Aegolius
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''Aegolius'' is a genus of small true owls. The genus name is from
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
, ''aegolius'', is a type of screech owl that was thought to be a bird of ill omen.


Taxonomy

The genus ''Aegolius'' was introduced in 1829 by the German naturalist
Johann Jakob Kaup Johann Jakob von Kaup (10 April 1803 – 4 July 1873) was a German naturalist. A proponent of natural philosophy, he believed in an innate mathematical order in nature and he attempted biological classifications based on the Quinarian system. Kaup ...
with the boreal owl (''Aegolius funereus'') as the
type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
. The genus name is
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
for a
screech owl Screech owls are typical owls (Strigidae) belonging to the genus ''Megascops'' with 23 living species. For most of the 20th century, this genus was merged with the Old World scops owls in ''Otus'', but nowadays it is again considered separate ...
, the word came from the
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
''aigōlios'' meaning "a bird of ill omen". In
Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical co ...
,
Aegolius ''Aegolius'' is a genus of small true owls. The genus name is from Latin, ''aegolius'', is a type of screech owl that was thought to be a bird of ill omen. Taxonomy The genus ''Aegolius'' was introduced in 1829 by the German naturalist Johann J ...
was originally a man whom
Zeus Zeus or , , ; grc, Δῐός, ''Diós'', label=Genitive case, genitive Aeolic Greek, Boeotian Aeolic and Doric Greek#Laconian, Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label=Genitive case, genitive el, Δίας, ''D ...
transformed into an owl.


Species

The genus contains five species: * Boreal owl or Tengmalm's owl, ''Aegolius funereus'' * Northern saw-whet owl, ''Aegolius acadicus'' *†
Bermuda saw-whet owl The Bermuda saw-whet owl (''Aegolius gradyi'') was a species of owl that was endemic to Bermuda. It was described from fossil records and explorer accounts of the bird in the 17th century. The cause of its extinct Extinction is the terminat ...
, ''Aegolius gradyi'' (
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
) *
Unspotted saw-whet owl The unspotted saw-whet owl (''Aegolius ridgwayi'') is a small "typical owl" in subfamily Surniinae. It is found in Costa Rica, El Salvador, Guatemala, Mexico, and Panama.HBW and BirdLife International (2020) ''Handbook of the Birds of the Worl ...
, ''Aegolius ridgwayi'' *
Buff-fronted owl The buff-fronted owl (''Aegolius harrisii'') is a small owl. It is found in widely separated areas in every South American country except French Guiana and Suriname.Remsen, J. V., Jr., J. I. Areta, E. Bonaccorso, S. Claramunt, A. Jaramillo, D. ...
, ''Aegolius harrisii''


Description

''Aegolius'' owls are small, stout, short-tailed, and broad-winged, with large, round facial discs. The species varies from 18 to 27 cm in length. These owls take mainly
rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...
s and other small
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s as their prey, but also feed on
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s,
bat Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
s,
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s, and other invertebrates according to what is available. They are dark brown or black above, and whitish or buff below, marked with streaks or spots in the two northern forms. The head is large, with yellow eyes and a well-defined facial disc. The flight is strong, agile, and direct. They breed in tree holes, laying several eggs. They have repetitive whistling calls in the breeding season.


Distribution

Three of the extant species are restricted to the
New World The term ''New World'' is often used to mean the majority of Earth's Western Hemisphere, specifically the Americas."America." ''The Oxford Companion to the English Language'' (). McArthur, Tom, ed., 1992. New York: Oxford University Press, p. 3 ...
, but the boreal owl has a circumpolar range through
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
,
Eurasia Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago a ...
, the
Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...
, and the
Rockies The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of western Can ...
. Essentially nocturnal woodland owls of temperate or colder climates; the two northern species breed in North America's
conifer Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...
ous forests, and the unspotted saw-whet owl and buff-fronted owl (
Central Central is an adjective usually referring to being in the center of some place or (mathematical) object. Central may also refer to: Directions and generalised locations * Central Africa, a region in the centre of Africa continent, also known as ...
and
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
, respectively) are species of mountain, cloud, or oak forests. They are mainly resident birds, but the northern species at least sometimes move south or to lower altitudes in autumn. The movements are difficult to monitor due to the problems of detecting these nocturnal owls outside the breeding season, when they are not calling.


Mating system

''Aegolius'' owls have a very diverse mating system containing multiple strategies. Although there are five species within this genus, one of which is extinct, the majority of the information known about the mating system is based on research from the boreal owl (''A. funereus)'' and the northern saw-whet owl (''A. acadicus'') in North America.Johnsgard, P. A. (1988). North American owls: biology and natural history. Both species have been shown to exhibit both
monogamous Monogamy ( ) is a form of Dyad (sociology), dyadic Intimate relationship, relationship in which an individual has only one Significant other, partner during their lifetime. Alternately, only one partner at any one time (Monogamy#Serial monogamy, ...
and
polygamous Crimes Polygamy (from Late Greek (') "state of marriage to many spouses") is the practice of marriage, marrying multiple spouses. When a man is married to more than one wife at the same time, sociologists call this polygyny. When a woman is ...
lifestyles with a focus on bigyny. However, the boreal owl has been show to exhibit biandry as well.


Mate choice


Vocalizations

Mate choice Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choic ...
in this genus is largely related to auditory vocalizations (a form of sensory exploitation). A quiet species through most of the year, the northern saw-whet owl male produces a bell-like or “series song” advertisement call. Generally, it can be heard early morning or on bright nights from March – May (the breeding season) to attract females. The calls are believed to be relatively different between individuals, suggesting they may play a role in individual identification by the females. After pairing with a mate, vocalizations are very limited and appear to be related to territory or offspring defence. In contrast, the boreal owl produces a “staccato song” starting around dusk, and ending around midnight in the late winter and early spring. The call serves to attract females and becomes increasingly persistent as time without a mate passes. Some research suggests that unlike the Northern saw-whet owl, a variation of the male call persists after mating throughout the reproductive cycle for defense, aggression, or to convey information about prey. This suggests that vocalization may be used in this species beyond mate attraction and possibly as a contact call.


