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== Behaviour == [[File:Common Loon (Gavia Immer).webm|thumb|Swimming]] The common loon is an expert fisher, catching its prey underwater by diving as deep as {{convert|60|m|abbr=on}}.<ref name="Wisconsin Natural Resources">{{cite web|title=The Uncommon Loon|url=http://dnr.wi.gov/wnrmag/2013/02/loon.htm|publisher=Wisconsin Department of Natural Resources|access-date=2 June 2013}}</ref> With its large webbed feet, the common loon is an efficient underwater pursuit predator and adroit diver. It needs a long run-up distance to gain momentum for flight take-off and is ungainly on land, sliding on its belly and pushing itself forward with its legs. Its clumsiness on land is due to the legs being positioned at the rear of its body; the pelvic muscles are well developed,<ref>{{Cite journal|last=Wilcox|first=Harry Hammond|date=1952|title=The pelvic musculature of the loon, ''Gavia immer'' | jstor=2422198 | journal=The American Midland Naturalist|volume=48|issue=3|pages=513–573|doi=10.2307/2422198}}</ref> ideal for swimming but not well-suited for walking. When it lands on water, it skims along on its belly to slow down, rather than braking with its feet, as they are set too far back. The common loon swims and dives well, and flies competently for hundreds of kilometres in migration. It flies with its neck outstretched, usually calling a particular [[tremolo]] that can be used to identify a flying loon. Its flying speed is as much as {{convert|120|km/h|abbr=on}} during migration.<ref name="Wisconsin Natural Resources" /> Particularly during the breeding season, common loons frequently engage in territorial disputes against other water birds, including ducks and geese, and will attack or drive off competitors and intruders to their territory.<ref>{{Cite journal|last1=Kirkham|first1=Ian R.|last2=Johnson|first2=Stephen R.|date=1988|title=Interspecific Aggression in Loons (Agresión Interespecífica en Somormujos (Gavia spp.))|url=https://www.jstor.org/stable/4513284|journal=Journal of Field Ornithology|volume=59|issue=1|pages=3–6|jstor=4513284 |issn=0273-8570}}</ref><ref>{{Cite journal|last=Sperry|first=Mark L.|date=25 November 1986|title=Common Loon Attacks on Waterfowl|journal=Minnesota Department of Natural Resources Wetland Wildlife Populations and Research Group|pages=5}}</ref> === Feeding === [[File:093 - COMMON LOON (12-17-11) moss landing, ca (2) (8720312789).jpg|thumb|Foraging]] Fish account for about 80% of the diet of the common loon. It forages on fish of up to {{convert|26|cm|abbr=on}} in length, including [[minnow]]s, [[Catostomidae|suckers]], [[Dorosoma|gizzard shad]], [[rock bass]], [[alewife (fish)|alewife]], [[northern pike]], [[Whitefish (fisheries term)|whitefish]], [[sauger]], [[brown bullhead]], [[pumpkinseed]], [[burbot]], [[walleye]], [[bluegill]], [[white crappie]], [[black crappie]], [[rainbow smelt]], and [[killifish]].<ref name=":8">{{Cite book|title=Birds of Ontario: Habitat Requirements, Limiting Factors, and Status: Volume 1–Nonpasserines: Loons through Cranes |last=Sandilands|first=Al|date=2011|publisher=University of British Columbia Press|isbn=978-0-7748-5943-1|location=Vancouver |page=171}}</ref> The young typically eat small minnows, and sometimes [[insect]]s and fragments of green vegetation.<ref name= MNFI/> The freshwater diet primarily consists of [[pike (fish)|pike]], [[perch]], [[Centrarchidae|sunfish]], [[trout]], and [[bass (fish)|bass]]; the saltwater diet primarily consists of [[Sebastes|rock fish]], [[flounder]], [[Brown trout|sea trout]], [[herring]], [[Sciaenidae|Atlantic croaker]], [[haddock]], and [[Atheriniformes|Gulf silverside]]. When there is either a lack of fish or they are difficult to catch, it preys on [[crustacean]]s, [[crayfish]], [[snail]]s, [[leech]]es, [[Larvae|insect larvae]], [[Mollusca|molluscs]], [[frog]]s, [[annelid]]s, and occasionally [[aquatic plant]] matter such as [[Potamogeton|pondweed]], roots, moss, [[Salix|willow shoots]], seeds, and [[algae]].