Few homeowners realize that when they are landscaping their yards, they are inadvertently creating habitats for birds, insects, and other animals that call urban areas home. Past CAP research has already established that residential landscaping in the Phoenix area has increased plant diversity compared to the native Sonoran desert due to the introduction of many exotic species of plants. Recent investigations have examined what residential landscaping choices mean for urban birds.
CAP scientists Susannah Lerman and Paige Warren have sought to understand bird distribution in residential yards. They focused on neighborhoods that are a part of the Phoenix Area Social Survey (PASS), a survey conducted every five years in 40 neighborhoods in the metropolitan Phoenix area.
With their research team, Lerman and Warren conducted bird surveys in selected PASS neighborhoods to determine the bird species present and characterized the landscape types in the neighborhoods. They also analyzed the results of questions on the PASS that dealt with people’s satisfaction with bird diversity in their yards and compared that with their bird survey results. They found that residents were more satisfied with the existing bird variety in their yards when their neighborhood had more native bird species present (Figure 1).
While this indicates a value for native bird species, these species are not spread equally across the metropolitan area. Lerman and Warren’s analyses indicated that native birds were mostly found in neighborhoods that contained native plants and shrubs, neighborhoods closer to desert tracts, and higher income neighborhoods where native vegetation was present. In low income and predominately Hispanic neighborhoods, which tend to have less vegetation (native and non-native) overall, there were very few birds species present, and none of these birds were native to the Sonoran desert. Not only do these results indicate the importance of native vegetation for retaining native bird species in the city, they also suggest unequal access to a valued aspect of the environment, native birds.
Lerman and Warren, joined by CAP colleagues Eyal Shochat and Hilary Gan, sought to better understand why native birds were associated with native vegetation in residential yards. Their investigation focused on foraging or feeding behavior by birds in xeric (drought tolerant and native vegetation) and mesic (grass and exotic species of vegetation) backyards. The team conducted an experiment with trays filled with seeds and sand, which were placed on stools in mesic and xeric backyards. They videotaped birds feeding at the trays and weighed the amount of seed left in the trays after 24 hours to determine the giving up density (GUD: the amount of seeds remaining when the final bird quits feeding). Using the videotapes, the researchers also recorded the number of pecks each bird made at the seed trays.
Their experiment was based on the assumption that birds would stop feeding at the seed trays once energy gains from food equal the costs of foraging, which include the risk of predation by domestic cats as well as missed opportunities to forage elsewhere. Over time as the number of seeds mixed with sand in the trays becomes depleted, birds have to spend more time feeding at the trays. In environments with relatively more high-quality natural food sources, birds will give up feeding from the trays sooner than in yards that lack higher quality, alternative food sources, resulting in a higher GUD.
Among the bird species visiting seed trays in both yard types, those visiting the xeric yards had higher GUDs and spent less time feeding at the trays (Figure 2). This indicates that the xeric yards had a greater abundance of alternative food sources. Statistical tests did not find significant GUD differences among bird species, although curved-bill thrashers consumed seed faster than other species. The research team concluded that yards with native-like landscaping support birds better than those with grass lawns and exotic plant species.
While xeric landscaping is already promoted by local governments in the Phoenix metropolitan area as means of conserving water, homeowners in some communities do not have absolute control over how they landscape their property, particularly their front yards. Homeowners Associations (HOAs) are a common feature of newer master-planned development in the Phoenix area. Individuals purchasing homes in a HOA must sign a legally-binding document outlining the Covenants, Conditions, and Restrictions (CCRs) of the HOA, which typically deal with exterior features of the property, including landscaping, physical structures, and even paint colors. Adherence to the CCRs is ensured through a system of fines and legal actions.
Some of the landscaping guidelines in CCRs involve regulations on thorny plants in yards. Former CAP graduate student Bobby Fokidis investigated thorny plant regulations and their potential impact on native birds. He found that of the 43 HOAs surveyed, all had restrictions on thorny (non-cacti) plants in the front and/or back yards. Nineteen of the HOAs also had restrictions on the size of certain cacti, such as barrel cacti, organ pipe cacti, and saguaros. Given that many native bird species use thorny vegetation for nesting and habitat, HOAs with restrictions on this type of vegetation may limit the native bird populations in these areas.
Lerman and CAP researchers Kelly Turner and Christofer Bang take the position that HOAs could be sites for encouraging biodiversity in the city. Using data from CAP’s long-term bird monitoring and Survey 200 data on plants and arthropods, they compared bird, arthropod, and plant diversity between HOA neighborhoods and non-HOA neighborhoods. They found that neighborhoods belonging to HOAs had significantly greater bird and plant diversity but that there was no difference in arthropod diversity between the two neighborhood types. Native bird diversity trended toward increased diversity in HOA neighborhoods.
The team hypothesized that either the institutional structure of the HOAs or the landscape management regime imposed by the CCRs helps promote biodiversity. To further biodiversity in the city, they recommend HOAs to adopt the Sustainable Sites Initiative guidelines for creating sustainable landscapes, an action that could be much more easily accomplished with communities in the planning stage compared to existing communities with legally-binding CCRs already in place.
Fokidis, H. B. 2011. Homeowners associations: Friend or foe to native desert avifauna? Journal of Arid Environments 75:394-396
Lerman, S. B., P. S. Warren, H. Gan and E. Shochat. 2012. Linking foraging decisions to residential yard bird composition. PLoS One 7:e43497. doi:10.1371/journal.pone.0043497.g002
Lerman, S. B., V. K. Turner and C. Bang. 2012. Homeowner associations as a vehicle for promoting native urban biodiversity. Ecology and Society 17:Art. 45.
Lerman, S. B. and P. S. Warren. 2011. The conservation value of residential yards: Linking birds and people. Ecological Applications 21:1327-1339.