Research

Corvids of Hastings

Food Storers with complex social lives

There are four members of the bird family Corvidae (jays, crows, magpies, and nutcrackers) that are regularly found at Hastings. Western scrub-jays (Aphelocoma californica), American crows (Corvus brachyrhynchos), yellow-billed magpies (Pica nutalli), and Steller’s jays (Cyanocitta cristata).

Why Study Corvids

Western scrub-jays interest scientists because they are pair-breeders while many of their closely-related species, such as the Florida scrub-jay (A. coerulescens), the unicolored jay(A. unicolor) and the Mexican jay (Aphelocoma ultramarina) are cooperative breeders, meaning that offspring from previous years help raise the young at the nest. Because of this difference, comparative studies are important to investigate the evolutionary and ecological context of pair-breeding and cooperative breeding.

Like many corvids, western scrub-jays are scatter-hoarders — they store small quantities of food in caches throughout their home range. Even though they are opportunistic generalists that feed on a broad diet of arthropods (insects, spiders, etc.), small vertebrates, fruits and nuts, scrub-jays at Hastings mainly scatter-hoard acorns. When the acorns hoarded in the ground are forgotten, they have a chance to germinate and establish as a seedling. As a result, scatter-hoarding corvids become important seed dispersers for large-seeded trees such as the oaks found at Hastings.

Yellow-billed magpies are one of only two bird species endemic to California (that is, found only within the state boundaries). They breed in loose colonies, a relatively uncommon phenomenon that is of considerable ecological interest. Like the scrub-jays, the magpies are closely associated with oak habitat and have been studied extensively at Hastings.

The research

Starting in the late 1960s, several UC Berkeley graduate students investigated the biology and behavior of corvids at Hastings Reservation. Initially the work investigated the behavior of yellow-billed magpies, with an emphasis on foraging. Nicolaas A.M. Verbeek, who went on to pursue a productive career in ornithology, followed magpies, and to a limited degree western scrub-jays, from 1968 – 1970 (Verbeek, 1970). His detailed behavioral observations still inspire current research — he is widely credited with discovering magpie “funerals” a corvid behavior that resembles the mourning of dead companions.

From 1981 to 1985, William Carmen conducted intensive research on the breeding and foraging behavior of western scrub-jays. Carmen and his field-assistants banded nearly 800 individual jays and found 350 nests. He found that the jays are indeed exclusive pair-breeders that exclude their young from their territories once the next breeding season begins, differentiating western scrub-jays from their cooperatively breeding relatives.

Western scrub-jays can cache around 5,000 acorns per year.

Carmen’s work also highlights the strong dependence of western scrub-jays on acorn production of local oaks. Following an acorn crop failure in 1983 more than a third of territorial jays abandoned breeding territories. In good acorn years jays spent almost 70% of their time foraging on acorns throughout the fall and were able to reduce overall foraging time. Based on his observations, Carmen also calculated that individual jays cache around 5,000 acorns per year.

In the same time period, Mark Reynolds, now with The Nature Conservancy, studied the spacing behavior of breeding yellow-billed magpies. To test the predictions of a theoretical model for the formation of breeding colonies, Reynolds (1990) determined whether magpie colonies are found at central locations relative to food sources and whether home ranges of individual birds overlapped significantly without defense of foraging areas.

In the late 1980s, long-time resident researcher at Hastings – Walt Koenig, began conducting studies on the role of acorns in the diet of western scrub-jays. For example, in Koenig & Heck (1988) feeding trials showed that the jays could not survive on an acorn-only diet. He later analyzed how oak-dependent birds respond to sudden oak death throughout California (Monahan & Koenig 2006). Most recently, he has investigated how responsive scrub-jay population numbers are to the oak diversity and variability in seed production (Koenig et al. 2009).

From 1994 – 1997, Ginger Bolen and her field assistants banded over 250 yellow-billed magpies at 45 nests to collect DNA samples to investigate the occurrence of extra-pair paternity — the biological father is not part of the social pair — an occurrence often found to be a cost of colonial breeding. Despite behavioral observations that suggested extra-pair interactions, Bolen (1999) found no DNA evidence for extra-pair paternity. The birds did reject mimed cuckoo eggs, suggesting that egg-rejection is still present in the species despite the absence of nest parasitism.

As of 2014, Mario Pesendorfer has been investigating the role of acorn crop variation in driving acorn hoarding by western scrub-jays and acorn woodpeckers. Using behavioral data from over 400 hours of observations, Pesendorfer & Koenig (2016) found that the jays attend trees with large acorn crops in larger numbers, and thus provide higher quality seed dispersal than for trees with small acorn crops.

References

Bolen, G.M., 1999. Extra-pair behavior in Yellow-billed Magpies (Pica nuttalli). Ph.D. Thesis. University of California, Berkeley

Carmen, W.J., 1988. Behavioral ecology of the California scrub jay (Aphelocoma coerulescens Californica): a non-cooperative breeder with close cooperative relatives. Ph.D. Thesis. University of California Berkeley

Koenig, W.D. & Heck, M.K., 1988. Ability of two species of oak woodland birds to subsist on acorns.  Condor 90: 705-708.

Koenig, W.D., Krakauer, A.H., Monahan, W.B., Haydock, J., Knops, J.M. & Carmen, W.J., 2009. Mast‐producing trees and the geographical ecology of western scrub‐jays. Ecography 32:561-570.

Monahan, W. B., & Koenig, W. D. 2006. Estimating the potential effects of sudden oak death on oak-dependent birds. Biological Conservation 127: 146-157

Pesendorfer M.B., Koenig, W.D., 2016. The effect of within-year variation in acorn crop size on seed predation and dispersal by avian hoarders. Oecologia 181: 97-106

Reynolds, M.D., 1990. The Ecology of Spacing Behavior in the Yellow-billed Magpie (Pica Nuttalli). Ph.D. Thesis. University of California, Berkeley

Verbeek, N.A.M., 1970. The exploitation system of the Yellow-billed Magpie. Ph.D. Thesis. University of California Berkeley

Recently Published

Pesendorfer MB, Sillett TS, Koenig WD, Morrison SA. Scatter-hoarding corvids as seed dispersers for oaks and pines: a review of a widely distributed mutualism and its utility to habitat restoration. The Condor: Ornithological Applications 118: 215-237. [pdf]  

Pesendorfer MB, Koenig, WD  The effect of within-year variation in acorn crop size on seed predation and dispersal by avian hoarders. Oecologia