Great Blue Heron Science

Eagles as Heron Predators

In the early 1990s when eagles were scarce, small heron colonies in British Columbia produced on average from zero to four offspring per nest, whereas large colonies consistently produced an average about two offspring per nest. Over a period of many years, the average number of offspring was the same across all colony sizes and this statistic probably explained why a range of colony sizes persisted.   

The subspecies of great blue heron Ardea herodias fannini, resides year round along southwestern British Columbia’s coastline. Thousands of bald eagles (Haliaeetus leucocephalus) eagles gather along streams and rivers in autumn and winter to eat post-spawning salmon carcasses, and hundreds remain through spring and summer to breed. The diet of breeding eagles in British Columbia is mostly scavenged fish, some birds and mammals.

Two decades ago, eagles rarely preyed on herons (Butler 1997, Harris et al. 2002). In the 1980s, biologists noted that the number of bald eagles nesting on the south coast of British Columbia had begun to increase, a trend that has continued up to today (Harris et al. 2002). At the same time, they began to witness eagles preying on young herons (Norman et al. 1989). By 1999, nearly 60% of all heron nesting attempts in southern British Columbia failed to raise young and the losses were attributable largely to eagle predation and disturbance (Vennesland 2000, Vennesland and Butler 2004). Herons are likely adjusting their behaviour in response to a recovery of eagles to former pre-DDT population levels.

The research of Dr. Butler and his colleagues at the Canadian Wildlife Service will be of interest to biologists in other parts of North America who will soon witness the recovery of eagles in their regions.  Along with Dr. Barry Smith and graduate student Jamie Kenyon, a theoretical model of how herons might nest in a changing predator landscape has been developed. Graduate student Iain Jones is exploring how eagles and herons interact near and far from nesting eagles. Their research examines how herons might respond to an increasing risk of falling victim to eagle attacks. Butler and Vennesland (2000) laid out how the interactions might unfold. Most herons nest in colonies. Some join a small colony but most settle in large colonies. Herons in small colonies might be able to hide their location from eagles but they will be quickly dispatched once discovered. Large and boisterous colonies are very noticeable and draw the attention of eagles. But the many nests reduce the chance that any individual nest will become a victim. An additional twist to the story arises from the territorial behaviour of eagles. If eagles defend against other eagles near their nests, then herons might want to nest close by to get the added protection, especially if the eagle does not prey on herons.

References

Butler, R.W., P. E. Whitehead, A. M. Breault and I. E. Moul. 1995. Colony effects on fledging success of Great Blue Herons (Ardea herodias) in British Columbia. Colonial Waterbirds 18: 159-165.

Butler, R.W. and R. G. Vennesland. 2000. Integrating climate change and predation risk with wading bird conservation research in North America. Waterbirds 23: 535-540.

Harris, M.L., J.E. Elliott, R.W. Butler and L.K. Wilson. 2002. Reproductive success and chlorinated hydrocarbon contamination of resident great blue herons (Ardea herodias) from Coastal British Columbia, Canada, 1977 to 2000. Environmental Pollution 121: 207-227.

Norman, D.M., A.M. Breault and I.E. Moul. 1989. Bald Eagle incursions and predation at Great Blue Heron colonies.  Colonial Waterbirds 12:143‑230.

Vennesland, R. G. 2000. The effects of disturbance from humans and predators on the breeding decisions and productivity of the Great Blue Heron in south-coastal British Columbia. MS thesis, Simon Fraser University, Burnaby, Canada.

Scientific Publications

Bayer, R.D. 1978. Aspects of an Oregon estuarine Great Blue Heron population. IN: A. Sprunt IV, J.C. Ogden and S. Winkler. Wading birds. Nation. Audub. Soc. Res. Rpt. No. 7, N.Y. Pp:213‑218.

Bayer, R.D. 1981. Regional variation of Great Blue Heron longevity. J. Field Ornithol. 52:210‑213.

Bayer, R.D. 1985. Bill length of herons and egrets as an estimator of prey size. Colonial Waterbirds 8:104‑109.

Blus, L.J, C.J Henny, and T.E. Kaiser. 1980. Pollution ecology of breeding Great Blue Herons in the Columbia Basin, Oregon and Washington. Murrelet 61: 63-71.

