An effective method for ecosystem-scale manipulation of bird abundance and species richness

Manipulation experiments are a cornerstone of ecological research, but can be logistically challenging to execute-particularly when they are intended to isolate the ecological role of large, vagile species, like birds. Despite indirect evidence that birds are influential in many ecosystems, large-scale, multi-year bird manipulation experiments are rare. When these studies are conducted, they are typically realized with caged or netted exclosures, an approach that can be expensive, risky for wildlife, and difficult to maintain. In cases where caged exclosures are not appropriate, alternate approaches are needed to allow rigorous empirical studies on the ecological role of birds. Here, we present and validate a method for experimentally increasing the abundance and richness of birds at the scale of entire aquatic ecosystems. Unlike bird exclusion, this approach is experimentally tractable, appealing to land managers, and possible to deploy over large spatial scales. We tested the efficacy of our approach for increasing bird abundance and species richness at 16 central California ponds. Based on bird visitation data obtained by summer camera trapping, our approach significantly increased bird species richness and abundance at manipulated ponds compared to control ponds. Attractant treatments mitigated the negative effects of a major drought on bird species richness and generated a near doubling of bird abundance in the presence of attractants. Treatments had no effect on most mammal species, with the exception of ground squirrels, which increased in abundance in the presence of attractants. These results suggest that attractants are effective in increasing bird abundance and richness. We encourage researchers to consider this approach for experimentally isolating the ecological role of birds in aquatic and open terrestrial ecosystems, especially in cases where cost or logistical constraints preclude the use of caged or netted exclosures.

Better living through conifer removal: A demographic analysis of sage-grouse vital rates

Sagebrush (Artemisia spp.) obligate wildlife species such as the imperiled greater sage-grouse (Centrocercus urophasianus) face numerous threats including altered ecosystem processes that have led to conifer expansion into shrub-steppe. Conifer removal is accelerating despite a lack of empirical evidence on grouse population response. Using a before-after-control-impact design at the landscape scale, we evaluated effects of conifer removal on two important demographic parameters, annual survival of females and nest survival, by monitoring 219 female sage-grouse and 225 nests in the northern Great Basin from 2010 to 2014. Estimates from the best treatment models showed positive trends in the treatment area relative to the control area resulting in an increase of 6.6% annual female survival and 18.8% nest survival relative to the control area by 2014. Using stochastic simulations of our estimates and published demographics, we estimated a 25% increase in the population growth rate in the treatment area relative to the control area. This is the first study to link sage-grouse demographics with conifer removal and supports recommendations to actively manage conifer expansion for sage-grouse conservation. Sage-grouse have become a primary catalyst for conservation funding to address conifer expansion in the West, and these findings have important implications for other ecosystem services being generated on the wings of species conservation.

Providing perches for predatory and aggressive birds appears to reduce the negative impact of frugivorous birds in vineyards

Context Birds active in vineyards in south-eastern Australia can reduce or enhance crop yields via their foraging activities (e.g. by consuming grapes or by preying on grape-eating species). Aims We examined the effectiveness of artificial perches in encouraging predatory birds into vineyards to scare frugivorous birds and consequently reduce the damage they cause to grapes. Methods We monitored 12 artificial perches for 4 months during the growing season, spread over six vineyards in north-eastern Victoria, and compared bird damage to grapes at these sites with control sites without perches. Key results We found that raptors did not use the artificial perches. However, the large and aggressive Australian magpie (Cracticus tibicen) commonly used perches and we recorded 38513 perch visits by this species. Grapevines around perch sites suffered >50% less grape damage (4.13% damage per bunch) than control sites (8.57% damage per bunch). Conclusions Our results suggest that providing artificial perches in vineyards can play a role in reducing frugivore damage to grapes. However, the effectiveness of perches can vary under different environmental conditions and certain perch types are not suitable for all predatory or aggressive birds. Implications Future research should focus on the potential role of large-bodied and competitively aggressive species such as the Australian magpie in altering the activity of smaller frugivorous birds in vineyards, and also on the optimum height and location of artificial perches within vineyards to increase visitation by other predatory or aggressive bird species.

Effects of introducing threatened falcons into vineyards on abundance of passeriformes and bird damage to grapes

Agricultural landscapes are becoming an important focus of animal conservation, although initiatives to conserve predators to date have rarely provided economic benefits to agricultural producers. We examined whether introduction to vineyards of the New Zealand Falcon (Falco novaeseelandiae), a species listed as threatened by the New Zealand Department of Conservation, affected the abundance of 4 species of Passeriformes that are considered vineyard pests or affected the amount of economic loss due to grape (Vitis vinifera) damage. Three of the species were introduced and remove whole grapes from bunches (Blackbird [Turdus merula], Song Thrush [Turdus philomelos], and Starling [Sturnus vulgaris]), whereas the one native species (Silvereye [Zosterops lateralis]) pecks holes in grapes. The introduction of falcons to vineyards was associated with a significant decrease in the abundance of introduced passerines and with a 95% reduction in the number of grapes removed relative to vineyards without falcons. Falcon presence was not associated with a change in the number of Silvereyes, but there was a 55% reduction in the number of grapes pecked in vineyards with falcons. Our results indicate that, relative to damage in vineyards without falcons, the presence of a falcon could potentially result in savings of US$234/ha for the Sauvignon Blanc variety of grapes and $326/ha for Pinot Noir variety of grapes.

Effects of artificial perches and interspecific interactions on patch use by wintering raptors

We used an experimental approach to investigate the effects of woody vegetation and interspecific interactions on patch use by three wintering raptor species in the coastal prairie in south Texas. We manipulated perch type and density by the addition of artificial perches to patches of coastal prairie grasslands, varying perch height in the first year and canopy density in the second year. American Kestrels (Falco sparverius) used areas with artificial perches more often than they used natural woody vegetation and areas without woody perches. Northern Harrier (Circus cyaneus) patch use did not vary with treatment type. Loggerhead Shrikes (Lanius ludovicianus) used areas of natural woody vegetation more than patches with artificial perches and patches with no perches. Patch use by Loggerhead Shrikes was also influenced by the presence of complex woody vegetation and interspecific interactions, whereas American Kestrel use was limited by landscape matrix and ground cover density. We believe that predation by Northern Harriers limited Loggerhead Shrikes to patches with escape cover, while competition from American Kestrels prevented Loggerhead Shrikes from concurrent patch use.

Response of American kestrels and gray-tailed voles to vegetation height and supplemental perches

We tested the behavioral and demographic responses of American kestrels, Falco sparverius, and gray-tailed voles, Microtus canicaudus, to vegetation height and addition of perches. We conducted our experiment in sixteen 0.2-ha rodent enclosures with four replicates assigned to each of the following treatments: tall vegetation without perches, tall vegetation with perches, short vegetation without perches, and short vegetation with perches. The enclosures were stocked with 20 gray-tailed voles in early November 1998. Before perches were erected during the 12th week of the experiment, kestrels showed a preference for short-vegetation enclosures (P < 0.05). After perches were erected, kestrels used enclosures with perches, showing the greatest preference for short-vegetation enclosures with a perch. Vole populations and recruitment rates were higher in tall-vegetation enclosures than in short-vegetation enclosures, but supplemental perches did not affect vole populations or recruitment. In many agricultural areas where perches are not available, providing supplemental perches may increase accessibility to prey species that cause crop damage. Facilitating predation by raptors may reduce vole populations and reduce the need to use potentially harmful chemicals in pest population management.