Passerina caerulea

Geographic Range

Blue grosbeaks ( Passerina caerulea ) are migratory passerine birds. In early April, these birds migrate to the southern United States. Their northern border consists of southern California, Nevada and Utah, as well as Colorado, South Dakota, southern Missouri, Kentucky, parts of southern Illinois, Indiana, Ohio, Virginia and Massachusetts. Some blue grosbeaks have been seen as far north as Idaho. The extent of their southern border is not fully known, but they are frequently found throughout Texas. In rare occasions, some blue grosbeaks do not leave their breeding grounds in southern Texas until early winter. In mid-October, after the breeding season, blue grosbeaks migrate to their winter location in Mexico and the Caribbean Islands. In some parts of Central Mexico, they are permanent residents and do not migrate.

• Biogeographic Regions
• nearctic nearctic
  • native native
• neotropical neotropical
  • native native

Habitat

Generally, blue grosbeaks reside in open areas with limited shrub density. They prefer areas with low growth such as shrubs, brush and small trees. The edges of woodlands, particularly salt cedar and southern pine forests also serve as common habitats, as well as the occasional grassland. Other areas where blue grosbeaks are known to reside include open slashes after logging, stream edges and mesquite savannah. In any of their chosen habitats, blue grosbeaks select areas of diminished canopy density and limited tree growth. In their wintering grounds, blue grosbeaks tend to live in and on the edges of dry, tropical forests and in bushy shrubs.

• Habitat Regions
• temperate temperate
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• savanna or grassland savanna or grassland
• forest forest
• scrub forest scrub forest

Physical Description

Blue grosbeaks are large buntings; they are 15 to 16 cm long and weigh 26 to 31 g. As with many passerine species, males exhibit a much more vibrant coloration, in this case, bold purplish blue plumage, framed in brownish edges on their back. Males have black primary flight feathers and two brown wingbars on their middle and greater secondary coverts. Male blue grosbeaks have a silvery lower beak and a black upper beak. A strip of black extends from the males' upper mandible to their eye, and black lores extend from their lower mandible to their chin. Females are mostly brown, with occasional scattered blue feathers on their head and wings. Their wings are mostly black with two light brown wingbars. Their breast feathers tend to be a slightly lighter brown than the rest of their body. There is no significant size difference between male and female blue grosbeaks; however, this species is larger than other closely related species such as lazuli buntings and indigo buntings .

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male more colorful

Reproduction

The breeding season for blue grosbeaks extends from early April to late August. Males are believed to arrive on the breeding grounds before females. Blue grosbeaks are assumed to be monogamous, based on sightings of the same paired individuals multiple times per mating season. Little is known of their courtship displays, but males are known to guard their females by following closely as they feed.

• Mating System
• monogamous monogamous

Female blue grosbeaks build their nests about 1 to 4 m (3 to 10 feet) above the ground. Blue grosbeaks build predominantly cup nests, although there has been one documented case of cavity nesting by this species. The first nest is usually built in May, but some can be seen as early as mid-April. The second nest is typically built in August. No more than two nests with two broods have been observed. Approximately 3 to 5 eggs are usually laid per clutch. Eggs are pale blue to bluish-white with occasional brown spotting. Males feed the females during the 11 to 12 day incubation period. Fledging has been observed 9 to 10 days after hatching.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

In their monogamous breeding system, both parents invest in their young. After the chicks hatch, they are fed by the male and the female. When the female begins building her second nest, the male becomes the primary caretaker for the young.

• Parental Investment
• altricial altricial
• male parental care male parental care
• female parental care female parental care
• pre-fertilization
  • provisioning
  • protecting
    • male
• pre-hatching/birth
  • provisioning
    • male
    • female
  • protecting
    • male
• pre-weaning/fledging
  • provisioning
    • male
  • protecting
    • male

Lifespan/Longevity

There is little information available on the lifespan of blue grosbeaks. However, the longest documented lifespan for this species is 5 years and 11 months.

