The song sparrow (Melospiza melodia) is a medium-sized American sparrow. Among the native sparrows in North America, it is easily one of the most abundant, variable and adaptable species.
Description
Adult song sparrows have brown upperparts with dark streaks on the back
and are white underneath with dark streaking and a dark brown spot in
the middle of the breast. They have a brown cap and a long brown rounded
tail. Their face is gray with a streak through the eye. They are highly
variable in size across numerous subspecies (for subspecies details,
see below). The body length ranges from 11 to 18 cm (4.3 to 7.1 in) and
wingspan can range from 18 to 25.4 cm (7.1 to 10.0 in).[2][3] Body mass ranges from 11.9 to 53 g (0.42 to 1.87 oz),.[4] The average of all races is 32 g (1.1 oz) but the widespread nominate subspecies (M. m. melodia) weighs only about 22 g (0.78 oz) on average. The maximum lifespan in the wild is 11.3 years.[5] The
eggs of the Song sparrow are brown with greenish-white spots. Females
lay three to five eggs per clutch, with an average incubation time of
13–15 days before hatching.
In the field, they are most easily confused with the Lincoln's sparrow and the Savannah sparrow.
The former can be recognized by its shorter, grayer tail and the
differently-patterned head, the brown cheeks forming a clear-cut angular
patch. The Savannah sparrow has a forked tail and yellowish flecks on
the face when seen up close.
Distribution and life history
Though a habitat generalist, the Song sparrow favors brushland and marshes, including salt marshes, across most of Canada and
the United States. They also thrive in human dominated areas such as in
suburbs, agricultural fields, and along roadsides. Permanent residents
of the southern half of their range, northern populations of the song
sparrow migrate to the southern United States or Mexico during winter and intermingle with the native, non-migratory population. The song sparrow is a very rare vagrant to western Europe, with a few recorded in Great Britain and Norway.
These birds forage on the ground, in shrubs or in very shallow water.
They mainly eat insects and seeds. Birds in salt marshes may also eat
small crustaceans. They nest either in a sheltered location on the ground or in trees or shrubs.
Physiology
The song sparrow has been the subject of several studies detailing the
physiological reactions of bird species to conditions such as daylight
length and differing climatic conditions. Most birds gain mass in their
reproductive organs in response to some signal, either internal or
external as the breeding season approaches. The exact source of this
signal varies from species to species - for some, it is an endogenous
process separate from environmental cues, while other species require
extensive external signals of changing daylight length and temperature
before beginning to increase the mass of their reproductive organs. Male
specimens of M. melodia gain significant testicular mass in
response both to changes in the daily photoperiod and as a result of
endogenous chemical signals.[6] Females also undergo significant ovarian growth in response to both photo-period and endogenous signals. In this way, M. melodia is
amongst only a handful of birds that use both external and engodenous
signalling to dictate their breeding season. Hormone levels in both
males and females were found fluctuate throughout the breeding season,
having very high levels in March and late April and then declining until
May.[7]These
studies suggest that there are multiple factors at work that influence
when and how the song sparrow breeds other than just increasing day
length.
Due to the myriad subspecies of the song sparrow and the extremely varied climate of southern California,
where many of these subspecies make their homes, physiological studies
were undertaken to determine how climatic conditions and local
environment influenced the bill size of M. melodia subspecies.
The bill of a bird is highly important for thermoregulation as the bare
surface area makes a perfect place to radiate excess heat or absorb
solar energy to maintain homeostasis.[8] Knowing
this, comparisons of bill length between individual song sparrows
collected in different habitats were made with regard to the primary
habitat type or microclimate that they were collected in. Larger beaked
subspecies were strongly correlated with hotter microclimates - a
correlation that follows from the conditions of Allen's Rule.[9]
Song
The sparrow species derives it name from its colorful repertoire of
songs. Enthusiasts report that one of the songs heard often in suburban
locations closely resembles the opening four notes of Ludwig van Beethoven's Symphony No. 5. The male uses a fairly complex song to declare ownership of its territory and attract females.
Singing itself consists of a combination of repeated notes, quickly
passing isolated notes, and trills. The songs are very crisp, clear, and
precise, making them easily distinguishable by human ears. A particular
song is determined not only by pitch and rhythm but also by the timbre
of the trills. Although one bird will know many songs—as many as 20
different tunes with as many as 1000 improvised variations on the basic
theme,[citation needed]—unlike thrushes, the song sparrow usually repeats the same song many times before switching to a different song.
Song sparrows typically learn their songs from a handful of other birds
that have neighboring territories. They are most likely to learn songs
that are shared in common between these neighbors. Ultimately, they will
choose a territory close to or replacing the birds that they have
learned from. This allows the song sparrows to address their neighbors
with songs shared in common with those neighbors. It has been
demonstrated that song sparrows are able to distinguish neighbors from
strangers on the basis of song, and also that females are able to
distinguish (and prefer) their mate's songs from those of other
neighboring birds, and they prefer songs of neighboring birds to those
of strangers.
Predators and parasites
Common predators of the song sparrow include cats, hawks, and owls, however snakes, dogs, and the American kestrel are
treated ambiguously, suggesting that they are less of a threat. The
song sparrow recognizes enemies by both instinctual and learned patterns
(including cultural learning),
and adjusts its future behavior based on both its own experiences in
encounters, and from watching other birds interact with the enemies.
Comparisons of experiments on hand-raised birds to observation of birds
in the wild suggest that the fear of owls and hawks is instinctual, but
fear of cats is learned.[10]
Song sparrows' nests are parasitized by the brown-headed cowbird.
The cowbirds' eggs closely resemble song sparrows' eggs, although the
cowbirds' eggs are slightly larger. Song sparrows recognize cowbirds as a
threat and attack the cowbirds when they are near the nest. There is
some evidence that this behavior is learned rather than instinctual.[10] A
more recent study found that the behavior of attacking female cowbirds
near nests may actually attract cowbird parasitism because the female
cowbirds use such behavior to identify female song sparrows that are
more likely to successfully raise a cowbird chick.[11] One
study found that while cowbird parasitism did result in more nest
failure, overall there were negligible effects on song sparrow
populations when cowbirds were introduced to an island. The study
pointed to a number of explanatory factors including song sparrows
raising multiple broods, and song sparrows' abilities to raise cowbird
chicks with their own.
No comments:
Post a Comment