Butler, R.W. 1994. Population regulation in Ciconiiform wading birds. Colonial Waterbirds 17:166-176.

Butler, R. W. 1992. Great Blue Heron, Ardea herodias. Birds of North America No. 25,

American Ornithologists’ Union.

Butler. R.W. 1993. Time of breeding in relation to food availability of female Great Blue Herons (Ardea herodias). Auk 110: 110:693-701.

Butler, R.W. 1997. The Great Blue Heron. University of British Columbia Press, Vancouver.

Butler, R.W., P.E. Whitehead, A. Breault and I. Moul. 1995. Colony effects on fledging success of Great Blue Herons (Ardea herodias) in British Columbia. Colonial Waterbirds 18: 159-165.

Butler, R.W., J.A. Kushlan and I.J. Davidson. 2000. Herons in North America, Central America and the West Indies. Pages151-176 IN: J.A. Kushlan and H. Hafner (eds.).Heron conservation. Academic Press, N.Y.

Butler, R.W. and R. G. Vennesland. 2000. Integrating climate change and predation risk with wading bird conservation research in North America. Waterbirds 23: 535-540.

Dickerman, R. W. 2004. Characteristics and distribution of Ardea herodias fannini with

comments on the effect of washing on the holotype. Northwestern Naturalist 85: 130-133.

Elliott, J.E., R.W. Butler, R.J. Norstrom and P.E. Whitehead. 1989. Environmental contaminants and reproductive success of Great Blue Herons (Ardea herodias) in British Columbia, 1986‑87. Environ. Pollut. 59:91‑ 114.

Elliott, J.E., Butler, R.W., R.J. Norstrom and P.E. Whitehead. 1989. Environmental contaminants and reproductive success of Great Blue Herons (Ardea herodias) in British Columbia, 1986-87. Environmental Pollution 59:91-114.

English, S. 1978. Distribution and ecology of Great Blue Heron colonies on the Willamette River, Oregon. IN: A. Sprunt IV, J.C. Ogden and S. Winkler. Wading birds. National Audubon Soc. Res. Rpt. No. 7. Pp:235‑246.

Forbes, L.S. 1982. Prey manipulation in the Great Blue Heron. Murrelet 63:89.

Forbes, L.S., K. Simpson, J.P. Kelsall and D.R. Flook. 1985a. Reproductive success of Great Blue Herons in British Columbia. Can. J. Zool. 63:1110‑ 1113.

Hancock, J. and J. Kushlan. 1984. The herons handbook. Harper and Row, New York.

Henny, C.J. and M.R. Bethers. 1971. Population ecology of the Great Blue Heron, with special reference to western Oregon. Can. Field‑Natural. 85:205‑209.

Krebs, J.R. 1974. Colonial nesting and social feeding as strategies for exploiting food resources in the great blue heron (Ardea herodias). Behav. 51:99‑131.

Moul, I.E., R. G. Vennesland, M.L. Harris, and R.W. Butler.  2001. Standardizing and interpreting nesting records for Great Blue Herons in British Columbia. Canadian Wildlife Service Progress Note No. 217, Ottawa, Canada (Available on line at: http://www.cws-scf.ec.gc.ca/publications/pnotes/index_e.cfm

Norman, D.M., A.M. Breault and I.E. Moul. 1989. Bald Eagle incursions and predation at Great Blue Heron colonies.  Colonial Waterbirds 12:143‑230.

Simpson, K. 1984. Factors affecting reproduction in Great Blue Herons (Ardea herodias). Unpubl. M.Sc. thesis, Univ. Brit. Col., 90pp.

Simpson, K., J.N.M. Smith and J.P. Kelsall. 1987. Correlates and consequences of coloniality in Great Blue Herons. Can. J. Zool. 65:572‑577.

Vennesland, Ross G. 2000. The effects of disturbance from humans and predators on the breeding decisions and productivity of the Great Blue Heron in south-coastal British Columbia."  MSc thesis, Simon Fraser University. MSc thesis, Simon Fraser University, Burnaby, B.C.

Werschkul, D.F., E. McMahon and M. Leitschuh. 1977. Observations on the  reproductive ecology of the Great Blue Heron in western Oregon. Murrelet 58:7‑12.