Avian use of hedgerows and adjacent crops in Central California agricultural landscapes

Abstract

Background/Question/Methods

Hedgerows provide valuable ecosystem services including air and water quality protection, soil erosion control, promotion of pollinators and other beneficial insects, and biodiversity. Planting hedgerows at field margins is an increasingly common conservation measure in heavily transformed agricultural regions of California. Hedgerows may also provide critical habitat for numerous avian species that utilize the small, linear, wooded patches for resting, foraging, wintering and breeding. However, avian usage of hedgerows in Central California has not been well studied and farmers fear they may attract undesirable bird species. More complex vegetation structure and composition attract numerous songbirds that frequent field edges, but there is little information about the degree to which hedgerows are used by wintering and breeding birds. We used a paired study design to 1) evaluate the degree to which hedgerows are used by birds relative to nearby unenhanced field margins; 2) evaluate bird use of adjacent crops (standardized by crop type) to determine if hedgerows attract bird species considered crop pests, 3) determine if hedgerows in agricultural landscapes increase avian diversity and abundance relative to landscapes without hedgerows and 4) quantify and determine key habitat structures that are influencing the avian community on our sites.

Results/Conclusions

The pilot phase of our study was conducted during winter (Nov-Jan 2011) and spring (Apr-Jun 2012) to determine wintering and breeding bird use on paired hedgerow and reference plots in Yolo County, CA. We quantified vegetation diversity, composition, structure, and noted fruiting and flowering events to include in avian-habitat relationship models. Analysis of pilot data indicate that more than twice as many bird species and nearly four times as many individual birds use habitat created by native-planted hedgerows as opposed to unenhanced field margins. Our results also show that species richness, abundance, and bird use in adjacent crops was not significantly different when hedgerows were present, indicating that hedgerows are not attracting pest bird species. Additionally, results indicate that planting hedgerows with native species may act as refugia for wintering songbirds without attracting species considered to be crop pests. These results may help act as an incentive for farmers and landowners to replace non-native field margins with native species, which in turn, provide valuable ecosystem services. We anticipate that survey efforts will continue and expand to further understand the role that hedgerow vegetation structure and composition plays in structuring avian communities in agricultural landscapes.

Full text

1
Agriculture and Natural
Resources, UC Cooperative
Extension, Woodland, CA.
2
Audubon California, Landowner
Stewardship Program,
Sacramento, CA.
3
Department of
Plant Sciences, UC Davis, CA.
4
College of Biological Sciences,
UC Davis, CA.
www.sercal.org 5 Ecesis
Introduction
Wildlife conservation in agro-ecosystems is especially challenging
due to habitat loss and fragmentation, intensive human land use,
and the need to balance conservation goals with agricultural
production. Hedgerows are often incorporated into agro-ecosystem
conservation planning since they offer disproportionate
enhancement of valuable ecosystem goods and services (e.g., air and
water quality protection, weed control, soil erosion control,
promotion of pollinators and other beneficial insects, and
biodiversity) in exchange for minimal reductions in production
(Earnshaw 2004, Long and Anderson 2010, Morandin and Kremen
2012). Hedgerows consist of trees, shrubs, perennial grasses, forbs
and other species planted in narrow strips along field margins
(Long and Anderson 2010), with the use of native species advocated
for in recent years (Long and Anderson 2010, Morandin and
Kremen 2012).
Hedgerows are an increasingly common conservation measure in
heavily transformed agricultural regions of Central California, in
part because of their perceived potential to provide critical habitat
for numerous avian species that may utilize the small, linear,
wooded patches for resting, foraging, wintering and breeding
(Hinsley and Bellamy 2000; Earnshaw 2004). However, avian usage
of hedgerows in Central California has not been well-studied, and
data is lacking in regard to hedgerow effects on avian abundance
and diversity and how these effects differ across wintering and
breeding seasons. Understanding these effects will be critical to the
promotion of hedgerows as agro-ecological conservation measures
in Central California and beyond. In addition, the perception that
hedgerows may attract
undesirable bird species into
agricultural fields (Earnshaw
2004) cannot be addressed
without quantitative data on
avian use of hedgerows and
adjacent fields.
In order to address these gaps
in knowledge, we conducted a
pilot study of avian hedgerow
use in Yolo County from 2011-
2012. Our objectives were to
use a paired study design to quantify 1) hedgerow effects on avian
abundance, richness, and diversity in both wintering and breeding
seasons relative to unenhanced field margins (control sites); 2)
avian use of adjacent crop fields (standardized by crop type) to
determine if hedgerows attract avian crop pests; and 3) key habitat
characteristics that may be influencing avian use of hedgerows and
unenhanced field margins. In this article, we focus primarily on the
results of avian analyses while briefly describing vegetation in
hedgerow and control sites.
Methods
We selected four hedgerow sites in Yolo County that were similar in
age and structure and that were already being used in UC
Cooperative Extension research (Fig. 1). Nearby unenhanced field
margins served as control sites, and both hedgerow and control sites
were standardized by adjacent crop type. Six 20-min avian search
censuses were conducted at 2-week intervals along 0.25-mi stretches
of hedgerow and control habitat in both winter (Nov. 2011-Jan
2012) and breeding (Apr-June 2012) seasons. Vegetation surveys
were conducted in May 2012, and consisted of identification and
measurements of all trees and shrubs (hedgerows) and 1-m
2
percent
cover quadrats in understories of both hedgerow and control sites.
Statistical comparisons of means were conducted using program R,
and statistical significance was assessed at the P≤0.05 level.
Results
Avian abundance, richness, and diversity. Pooled across seasons, we
found that avian abundance was more than three times higher in
Figure 1 A native-planted hedgerow in Yolo County, California. Photograph courtesy H.M.White.
Avian Use of Hedgerows and Adjacent Crops in Central
California Agricultural Landscapes
by Hillary M. White
1
, Rachael F. Long
1
, Karen Velas
2
, Andrew P. Rayburn
3
, William L. Rockey
4
, and Rodd Kelsey
2
.
continued next page