<ref name="hbw2" /><ref name=":2" /><ref name=":9" /> It has also been known to eat ducklings.<ref name=":9" /> The common loon uses its powerful [[Hindlimb|hind legs]] to propel its body underwater at high speed to catch its prey, which it then swallows head-first. If the fish attempts to evade the common loon, the bird chases it down with excellent underwater manoeuvrability due to its strong legs.<ref name=":2" /> Most prey are swallowed underwater, where they are caught, but some larger prey are first brought to the surface. It is a visual predator, so it is essential to hunting success that the water is clear.<ref name= MNFI/> It normally dives {{Convert|4|to|10|m|abbr = on}}, but has been recorded to dive up to {{Convert|70|m|abbr = on}}.<ref name="hbw2" /> The average diving time is 42 seconds,<ref name="Wisconsin Natural Resources" /> but the maximum duration spent underwater is about {{Convert|1|minute||abbr = on}}.<ref name="hbw2"/> ===Breeding=== [[File:Gavia immer -Maine, USA -nest-8a (1).jpg|thumb|On a waterside nest in [[Maine]]]] [[File:Eggs of British Birds Seebohm 1896 Plate21 The Common Loon.jpg|thumb|Egg]] [[File:Milwaukee Public Museum March 2023 66 (Wisconsin Birds--Wisconsin Lake Country, Common Loon).jpg|thumb|Taxidermied common loon at the [[Milwaukee Public Museum]]]] The common loon's mating system is serially [[Monogamy in animals|monogamous]]; breeding pairs jointly defend a territory consisting of an entire small lake or a protected bay within a large lake.<ref name=":11">{{Cite journal|last1=Piper|first1=Walter H.|last2=Brunk|first2=Kristin M.|last3=Flory|first3=Joel A.|last4=Meyer|first4=Michael W.|year=2017|title=The long shadow of senescence: age impacts survival and territory defense in loons|journal=Journal of Avian Biology|volume=48|issue=8|pages=1062–1070|doi=10.1111/jav.01393|url=https://digitalcommons.chapman.edu/cgi/viewcontent.cgi?article=1140&context=sees_articles|url-access=subscription}}</ref> A given male and female remain together throughout a breeding attempt, rear their own biological offspring,<ref>{{Cite journal|last1=Piper|first1=Walter H.|last2=Evers|first2=David C.|last3=Meyer|first3=Michael W.|last4=Tischler|first4=Keren B.|last5=Kaplan|first5=Joseph D.|last6=Fleischer|first6=Robert C.|date=1997|title=Genetic monogamy in the common loon (''Gavia immer'')|journal=Behavioral Ecology and Sociobiology|volume=41|issue=1|pages=25–31|doi=10.1007/s002650050360|bibcode=1997BEcoS..41...25P |s2cid=33036050}}</ref> reunite each spring, and may breed together for many consecutive years. However, in the event of death or territorial eviction of one pair member by an intruding loon of the same sex, the other pair member quickly establishes a pair bond with the evicting bird.<ref>{{Cite journal|last1=Piper|first1=Walter H.|last2=Tischler|first2=Keren B.|last3=Klich|first3=Margaret|year=2000|title=Territory acquisition in loons: the importance of take-over|journal=Animal Behaviour|volume=59|issue=2|pages=385–394|doi=10.1006/anbe.1999.1295|pmid=10675261|s2cid=23085958}}</ref> Hence, most adult loons have two or more different mates during their lives. Evicting individuals tend to be young males and females (5 to 9 years old), while evicted adults are often those 15 years and older.<ref>{{Cite journal|last1=Piper|first1=Walter H.|last2=Mager|first2=John N.|last3=Walcott|first3=Charles|last4=Furey|first4=Lyla|last5=Banfield|first5=Nathan|last6=Reinke|first6=Andrew|last7=Spilker|first7=Frank|last8=Flory|first8=Joel A.|date=2015|title=Territory settlement in common loons: no footholds but age and assessment are important|journal=Animal Behaviour|volume=104|pages=155–163|doi=10.1016/j.anbehav.2015.03.013|s2cid=53404673}}</ref><ref name=piper2018>{{Cite journal|last1=Piper|first1=Walter H.