows frequently engage in a [[Mutualism (biology)|mutualist]] relationship with [[osprey]]s. Barn swallows will build their nest below an osprey nest, receiving protection from other birds of prey that are repelled by the exclusively fish-eating ospreys. The ospreys are alerted to the presence of these predators by the alarm calls of the swallows.<ref name= umich/>

Barn swallows will normally raise two broods, with the original nest being reused for the second brood and being repaired and reused in subsequent years. The female lays two to seven, but typically four or five, reddish-spotted white eggs.<ref name=Turner/> The clutch size is influenced by latitude, with clutch sizes of northern populations being higher on average than southern populations.<ref name="Moller1984">{{cite journal|last1=Moller|first1=Anders Pape|title=Geographical trends in breeding parameters of swallows ''Hirundo rustica'' and house martins ''Delichon urbica''|journal=Ornis Scandinavica|volume=15|issue=1|year=1984|pages=43–54|issn=0030-5693|doi=10.2307/3676002|jstor=3676002}}</ref> The eggs are {{convert|20|x|14|mm|in|frac=8|abbr=on}} in size, and weigh {{convert|1.9|g|gr|abbr=off}}, of which 5% is shell. In Europe, the female does almost all the [[avian incubation|incubation]], but in North America the male may incubate up to 25% of the time. The incubation period is normally 14–19 days, with another 18–23 days before the [[altricial]] chicks fledge. The fledged young stay with, and are fed by, the parents for about a week after leaving the nest. Occasionally, first-year birds from the first brood will assist in feeding the second brood.<ref name=Turner/> Compared to those from early broods, juvenile barn swallows from late broods have been found to migrate at a younger age, fuel less efficiently during migration and have lower return rates the following year.<ref>{{cite journal |author1= Raja-aho, S. |author2= Eeva, T. | author3= Suorsa, P. |author4= Lehikoinen, E. | year=2017| title= Juvenile Barn Swallows ''Hirundo rustica'' L. from late broods start autumn migration younger, fuel less effectively and show lower return rates than juveniles from early broods | journal=Ibis| volume=159| pages=892–901| doi=10.1111/ibi.12492 | issue=4| doi-access=free }}</ref>

[[File:Barn swallow (feeding) at Tennōji Park in Osaka, June 2016.jpg|Juvenile being fed|thumb|left|alt=A swallow is feeding an insect to another swallow]]

The barn swallow will [[mobbing behavior|mob]] intruders such as cats or [[accipiter]]s that venture too close to their nest, often flying very close to the threat.<ref name = Moller>
{{cite book |last=Møller |first=Anders Pape |year=1994 |title=Sexual Selection and the Barn Swallow |location=Oxford |publisher=Oxford University Press |isbn=978-0-19-854028-1| page=245}}</ref> Adult barn swallows have few predators, but some are taken by accipiters, [[falcon]]s, and [[owl]]s. [[Brood parasite|Brood parasitism]] by [[cowbird]]s in North America or [[cuckoo]]s in Eurasia is rare.<ref name=BWP/><ref name= umich/>

Hatching success is 90% and the fledging survival rate is 70–90%. Average mortality is 70–80% in the first year and 40–70% for the adult. Although the record age is more than 11 years, most survive less than four years.<ref name=Turner/> Barn swallow nestlings have prominent red gapes, a feature shown to induce feeding by parent birds. An experiment in manipulating brood size and immune system showed the vividness of the gape was positively correlated with T-cell–mediated immunocompetence, and that larger brood size and injection with an antigen led to a less vivid gape.<ref name="Saino03">{{cite journal|author=Saino, Nicola|author2=Ambrosini, Roberto|author3=Martinelli, Roberta|author4=Ninni, Paola|author5=Møller, Anders Pape |year=2003 |title=Gape coloration reliably reflects immunocompetence of barn swallow (''Hirundo rustica'') nestlings |journal=Behavioral Ecology |volume=14 |issue=1 |pages=16–22 |url=http://www.georgealozano.com/papers/Saino2K03.pdf|doi=10.1093/beheco/14.1.16 |url-status=dead |archive-url=https://web.archive.org/web/20110711084908/http://www.georgealozano.com/papers/Saino2K03.pdf |archive-date=11 July 2011|doi-access=free }}</ref>

The barn swallow has been recorded as [[hybrid (biology)|hybridising]] with the [[American cliff swallow|cliff swallow]] (''Petrochelidon pyrrhonota'') and the [[cave swallow]] (''P. fulva'') in North America, and the [[house martin]] (''Delichon urbicum'') in Eurasia, the cross with the latter being one of the most common passerine hybrids.<ref name = Moller/>