Ecesis 6 Winter 2012 Volume 22, Issue 4
hedgerows (1689 individuals) compared to control sites (514
individuals). This difference in abundance persisted when the data
were analyzed separately for wintering (N
hedgerows
= 1314
individuals and N
control
= 331 individuals) and breeding seasons
(N
hedgerows
= 375 individuals and N
control
= 183 individuals).
Similar results were found for species richness. A total of 41 species
were detected at hedgerow sites, compared to 22 species at control
sites. Pooled across seasons, average avian richness was nearly
double in hedgerows (27.50 ± 1.55 spp.) compared to control sites
(15.25 ± 0.95 spp.). Analysis by season showed that average avian
richness was significantly higher in hedgerows in both wintering
(20.00 ± 1.96 spp. in hedgerows, 8.00 ± 0.41 spp. in control sites)
and breeding seasons (17.50 ± 1.32 spp. in hedgerows, 9.25 ± 0.25
spp. in control sites).
Results were less clear for species diversity, which we calculated
using the Shannon-Wiener index that accounts for species richness
and relative abundance. During the breeding season, avian species
diversity was significantly higher in hedgerow sites (9.94 ± 1.15)
compared to controls sites (5.70 ± 0.82). No other significant
differences in diversity between hedgerow and control sites were
detected.
Interestingly, our results showed that avian abundance and richness
in adjacent crop fields were not significantly different when
hedgerows were present, suggesting that hedgerows are not
attracting avian pest species into nearby crops. For example, for
three of the most common avian crop pests (American crow
[Corvus brachyrhynchos], red-winged blackbird [Agelaius
phoeniceus], and Brewer’s blackbird [Euphagus cyanocephalus]),
there were between five and ten times more birds detected during
the study in agricultural fields adjacent to unenhanced field margins
Our many thanks to our generous
2012 conference sponsors…
compared to crop fields adjacent to hedgerows. Analysis of flyover
data collected during the study suggested that these species tended
to pass over hedgerows entirely and may have been more focused
on crop fields as a landscape feature.
Habitat characteristics. Overstory vegetation in hedgerows was
dominated by Coyotebush (Baccharis pularis), Black elderberry
(Sambucus nigra), and California coffeeberry (Frangula californica).
Mean abundance, richness, and diversity of trees and shrubs in
hedgerows were 83 ± 6.50 individuals, 9.25 ± 0.58 spp., and 5.17 ±
0.63 respectively. Understory vegetation in both hedgerows and
control sites was characterized by both native and exotic grasses and
forbs, although native cover was significantly higher in hedgerows
(30.65 ± 8.36% ) versus control sites (8.05 ± 3.17%) and exotic cover
was significantly higher in control sites (64.90 ± 3.42%) versus
hedgerow sites (47.90 ± 6.29%). Hedgerow sites also had
significantly more litter, while control sites had significantly more
bare ground. No differences in native understory species richness
were found, but control sites did have significantly more exotic
understory species.
Conclusions and future research directions
We found that avian abundance and richness were significantly
higher in native hedgerows compared to unenhanced field margins,
especially during the wintering season. During the breeding season,
avian diversity was higher in hedgerows compared to unenhanced
field margins. Our results strongly suggest that native hedgerows in
Central California agricultural landscapes may act as both refugia
for wintering songbirds and as habitat for breeding songbirds
without attracting avian pests into adjacent crop fields. These
findings may serve as an incentive for producers to plant hedgerows
at field margins, since native-planted hedgerows positively influence
a broad suite of valuable ecosystem goods and services in addition
to providing wildlife habitat.
The results of this pilot study are being used to scale up research
efforts to include a larger set of study sites across the Sacramento
Valley in conjunction with native pollinator researchers at
UC Berkeley, enabling us to further understand the effects
of hedgerows on avian communities in agricultural
landscapes.
References
Earnshaw, S. 2004. Hedgerows for California agriculture. CAFF.
caff.org/wp-content/uploads/2010/07/Hedgerow_
manual.pdf
Hinsley, S.A. and P.E. Bellamy. 2000. The influence of hedge
structure, management and landscape context on the value of
hedgerows to birds: A review. Journal of Environmental
Management 60:33-49.
Long R.F., and J. Anderson. 2010. Establishing Hedgerows on
Field Crop Farms in California’s Central Valley. UC ANR Pub
8390. Oakland, CA. 7 p.
Morandin, L.A. and C. Kremen. 2012. Bee preference for native
versus exotic plants in restored agricultural hedgerows.
Restoration Ecology. doi: 10.1111/j.1526-100X.2012.00876.x
Avian use of hedgerows and adjacent crops
continued