|last2=Brunk|first2=Kristin M.|last3=Jukkala|first3=Gabriella L.|last4=Andrews|first4=Eric A.|last5=Yund|first5=Seth R.|last6=Gould|first6=Nelson G.|year=2018|title=Aging male loons make a terminal investment in territory defense|journal=Behavioral Ecology and Sociobiology|volume=72|issue=6|page=95 |doi=10.1007/s00265-018-2511-9|bibcode=2018BEcoS..72...95P |s2cid=46956208}}</ref> Pairs do not remain together during winter;<ref name="hbw2" /><ref name="Evers2010" /> in addition, males usually precede females by a few days to a few weeks during spring migration, settling on their lake once a portion of it becomes ice-free.<ref>{{Cite book|title=The Common Loon: Spirit of Northern Lakes|last=McIntyre|first=Judith|publisher=University of Minnesota Press|year=1988|isbn=978-0-8166-1651-0|location=Minneapolis, MN|page=[https://archive.org/details/commonloon00judi/page/9 9]|url=https://archive.org/details/commonloon00judi/page/9}}</ref> Copulation takes place ashore, often on the nest site, repeated daily until the eggs are laid. The preceding courtship is very simple, with mutual bill-dipping and dives.<ref>{{Cite journal|last1=Sjölander|first1=Sverre|last2=Ågren|first2=Greta|date=1972|title=Reproductive behavior of the common loon|journal= Wilson Bulletin|volume=84|issue=3|pages=296–308|jstor=4160227| url=https://sora.unm.edu/sites/default/files/journals/wilson/v084n03/p0296-p0308.pdf }}</ref> The displays towards intruders, such as bow-jumping (an alternation of fencing and bill-dipping postures{{sfn|Johnsgard|1987|p=107}} and rushing (running "along the surface with its wings either folded or half-extended and flapping at about the same speed as when taking off"<ref>{{Cite journal | last1=Sjölander |first1=S.|last2=Ågren |first2=G.|date=1976|title=Reproductive behavior of the yellow-billed loon, ''Gavia adamsii''|url=https://sora.unm.edu/sites/default/files/journals/condor/v078n04/p0454-p0463.pdf|journal=The Condor| volume=78 | issue=4 | pages=454–463|doi=10.2307/1367094|jstor=1367094}}</ref>) are often misinterpreted as courtship.<ref name=":9">{{Cite book|title=The Eastman Guide to Birds: Natural History Accounts for 150 North American Species |last1=Eastman| first1=John|date=2000|publisher=Stackpole Books|isbn=978-0-8117-4552-9|location=Mechanicsburg, PA|page=219}}</ref> Nesting typically begins in early May.{{sfn|Cramp|1977|p=61}} Significantly more nesting sites are found on islands than on mainland shoreline.<ref>{{Cite journal|last1=Radomski|first1=Paul J. |last2=Carlson|first2=Kristin|last3=Woizeschke|first3=Kevin|year=2014|title=Common loon (''Gavia immer'') nesting habitat models for north-central Minnesota lakes | journal=Waterbirds |volume=37|issue=sp1|pages=102–117|doi=10.1675/063.037.sp113|doi-access=free}}</ref> Breeding pairs patrol their territories routinely, even at night,<ref>{{cite journal|author=Paruk, James D.|year=2008|title=Nocturnal behaviour of the common loon, Gavia immer|journal=Canadian Field-Naturalist|volume=122|pages=70–72|doi=10.22621/cfn.v122i1.548|doi-access=free}}</ref> defending the territory both physically and vocally.<ref>{{Cite web|url=http://animaldiversity.org/accounts/Gavia_immer/|title=''Gavia immer'' (common loon)|last=Rodriguez|first=R.|date=2002|website=Animal Diversity Web|archive-url=https://web.archive.org/web/20170714185921/http://animaldiversity.org/accounts/Gavia_immer/|archive-date=14 July 2017|access-date=14 July 2017}}</ref> Pairs that nested together the preceding year typically reuse the nest site from the previous year, if they hatched chicks successfully there. In contrast, pairs that lost their eggs to a predator usually shift the nest to a new location.<ref name=":12">{{Cite journal|last1=Piper|first1=Walter H.|last2=Walcott|first2=Charles|last3=Mager|first3=John N.|last4=Spilker|first4=Frank J.