Meet "the most interesting bird on the island"

Island scrub jay
(Photo from Channel Islands Restoration site)

"...the most interesting bird on the island" (Howell and van Rossem, 1911)

The Island scrub jay (Aphelocoma insularis), found only on Santa Cruz Island, was first described by scientists in 1886. Its nearest relative is the Western scrub jay (Aphelocoma californica), which is found in the western United States and Mexico. The island species differs morphologically from the mainland species by being larger, brighter blue, and having a larger bill.



Evolution

Island (L) and Western scrub jay (R)
(photo from Sibley website)

Two independent analyses have confirmed that the Island scrub jay is a distinct species and very different from its mainland relative genetically as well as morphologically. Genetic differences, coupled with the observation that genetic diversity of Island scrub jays is significantly lower than Western scrub jays, suggest that the two groups haven't exchanged genes for a very long time.  By using a "molecular clock", biologists estimate that the Island scrub jay has been isolated from the Western scrub jay for at least 140,000 years.

140,000 years is a long time. The mainland is only 20 miles away and was as close as 5 miles within the last 18,000 years.  And these are birds who can fly.  So why the long separation?  As it turns out, scrub jays are homebodies. This is supported by the observation that vagrant Island scrub jays have not been reported from any other Channel Island nor from the mainland. Likewise, genetic studies suggest that no Island scrub jays made the short hop to the mainland during the last ice age either.

Ecology

The birds prefer oak woodlands and chaparral, though they are sometimes found in pine woodlands on the island. They are found in grasslands or eucalyptus groves, but they do not breed there.

Island scrub jay
(Photo from Smithsonian site)

Island scrub jays are among the top avian predators on Santa Cruz Island, eating anything they can catch -- snakes, insects, spiders, mice, lizards and other birds (including Island scrub jay eggs and nestlings). The also eat plant material, particularly acorns, which are collected and buried for later consumption. Because they don't recover all of their stashed food, they are probably important acorn dispersers.

These monogamous birds appear to form permanent pair bonds, although I have found no genetic analyses of paternity (so they could be sneaking around a bit). The pair defends a territory of about 4 acres year-round; they rarely stray outside of this area. Nesting commences in mid-March as arthropod populations rise following the leafing out of oak trees after winter rains.

(Photo from Smithsonian site)

Both sexes build the nest which consists of an outer layer of sticks surrounding an inner layer of grass, small twigs, and rootlets. Sticks for the nest are not picked up from the ground, but are broken off of trees and shrubs. The nests are placed 3-25 feet above the ground in trees or shrubs, usually oaks but sometimes lemonadeberry or toyon. Females lay 3-5 eggs which she incubates. Just before egg-laying and during egg-brooding, the male provides the female with food. After the young hatch, both sexes feed them for about 20-25 days.

Somewhere between 50 and 90% of the nests fail to fledge young in any given year. Most nests are lost to predators, of which Island foxes and other Island scrub jays appear to be the most important. (See the Smithsonian website for videos of nest predation.) The low survival rate of eggs and young is offset by high adult survival (estimated at 81% annually) and an average life expectancy of about 18 years.

Delayed Reproduction

An interesting aspect of Island scrub jay ecology is that young birds often cannot obtain territories and must wait for several years for a chance to reproduce. Breeding opportunities usually arise through the death of a current breeder. Thus, the Island scrub jays experience an ecological situation similar to another cousin, the Florida scrub jay (Aphelocoma coerulescens): Both species have limited habitat, few breeding opportunities, and  high nest predation rates. However, the Florida scrub jay is a cooperative breeder while the Island scrub jay is not.

Florida scrub jay
(Photo from Byways site)

The Florida scrub jay splintered from the Western scrub jay about a million years ago (really, 1,000,000 years) and is restricted to central Florida. Like other scrub jays, Florida scrub jays are dependent on oak woodlands, a habitat that is very hard to come by throughout their range. Because breeding opportunities are few and far between, young birds stay home and help their parents raise young, a system called cooperative breeding. Helpers might wait years for a breeding opportunity, usually by replacing a dead breeder in an existing territory (usually their home territory, mating with a step-parent), but sometimes by budding off a new territory from the family's existing territory, which requires help from their family.

Island scrub jays do not breed cooperatively, apparently because they are more flexible ecologically than Florida scrub jays. Both species nest in oak woodlands, but unlike Florida scrub jays, unmated Island scrub jays spend their "waiting time" living in flocks in marginal habitats such as grassland and coastal sage scrub.


Everything sounds hunky-dory in Island scrub jay land.  Or does it?  My next post will focus on conservation of the Island scrub jay!

References:

Smithsonian Institution website -- researchers at the Migratory Bird Center are studying the Island scrub jay. Scroll down the page to find videos of predation on Island scrub jay nests. Navigate around the site to find other information about scrub jays.

Atwood, J. L. 1978. Breeding biology of the Santa Cruz Island scrub jay. In: Power, D. M. (ed.). The California Channel Islands: Proceedings of a Multidisciplinary Symposium. Pp. 675 - 688. 

Atwood, J. L. 1980. Social interactions in the Santa Cruz Island scrub jay. Condor 82: 440-448.

Collins, C. T. and K. A. Corey. 2003. Territory acquisition by island scrub-jays: How to become a breeder.  In: Proceedings of the 6th California Islands Symposium. Pp. 257-262.

Delaney, K. S. and R. K. Wayne. 2005. Adaptive units for conservation: Population distinction and historic extinctions in the island scrub-jay. Conservation Biology 19: 523-533.

Delaney, K. S., S. Zafar, and R. K. Wayne. 2008. Genetic divergence and differentiation within the Western scrub-jay (Aphelocoma californica). The Auk 125: 839-849.

Howell, A. B. and A. J. van Rossem. 1911. Further notes from Santa Cruz Island. Condor 13: 208-210.

Photo credits:

http://www.sibleyguides.com/2010/04/the-next-10-north-american-bird-splits/
http://www.channelislandsrestoration.com/sci/IslandScrubJay.htm
http://nationalzoo.si.edu/scbi/migratorybirds/featured_birds/default.cfm?bird=Island_Scrub-Jay
http://library.byways.org/assets/57054