Reversing the Trend: The Fight to Save Santa Clara's Burrowing Owls
(Produced by Caroline Armer and Jonathan Armer)
The Western burrowing owl (Athene Cunicularia Hypugea meaning "little digger") is a diminutive raptor of short grasslands and semi-arid areas whose life history is centered in and around an underground burrow, a unique aspect among owl species. The burrow is used for nesting, escape from predators, shelter during inclement weather, a food supply, thermoregulation and social interaction (Coulombe 1971 and Thomsen 1971).
Despite their dependence on burrows, the owls rely on fossorial mammals to provide the burrows, whereby they live in the same colony as colonial mammals but not in the same burrow (Thomsen 1971), so great is the need for burrows that the owls are dependant on burrowing mammals, especially ground squirrels (Spermophilus spp.) and prairie dogs (Cynomys spp.) (Haug, Millsap, and Martell. 1993).
The burrowing owl is approximately 9 inches tall and weighs up to 6 ounces. They have a rounded head, lack ear tufts, but are easily distinguishable by their white eyebrows, bright yellow irises and long stilt-like legs. Their plumage is a mottled brown and white coloring. The female is slightly smaller than the male and a little darker in coloring.
The range of the western burrowing owl includes most of western North America from southern Canada to northern Mexico, and west of the Mississippi river (Figure 1). Habitat loss, alteration of existing habitat, and reduction, and removal of colonial mammal populations have resulted in burrowing owls being listed as endangered in Canada, Minnesota and Iowa, and listed as a Species of Special Concern in most other US states, including the state of California (Sheffield 1997).
Once abundant throughout its range, this species is now experiencing a population decline as a direct result of habitat destruction and fragmentation (Dyer 1987, Sheffield 1997). This population decline has become so severe at some locations that the burrowing owl has become extirpated in parts of its range especially in California (Figure 2) (Holroyd, Rodriguez-Estrella and Sheffield 2001, DeSante, Ruhlen and Rosenberg 2004).
Burrowing owls have a symbiotic relationship with ground squirrels. They take over vacated squirrel burrows and live in the same colonies as the ground squirrels but not in the same burrow. The ground squirrels gain an extra pair of eyes to watch for predators from this relationship. The birds normally have satellite burrows near the main burrow which are used to escape predators.
- Unlike most owls, the burrowing owls are active both day and night, especially during dawn and dusk.
- Out of 171 species of owls worldwide, the burrowing owl is the only one that lives in burrows.
These birds can usually be seen standing at the entrance to their burrows, often perched on one leg only. They are year-round residents at Mission College. Burrowing owls are found in open prairie with few trees. They prefer the vegetation to be shorter than they are, so they can see any predators approaching.
Burrowing Owls in Santa Clara County
Urban areas are especially prone to burrowing owl declines, as suitable habitat is replaced with urban sprawl, and no where is this more prevalent than Santa Clara County, where present burrowing owl populations are experiencing a severe decline. From 1991-1994, 60% of sites containing burrowing owls in Santa Clara County alone were destroyed as a direct result of urban development (Buchanan 1996).
Habitat loss due to urban development has resulted in a 57% loss of open grassland patches that once contained owls since 1988 (Trulio 2003). Another study by Trulio and Chromczak (2004) found that there was a 37% decline in the number of burrowing owl nests at seven sites in Santa Clara County during the period 1999 (64 nests) to 2003 (42 nests).
Burrowing Owls at Mission College
The burrowing owl population at Mission College has experienced a huge decline since owls were first documented in 1987. This decline has severely dropped as the campus has expanded especially with the construction of the Mercado Center. From 1998 to the present, the burrowing owl population has been monitored and the trend in population for adult burrowing owls and for number of chicks over the same time period is displayed in Figures 3 & 4.
During two breeding seasons (2005 & 2007) no reproduction occurred on campus. In 2009 the breeding success of burrowing owls for the first time in many years was exceptionally successful. Two pairs each produced chicks. All chicks and the four adults were banded to monitor their movements. Some of the chicks have remained at Mission College in 2010, while others have migrated to other areas within Santa Clara County (Table 1).
So far, in 2010, two pairs have been observed on campus at two locations (Figure 5).
Burrowing Owl Diet
Burrowing owls are opportunistic feeders whose diet includes:
- small rodents such as mice, voles and shrews.
- eptiles such as frogs and lizards. The Environmental Awareness Association (EAA), a Mission College student club, has seen remains of frogs outside of a few of the burrows at Mission College, most likely from feeding at the Calabasas Creek.
- insects, especially Jerusalem crickets and earwigs, which comprise the vast majority of their diet.
Burrowing owls hunt by running rapidly along the ground and catching their prey or by hovering above prey and pouncing downward. The owls at Mission College have been observed perching on lamp posts at night and feasting on the insects that are attracted to the light.
During 2005 and 2006 Higgins (2007) conducted a study at five sites in Santa Clara County on the diet of burrowing owls, including the owls at Mission College. Analysis of 3,092 owl pellets, and prey remains at owl burrows revealed 7,227 prey items. Dermaptera (earwigs) dominated the diet, and invertebrates outnumbered vertebrates in the ratio 94:6. However, rodents accounted for 82.53% of the biomass (Figures 6 & 7). Hence, although insects were more common in the diet, rodents provided a far superior source of nutrition, as owls capture one prey species at a time. Most importantly, more vertebrates in pellets were associated with successful nests.
Dietary differences occurred among the five sites, with the diet of owls at Mission College lacking Microtus californicus (California voles), this species was the most numerous rodent overall at the other four sites. The lack of voles in the diet of burrowing owls at Mission College is probably an indication of the fragmentation and isolation of Mission College compared to the other four sites: Shoreline Park, Moffett Federal Airfield, Tasman Drive and Sunnyvale Park.
Based on these findings the burrowing owl prey base in this urban area can be improved by maintaining a mosaic of different habitat types to support prey species, planting native perennial plants, removing feral cats, installing irrigated areas adjacent to owl habitat, and providing supplemental feeding to the owls.
Breeding and Habitat
The breeding season for burrowing owls starts in February when pairs will bond together and search for an appropriate burrow. Egg laying begins in March and a female can lay as many as 6-12 eggs although the success rate for survival is much lower. Both parents feed the young and by September the young will have fledged.
Burrowing owls usually choose their homes in a flat grassy area. Burrowing owls, being only 9 inches tall, do not like vegetation in close proximity to their burrows to be taller than 6 inches which would interfere with the owls' ability to spot predators. They will forsake their burrows if the vegetation gets too tall.
Since there is a major lack of large flat areas, there are ways to accommodate these creatures right in your back yard. These furry looking birds also love native vegetation. Native plants and flowers encourage local insects liked by the owls.
Burrowing owls are experiencing significant declines throughout their range. The population of burrowing owls at Mission College has gone from a peak of 60 adults in 1988 to only 4 adult burrowing owls in 2010. If sufficient habitat is not provided for burrowing owls at Mission College over the next few years, burrowing owls will become an extirpated species on campus.
No longer will we enjoy their presence on campus if we do not protect the burrowing owls today, all that will be left is photographs of this very special species and memories of a bygone age and a bygone species.