|year=2008|title=Nestsite selection by male loons leads to sex-biased site familiarity|journal=Journal of Animal Ecology|volume=77|issue=2|pages=205–210|doi=10.1111/j.1365-2656.2007.01334.x|pmid=17976165|doi-access=free|bibcode=2008JAnEc..77..205P }}</ref> This logical behaviour pattern appears to depend upon the male, because breeding pairs consisting of last year's male and a female not present during the preceding year continue to exhibit the behaviour; pairs composed of last year's female and a new male tend to select a new nest site, regardless of the success or failure of the previous year's attempt.<ref name=":12" /> Despite the lead role of males in nest site selection, both sexes contribute substantially to [[nest]] construction.<ref name=":2" /> The nest is about {{Convert|56|cm|abbr=on}} wide and is constructed out of dead [[Hymenachne|marsh grasses]] and other indigenous plants, and formed into a mound along the vegetated coasts of lakes greater than {{convert|3.7|ha|abbr=on}}.<ref name=":72" /><ref name=":2" /> After a week of construction in late spring, one parent climbs on top to mould the interior of the nest to the shape of its body.<ref name=":2" /> Based on a number of studies, nesting success averages about 40%, and most newly hatched young survive due to parental care.<ref name=":13">{{Cite book|title=Birds of Lake, Pond, and Marsh: Water and Wetland Birds of Eastern North America|last=Eastman|first=John Andrew| year=1999 |location=Mechanicsburg, PA | publisher=Stackpole Books |isbn=978-0-8117-2681-8|page=216}}</ref> Eggs from first clutches are typically laid in May or early June, the timing depending largely upon the date that lakes become ice-free and inhabitable.<ref name=":2" /> A clutch consists of two (occasionally one) olive-brown oval [[egg]]s with dark brown spots.<ref name=":72" /> Incubation is carried out jointly by male and female and lasts about 28 days.<ref name=Evers2010/><ref name="MNFI" /> Loons often place nests along steep lake shorelines where adults can quickly dive underwater when approached by predators.<ref name=":1">{{Cite journal|last=McIntyre|first=Judith W.|date=1983|title=Nurseries: a consideration of habitat requirements during the early chick-rearing period in common loons|jstor=4512827|journal=Journal of Field Ornithology|volume=54|issue=3| pages=247–253}}</ref> The eggs are about {{Convert|88|mm|abbr = on}} long and {{Convert|55|mm|abbr = on}} wide<ref name=":2" /> and the two eggs are laid with an interval of one to three days between them,<ref name=":8" /> and hatch [[wikt:asynchronous|asynchronously]].<ref name=":13" /> Newly hatched chicks are dark chocolate brown in colour and have a white belly. Within hours of hatching, the young begin to leave the nest with the parents, swimming close by and sometimes riding on one parent's back.<ref name=":2" /> Parents and chicks initially stay in shallow, isolated bays where the parents are able to defend the chicks better from intruding loons and eagles, which are their main predators.<ref name=":1" /><ref name="Jukkala 2015 551–558">{{Cite journal|last1=Jukkala|first1=Gabriella|last2=Piper|first2=Walter|year=2015|title=Common loon parents defend chicks according to both value and vulnerability|journal=Journal of Avian Biology|volume=46|issue=6|pages=551–558|doi=10.1111/jav.00648}}</ref> Male parents defend broods consisting of two chicks more vigorously than singleton chicks, chiefly with the territorial yodel call.<ref name="Jukkala 2015 551–558"/> The chicks are capable of making shallow dives from their first day<ref name=":7">{{Cite journal|last=Barr|first=J.F.|year=1996|title=Aspects of common loon (''Gavia immer'') feeding biology on its breeding ground| journal=Hydrobiologia |volume=321|issue=2|pages=119–144|doi=10.1007/bf00023169|s2cid=32804759}}</ref> but make deeper dives as they grow.