The importance of street trees to urban avifauna

Abstract

Street trees are public resources planted in a municipality's right-of-way and are a considerable component of urban forests throughout the world. Street trees provide numerous benefits to people. However, many metropolitan areas have a poor understanding of the value of street trees to wildlife, which presents a gap in our knowledge of conservation in urban ecosystems. Greater Los Angeles (LA) is a global city harboring one of the most diverse and extensive urban forests on the planet. The vast majority of the urban forest is nonnative in geographic origin, planted throughout LA following the influx of irrigated water in the early 1900s. In addition to its extensive urban forest, LA is home to a high diversity of birds, which utilize the metropolis throughout the annual cycle. The cover of the urban forest, and likely street trees, varies dramatically across a socioeconomic gradient. However, it is unknown how this variability influences avian communities. To understand the importance of street trees to urban avifauna, we documented foraging behavior by birds on native and nonnative street trees across a socioeconomic gradient throughout LA. Affluent communities harbored a unique composition of street trees, including denser and larger trees than lower-income communities, which in turn, attracted nearly five times the density of feeding birds. Foraging birds strongly preferred two native street-tree species as feeding substrates, the coast live oak (Quercus agrifolia) and the California sycamore (Platanus racemosa), and a handful of nonnative tree species, including the Chinese elm (Ulmus parvifolia), the carrotwood (Cupaniopsis anacardioides), and the southern live oak (Quercus virginiana), in greater proportion than their availability throughout the cityscape (two to three times their availability). Eighty-three percent of street-tree species (n = 108, total) were used in a lower proportion than their availability by feeding birds, and nearly all were nonnative in origin. Our findings highlight the positive influence of street trees on urban avifauna. In particular, our results suggest that improved street-tree management in lower-income communities would likely positively benefit birds. Further, our study provides support for the high value of native street-tree species and select nonnative species as important habitat for feeding birds.

Keywords: California; Los Angeles; bird; foraging behavior; migratory; native vegetation; nonnative vegetation; socioeconomic; urban forest; wildlife.

Fig. 1

Street trees in a suburban neighborhood in Los Angeles County, California, USA (Photo credit, E. Wood).

Fig. 2

(a) Sampling design depicting 36 survey locations distributed across a socioeconomic gradient throughout the Los Angeles basin and surrounding valleys and mountains, Los Angeles County, California. (b) Inset map highlights a walking route (yellow line), where observers documented bird‐feeding behavior in street trees, twice during each of the 2016–2017 and 2017‐2018 winter seasons. Further, observers identified, recorded location, and measured diameter at breast height for all street trees throughout each route. Photos highlight typical differences in street trees from low‐, to medium‐, to high‐income areas of Greater Los Angeles (Photo credits, E. Wood).

Fig. 3

Box‐plot summaries of street‐tree density (number of street trees per 1 km of survey route), total street‐tree basal area (m²) per km, and migratory and year‐round feeding bird density within 36 residential communities situated across a socioeconomic gradient of low (<US$53,219, median household income), medium (US$53,220–US$70,719), and high‐income residential communities (>US$70,720) throughout Greater Los Angeles. In all cases, high‐income residential communities harbored significantly greater tree density, tree basal area, and density of migratory and year‐round feeding birds than medium and low‐income residential communities based on a one‐way ANOVA or Kruskal‐Wallis analysis followed by a multiple comparisons analysis. The boxplot figures display the median values, the first and third quartile, and the minimum and maximum values, while circles denote outliers.

Fig. 4

Scatterplots depicting the relationships between density of feeding migratory, year‐round, and total birds (migratory and year‐round feeding birds combined) with street‐tree density and street‐tree size. We derived the fitted smoothed line and estimated prediction intervals from a generalized linear model analysis using a negative binomial error distribution. The color scheme represents survey areas located in 36 residential communities situated across a socioeconomic gradient of low (US$70,720) throughout Greater Los Angeles.

Fig. 5

(a) Relative importance values of common street‐tree species (IV), grouped by whether they were native or nonnative in geographic origin, and the proportional use of native and nonnative trees by migratory, year‐round, and total birds (five migratory and five year‐round species combined) during the 2016–2017 and 2017–2018 winter field seasons throughout Los Angeles. (b) Inset figure depicts the relative importance values of grouped native and nonnative street‐tree species, and the proportional use of native and nonnative trees species by migratory, year‐round, and total birds. The street‐tree importance values represent a tree species’ or tree group’s availability as a foraging substrate to birds. Bars depicting bird‐foraging proportion that are greater than street‐tree importance values (horizontal dashed lines provided for reference in inset) suggest bird‐feeding preference, whereas bars below street‐tree importance values suggest bird‐feeding avoidance.