Foraging decisions, bird community structure, and residential landscapes: A mechanistic approach to explain the decline of urban bird diversity

Background/Question/Methods

Patterns emerging from cities indicate that bird species richness and evenness decline while biomass and density increase. However, recent studies have shown that within cities, invasive and synanthropic species had a strong association with residential yards and gardens landscaped with exotic vegetation while native bird diversity increased in yards with native plantings. Working within the CAP-LTER study site, we tested whether invasive and synanthropic species exhibited behavioral advantages that enabled them to exploit yards landscaped with exotic vegetation. We compared foraging decisions (Giving Up Density – GUD) between two markedly different landscape designs (mesic: lush and exotic vegetation; xeric: drought-tolerant and native vegetation) within residential yards in the Sonoran desert city of Phoenix, AZ. We focused our study in residential yards since the management of residential landscapes could greatly enhance urban diversity. In addition, to improve the GUD method for evaluating artificial food patches with multiple species, we developed an experimental and analytical method for estimating the GUD for all species visiting each artificial food patch. We used video monitoring to quantify the cumulative number of foraging pecks at the point when each species quit foraging the artificial food patch. We then modeled a GUD - peck relationship for the final forager to estimate all species’ foraging decisions.

Results/Conclusions

Foraging decisions differed between mesic and xeric yards: birds foraging in mesic yards depleted the artificial food patches to a lower level than birds foraging in xeric yards. After accounting for bird densities, the lower GUDs in mesic yards appeared to be largely driven by invasive and synanthropic species. Furthermore, individual species exhibited a degree of behavioral plasticity. Seven of the ten species visited trays in both mesic and xeric yards, and on average, these species had lower GUDs in mesic yards compared with xeric yards. Our results suggested that birds associated different foraging costs with the mesic and xeric yards. The consistently higher GUDs from the artificial patches in xeric yards implied that alternative resources were more abundant, and the quality of these yards may be superior for native birds. Our GUD-peck model enabled us to explore how individual species and bird densities influenced the GUD, thus providing greater information on how foraging behavior shapes the urban bird community. Our results lend additional support for the implementation of native and natural landscapes in yards and gardens to help reverse the loss of urban bird diversity.