<ref name="Evers2010">{{cite journal|url=https://birdsna.org/Species-Account/bna/species/comloo|title=Common Loon (''Gavia immer'') | last1=Evers|first1=D.C. | last2=Paruk | first2=J.D. | year=2021 |editor-last=Poole |editor-first=A.F. |journal=The Birds of North America | publisher=Cornell Lab of Ornithology |url-access=subscription | last3=McIntyre | first3=J.W. | last4=Barr | first4=J.F. |doi=10.2173/bow.comloo.02 }}</ref> [[Fledge|Fledging]] takes 70 to 77 days.<ref name="hbw2" /> Usually, only one brood is raised.<ref name=":2" /> Both parents feed the chicks live prey from hatching to fledging. As they grow, chicks are able to catch an increasingly large proportion of their diet by themselves; they can feed and fend for themselves after about two months, although many juveniles continue to beg from adults well beyond this age. The parent birds capture small fish and hold them crosswise in their bill, call and approach the chicks with their head lowered so that the chicks can grasp them.<ref name=":7" /> If food is scarce, the larger chick may peck its small sibling incessantly; on small lakes with limited food, only one chick often survives.<ref name=":14">{{Cite journal|last1=Piper|first1=Walter H.|last2=Grear|first2=Jason S.|last3=Meyer|first3=Michael W.|year=2012|title=Juvenile survival in common loons ''Gavia immer'': effects of natal lake size and pH |journal=Journal of Avian Biology |volume=43|issue=3|pages=280–288|doi=10.1111/j.1600-048x.2012.05633.x}}</ref> Juveniles leave the breeding ground before ice formation in the fall, weeks after their parents.<ref name=":72" /> A pair of loons raising two chicks have been estimated to feed on {{convert|423|kg}} of fish during the five and a half months that they spend in their breeding territory.<ref name=":7" /> === Habitat selection === Loons exhibit a strong tendency to settle as breeders on a lake that resembles their natal one, a phenomenon termed natal habitat imprinting. This preference is based on two lake attributes: size and pH.<ref name=":15">{{Cite journal|last1=Piper|first1=Walter H.|last2=Palmer|first2=Michael W.|last3=Banfield|first3=Nathan|last4=Meyer|first4=Michael W.|date=2013|title=Can settlement in natal-like habitat explain maladaptive habitat selection? |journal=Proceedings of the Royal Society of London B: Biological Sciences|volume=280|issue=1765|pages=20130979|doi=10.1098/rspb.2013.0979 |pmid=23804619|pmc=3712445}}</ref> The behaviour is puzzling, because it is as strong in loons hatched on small, acidic lakes as those from large lakes of neutral pH. Hence, the former group is exhibiting active preference for lakes that have been shown to result in higher chick mortality and lower breeding success.<ref name=":14" /><ref name=":15" /> ===Vocalizations=== The common loon produces a variety of vocalizations, the most common of which are the tremolo, the yodel, the wail, and the hoot. Each of these calls communicates a distinct message. The frequency at which it vocalizes has been shown to vary based on time of day, weather, and season. It is most vocally active between mid-May and mid-June. The wail, yodel, and tremolo calls are sounded more frequently at night than during the day; calls have also been shown to occur more frequently in cold temperatures and when there is little to no rain.<ref name="Mennill2014" /> {{Listen|filename=Common loon tremolo.ogg|pos=right|title=Common loon tremolo call}} The tremolo call—sometimes called the "laughing" call—is characterized by its short, wavering quality. It often uses this call to signal distress or alarm caused by territorial disputes or perceived threats.<ref name="Mennill2014">{{cite journal|last1=Mennill|first1=Daniel J.|date=2014|title=Variation in the vocal behavior of common loons (''Gavia immer''): Insights from landscape-level recordings|journal=Waterbirds|volume=37|issue=sp1|pages=26–36|doi=10.1675/063.037.sp105 | doi-access=free }}</ref> It emits a tremulous series of up to 10 rather high notes ''(hu)-heheheheheheha''.