Behavior

In the spring, before females arrive, males flock and feed. During breeding season, the ultraviolet structural ornamentation in the feathers of males likely indicates his quality and factors into female selection. During the summer months, flocks of males and females feed together. While foraging, they may hop awkwardly on occasion, but are more commonly seen flying low over the ground. Mating pairs are often seen together, and males follow the females very closely. This species is very shy around humans, which makes observation very difficult. Tail flicking is a common behavior observed in both sexes. However, the purpose of this behavior is unknown. Blue grosbeaks have also been known to sidle along branches, as seen in parrots.

• Key Behaviors
• arboreal arboreal
• flies
• motile motile
• migratory migratory
• social social

Home Range

The home range size of blue grosbeaks has been estimated at 6,000 to 61,900 meters squared (0.6 to 6.19 hectares). Males body size and coloration is correlated to their range size. Larger males with more intense coloration tend to have larger territories.

Communication and Perception

Only male blue grosbeaks are known to sing. Their song is a long, rich warble lasting around 2.5 seconds. Notably, the song lacks burry qualities common in related species, such as house finches and indigo buntings . The beginning of the song does not vary greatly from male to male, but the song endings can be variable and can consist of approximately 11 to 19 unique elements. Older males tend to have longer songs. There is a strong correlation between increased song complexity and female fertility periods. Males tend to use more song variants with a wide variety of arrangements and elements during times of female fertility. This correlation supports theories that song complexity is a sexually selected trait. Their flight call is described as 'zit-zit-zu-zit-zit-zu zoo-zieet zieet zieet zi-zi-zi-zi-zi-zi-zi-zi'. Their general call is described as a low-pitched buzz or a metallic tink. During mating periods, females have been observed to erect their crown feathers. Both sexes exhibit frequent tail flicking and tail spreading, although the purpose of these movements is unknown. The blue coloring of male blue grosbeaks is maximally reflected in the blue-ultraviolet range. Most of the coloring present on these birds is invisible to the human eye. Male coloration may indicate the quality of the male and is strongly correlated to female mate choice.

• Communication Channels
• visual visual
• acoustic acoustic

• Perception Channels
• visual visual
• ultraviolet
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Blue grosbeaks gather the majority of their food through foraging in agricultural fields and pastures. In the fall, blue grosbeaks often gather in large flocks to feed in grain fields. These birds are opportunistic feeders, and their diet consists mostly of invertebrates including snails, beetles, moths, grasshoppers, spiders and worms. Blue grosbeaks also eat seeds, particularly grass and waste grain in abandoned agricultural fields. When feeding insects to their young, the adults remove the head, legs and wings.

• Primary Diet
• carnivore carnivore
  • insectivore insectivore

• Animal Foods
• insects
• terrestrial non-insect arthropods
• mollusks
• terrestrial worms

• Plant Foods
• seeds, grains, and nuts

Predation

There is no information on predators of this species, aside from general bird predators such as raptors as well as snakes and cats.

Ecosystem Roles

The nests of blue grosbeaks are heavily parasitized by brown-headed cowbirds . Feather mites from genus Proctophyllodes have also been found on 20% of selectively studied blue grosbeaks. These birds may assist in the spread of diseases specifically arboviral diseases between birds and mosquitoes. Their range is expanding northward where there are a growing numbers of new and reemerging diseases. This includes Eastern equine encephalitis, whose vertebrate hosts are often passerine birds, such as red-eyed vireos . The role of blue grosbeaks as a vector for this and other arboviral diseases has not been studied, but this is a topic of concern that needs attention.

• Ecosystem Impact
• disperses seeds

Economic Importance for Humans: Positive

There is little information about the interaction of this bird with humans. Because blue grosbeaks eat primarily insects, they could provide benefits for crops through insect control. However, there has been no official information to support this theory.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

Because blue grosbeaks are passerine birds, they could become vectors of arboviral and other diseases. However, there is no documentation of this species spreading diseases.

Conservation Status

The summer breeding range of blue grosbeaks is slowly expanding northward, and their population is stable and even increasing slightly. The conservation status of this bird is “Least Concern”, indicating there is no danger of extinction in the near future. Blue grosbeaks avoid suburban environments. As with many bird species, increasing development of their habitat could lead to population decreases. However, their habitats, most notably abandoned agricultural fields, are plentiful at this point.

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

Lauren Smith (author), Northern Michigan University, Alec Lindsay (editor), Northern Michigan University, Leila Siciliano Martina (editor), Texas State University.

References

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