In deciding to focus on owls for this month’s posts, I clearly had forgotten about one of the biggest debates over timber harvesting- the northern spotted owl (Strix occidentalis caurina) gained lots of publicity as a bone of contention in the discussion of what to do with old-growth forest in the Pacific Northwest.
As birds that depend upon old (meaning more than 150 years) forest for both roosting and nesting, northern spotted owls are very sensitive to habitat loss. And the debate over what our priorities are with regard to management of these forests was very vocal starting in the late 1970s (Noon & McKelvey 1996). These are decisions with wide-ranging impacts, both for a bird whose total population in California, Oregon, and Washington was estimated to be between 4000 and 6000 animals by 1987 (Simberloff 1987), and for the economics of the region where some of the land in question was valued at $4,000 an acre. Additionally, other species, such as northern flying squirrels, red tree voles, and Vaux’s swift, require these same habitat conditions, which made spotted owls something of a poster child for the entire old-growth ecosystem. I’m not going to recount the entire saga step by step here (see Noon & McKelvey 1996 below for more details), but there was a lot going on.
As it turns out, spotted owls are threatened by more than just habitat loss and small population size (although these are both important issues). They are also at risk from a fellow owl who has been expanding into their territory for several decades. Barred owls (Strix varia) were historically present in eastern North America, but have been spreading west since at least the 1940s (Dark et al. 1998). Barred owls are similar in size and appearance to spotted owls, but they are more aggressive and can displace spotted owls from their territories. More importantly, barred owls and spotted owls can produce fertile hybrids (Hamer et al. 1994). Why is it so important that these two species can hybridize? Well, there are a couple of issues here: to begin with, there is dilution of a species’ genome when one species dominate the genes of the other (for example, these owl hybrids had white bands very similar to barred owls, rather than looking like a combination of the two); additionally, the hybrids observed by Hamer et al. successfully bred with barred owls, rather than spotted owls, meaning that those offspring were lost from the spotted owl population; and the big problem tends to come down to conservation efforts- even though northern spotted owls are listed as threatened under the Endangered Species Act, there is no protection within the legislation for hybrids- if enough barred owls interbreed with spotted owls, there may be no grounds to continue protecting the species.
Burrowing owls are generally dependent on other animals to construct the burrows they use for nesting.
It took me a while to work my way through the various studies on spotted owls (the Mexican spotted owl is also threatened, the California spotted owl is a species of concern…), but when I reached the other side, I discovered an entire suite of literature on other owl species in decline but without the same level of publicity. With the exception of Florida (although they have other issues there), burrowing owls (Athene cunicularia) are in decline across much of their range (and listed as endangered in Canada) due to limited nest sites and reproductive success (Desmond et al. 2000). As prairie dog colonies have disappeared, burrowing owls have found themselves without suitable burrows for nesting and more exposed to predators. In Florida where the owls create their own burrows, they have generally done well in the partially developed areas humans made by filling in wetlands- however, when the percent of developed land goes above 60% in a given area, owls experience reduced breeding success due to human disturbance, difficulty burrowing under lawn sod, and burrow collapse during mowing operations (Millsap & Bear 2000). Cactus ferruginous pygmy-owls (Glaucidium brasilianum) in Arizona were listed as endangered in 1997 following decades of decline (Johnson et al. 2003). When researchers looked back through 100 years of sighting records, they realized that the decline became noticeable starting in the 1920s, a time-scale that aligns suspiciously well with the major water reclamation projects carried out between 1902 (The National Irrigation Act) and the 1930s- the loss of cottonwood habitat as rivers were turned into lakes is a likely cause of the owl’s decline. Farther afield, in the last two decades of the 20th century eagle owls (Bubo bubo) in Italy experienced high rates of mortality from electrocution when individuals tried to land on power line pylons which have become ever more of a presence in the countryside (Sergio et al. 2004).
So owls around the world have certainly had their share of problems during the 20th century, but there has also been a lot of work to better understand owl ecology and address some of the causes of population decline. In my next post I’ll take a look at how things have progressed over the last 15 years or so and how researchers feel about their future prospects.
Works cited:
Dark, SJ, Gutierrez, RJ, and GI Gould, Jr. 1998. The barred owl (Strix varia) invasion in California. The Auk 115: 50-56.
Desmond, MJ, Savidge, JA, and KM Eskridge. 2000. Correlations between burrowing owl and black-tailed prairie dog declines: a 7-year analysis. The Journal of Wildlife Management 64: 1067-1075.
Hamer, TE, Forsman, ED, Fuchs, AD, and ML Walters. 1994. Hybridization between barred and spotted owls. The Auk 111: 487-492.
Johnson, RR, Cartron, JE, Haight, LT, Duncan, RB, and KJ Kingsley. 2003. Cactus ferruginous pygmy-owls in Arizona, 1872-1971. The Southwestern Naturalist 48: 389-401.
Millsap, BA and C Bear. 2000. Density and reproduction of burrowing owls along and urban development gradient. The Journal of Wildlife Management 64: 33-41.
Noon, BR and KS McKelvey. 1996. Management of the spotted owl: a case history in conservation biology. Annual Review of Ecology and Systematics 27: 135-162.
Sergio, F, Marchesi, L, Pedrini, P, Ferrer, M, and V Penteriani. 2004. Electrocution alters the distribution and density of a top predator, the eagle owl Bubo Bubo. Journal of Applied Ecology 41: 836-845.
Simberloff, D. 1987. The spotted owl fracas: mixing academic, applied, and political ecology. Ecology 68: 766-772.