<ref name="hbw2"/> It also uses the tremolo to communicate its presence to other loons when they arrive at a lake, often when they are flying overhead. It is the only vocalization used in flight.<ref name="vtfishandwildlife">{{cite web|url=http://www.vtfishandwildlife.com/get_the_lead_out_looncall.cfm|title=Loon Vocalizations: What are you hearing and what does it mean?|publisher=Vermont Fish and Wildlife|archive-url=https://web.archive.org/web/20140913011626/http://www.vtfishandwildlife.com/get_the_lead_out_looncall.cfm|archive-date=13 September 2014|access-date=27 August 2017}}</ref> The tremolo call has varying three levels of intensities that correlate with a loon's level of distress, and the types are differentiated by increasingly higher pitch frequencies added to the call.<ref name="Barklow">{{cite journal|last1=Barklow|first1=William E.|year=1979|title=Graded frequency variations of the tremolo call of the common loon (''Gavia immer'')|journal=The Condor|volume=81|issue=1|pages=53–64| url=https://sora.unm.edu/sites/default/files/journals/condor/v081n01/p0053-p0064.pdf | doi=10.2307/1367857|jstor=1367857}}</ref> {{Listen|filename=Loon yodel.ogg|pos=right|title=Male Common loon yodel call}} The yodel is a long and complex call made only by the male. It is used in the establishment of territorial boundaries and in territorial confrontations, and the length of the call corresponds with the loon's level of aggression.<ref name="Mager2012">{{cite journal|last1=Mager III|first1=John N|last2=Walcott|first2=Charles|last3=Piper|first3=Walter H|date=2012|title=Male common loons signal greater aggressive motivation by lengthening territorial yodels|url=https://www.researchgate.net/publication/257920946 |journal=Wilson Journal of Ornithology|volume=124|issue=1|pages=73–80|doi=10.1676/11-024.1|s2cid=85216811}}</ref> The dominant frequencies in the yodel indicate the body mass and thereby the health of males.<ref>{{Cite journal|last1=Mager|first1=John N.|last2=Walcott|first2=Charles|last3=Piper|first3=Walter H.|title=Male common loons, ''Gavia immer'', communicate body mass and condition through dominant frequencies of territorial yodels |journal=Animal Behaviour|volume=73|issue=4|pages=683–690|doi=10.1016/j.anbehav.2006.10.009|citeseerx=10.1.1.483.1889|year=2007|s2cid=43610342}}</ref> A male that occupies a new territory appears to alter its yodel to be clearly distinguishable from the call of the previous territory owner.<ref>{{Cite journal|last1=Walcott|first1=Charles|last2=Mager|first2=John N.|last3=Piper|first3=Walter|date=2006|title=Changing territories, changing tunes: male loons, ''Gavia immer'', change their vocalizations when they change territories|url=https://www.fws.gov/uploadedFiles/Walcott2005.pdf|journal=Animal Behaviour|volume=71|issue=3|pages=673(11)|doi=10.1016/j.anbehav.2005.07.011|s2cid=43750841}}</ref> A loon's wail is a long call consisting of up to three notes, and is often compared to a [[wolf]]'s [[Howling|howl]]. It uses this call to communicate its location to other loons. The call is given back and forth between breeding pairs or an adult and its chick, either to maintain contact or in an attempt to move closer together after being separated.<ref name="vtfishandwildlife" /> It is a loud ''aaoo'', ''weee-wea weee-wea weee-wea'', or ''ooo-aaah-éééé''.<ref name="hbw2"/> The hoot is a short, soft call and is another form of contact call. It is a more intimate call than the wail and is used exclusively between small family groups or flocks.<ref name="Mennill2014" /> The common loon hoots to let other family or flock members know where it is. This call is often heard when the adult loon is summoning its chicks to feed.<ref name="vtfishandwildlife" />
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