Nest and territory quality

Aside from vocalization, females in this genus may choose males based on nest or territory quality. For example, in boreal owls, the number and quality of nest holes or boxes defended by males is a factor in female reproductive effort.Carlsson, B. G. (1991). Recruitment of mates and deceptive behavior by male Tengmalm's owls. ''Behavioral Ecology and Sociobiology'', ''28''(5), 321-328. The number and quality of nests appear to serve a direct benefit to females (direct benefit hypothesis), as it provides protection from
predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
, as well as access to food resources, as good nest quality can be indicated by access to prey (i.e. small mammals). Female boreal owls’ reproductive success can decline in polyterritorial polygamous situations, as females are unable to discriminate previously paired and unpaired males (also known as the deception hypothesis). It is also important to note that increasing polygamy levels leads to decreased reproductive success in secondary females. Limited research has been conducted into mate choice of the Northern saw-whet owl in relation to territory quality. Some researchers suggest they follow a similar trend to boreal owls, where the quality of nests is a direct benefit to the female and therefore a determining factor.Marks, J. S., Nightingale, A., & McCullough, J. M. (2015). On the breeding biology of Northern Saw-whet Owls (''Aegolius acadicus''). ''Journal of Raptor Research'', ''49''(4), 486-497. Northern saw-whet owls' nest locations appear less linked to the availability of small mammals, as they have a greater variety of prey options and tend to be more nomadic after each breeding season.


Reversed size dimorphism

Lastly, size of males may play a role in mate choice in some species. The males in this genus provide direct benefit to females such as protection, territory, and resources. As a result, large male size (and smaller female size) would theoretically be more advantageous. However, in the boreal owl, this is not the case. Females tend to be larger and heavier than the male mates they choose, some suggesting this is the result of the starvation hypothesis (selection acts on females to increase ability to withstand long periods of time without food in polygamous lifestyles).Korpimäki, E. (1986). Reversed size dimorphism in birds of prey, especially in Tengmalm's Owl Aegolius funereus: a test of the" starvation hypothesis". ''Ornis Scandinavica'', 326-332. Another theory suggests that females purposely choose short-winged, light males, as they would be able to hunt at lower energetic costs and compete for territory more efficiently than larger, long-winged individuals.Hakkarainen, H., & Korpimäki, E. (1991). Reversed sexual size dimorphism in Tengmalm's Owl: Is small male size adaptive?. ''Oikos'', 337-346. This would provide the female with greater material benefits for her offspring. Reversed
size dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different Morphology (biology), morphological characteristics, particularly characteristics not directly involved in reproduction. The conditio ...
is also observed in northern saw-whet owls, but the differences are much more difficult to observe, as the species is smaller overall; the role of this in mate choice is unclear.


Contests

Relating to the observed size dimorphism in males, most species compete for territory in this genus, as it indirectly relates to better female mates. The intraspecific competition tends to be dependent on individual flight ability, individuals that can fly more efficiently and effectively tend to get better territory. However, interspecific competition also plays a role. Species in this genus are often smaller than other owls that have similar ranges; as a result, they often are outcompeted for territory by other species.Hakkarainen, H., & Korpimaki, E. (1996). Competitive and predatory interactions among raptors: an observational and experimental study. ''Ecology'', ''77''(4), 1134-1142. This is especially true in boreal owls, as their range overlaps with the much larger Ural ('' Strix uralensis)'' and Eurasian eagle owls (''
Bubo bubo The Eurasian eagle-owl (''Bubo bubo'') is a species of eagle-owl that resides in much of Eurasia. It is also called the Uhu and it is occasionally abbreviated to just the eagle-owl in Europe. It is one of the largest species of owl, and female ...
)''. As a result, when these species are present, the boreal owl often mates with less desirable subdominant females.


Parental Care

Parental care in the boreal and northern saw-whet owls is quite similar; females incubate and brood eggs, while males often provide protection, and resources such as food and nesting sites. This trend likely applies across the species in this genus, as some species may be conspecifics (i.e. northern saw-whet and unspotted saw-whet owls). In boreal owls, the level of investment provided by the male is directly related to the level of parental care effort in brooding by the female.Zárybnická, M., & Vojar, J. (2013). Effect of male provisioning on the parental behavior of female Boreal Owls Aegolius funereus. ''Zoological Studies'', ''52''(1), 36. Increased resources (i.e. food) provided by the male during the nesting stage increases female effort in brooding of the offspring. This is similar to the differential allocation hypothesis, which suggests that females increase their reproductive effort overall when offspring are the result of ideal mates. Brooding behavior is key to the success of owl fledglings, so when males provide adequate food, females provide adequate care. Fewer or lighter individual fledglings are produced as polygamy rate increases and the amount of male effort becomes increasingly important in predicting female brooding effort. Information on parental care effort is limited on the northern saw-whet owl, and likely related to male provisioning rates, the extent, however, is unclear. Current research examines the effect of habitat loss on the male provisioning rates.Hinam, H. L., & Clair, C. C. S. (2008). High levels of habitat loss and fragmentation limit reproductive success by reducing home range size and provisioning rates of Northern saw-whet owls. ''Biological Conservation'', ''141''(2), 524-535.


References

{{Taxonbar, from=Q310862 Bird genera Taxa named by Johann Jakob Kaup