Using Effigies to Deter American Crows and Common Ravens In

USING EFFIGIES TO DETER AMERICAN CROWS AND COMMON RAVENS IN

SNOWY PLOVER BREEDING HABITAT

Sara A. Peterson

A Thesis Presented to

The Faculty of Humboldt State University

In Partial Fulfillment of the Requirements for the Degree

Masters of Science in Natural Resources: Wildlife

Committee Membership

Dr. Mark Colwell, Committee Chair

Dr. Luke George, Committee Member

Dr. Micaela Szykman Gunther, Committee Member

Dr. Rob Van Kirk, Natural Resources Graduate Program Coordinator

May 2013 ABSTRACT

Using Effigies to Deter American Crows and Common Ravens in Snowy Plover Breeding Habitat

Sara A. Peterson

Common Ravens (Corvus corax) and American Crows (C. brachyrhynchos) are the principal predators of eggs and chicks of the Western Snowy Plover (Charadrius nivosus), which compromise population recovery of this federally listed species. I used a before-after/control-impact experimental design to examine the change in corvid activity in response to corvid effigies, a non-lethal predator control method. I conducted my study at Clam Beach, where corvids are more abundant than at any other plover breeding site in northern California. I conducted 18 trials, consisting of one “before” day and three days post-treatment, during the months of September through February. I recorded the average number and incidence of corvids that visited control and treatment plots within 1 m, 10 m, and 50 m of the center of the plot. Corvids were attracted to bait (trash and food) I placed in the study plots within 1-2 hours of the start of the experiment, and they continued to visit plots for four consecutive days (i.e., a trial). Effigies significantly reduced average corvid abundance and incidence (percentage of observations with at least one corvid) within a 50 m radius, but corvids still frequented plots. The effectiveness of effigies appeared to diminish over time; however this result was not statistically significant, and there was no significant difference in treatment effect among days 2, 3, and 4. I conclude that effigies may not be an effective predator management tool for deterring corvids in the vicinity of Snowy Plover nests.

ii ACKNOWLEDGMENTS

I would first like to thank my advisor, Dr. Mark Colwell, for allowing me the opportunity to work with Snowy Plovers as an undergrad which led me to this project. I will forever be grateful for the opportunity he afforded me by allowing me to pursue grad school and study a topic about which I am deeply passionate. I am humbled and greatly appreciative of his seemingly endless support, encouragement, and advice offered throughout my experience at Humboldt State University. I thank my committee members, Dr. Luke George and Dr. Micaela Szykman Gunther, for their valuable comments and review of this manuscript.

I would like to thank my shorebird lab mates Noah Burrell, Luke Eberhart-

Phillips, Michael Hardy, Jordan Muir, Allie Patrick, Wendy Pearson, and Kristin Sesser for inspiring, encouraging, and helping me throughout this process. A special thank you to Courtney Nicks, David Orluck, and Jessicca Patterson for their assistance in the field.

I am also thankful for the generous support and assistance offered from Tamar Danufsky and Anthony Desch, who provided invaluable guidance with the preparation of effigies and field equipment. I also would like to thank the management and staff of the

McKinleyville McDonald’s for generously supplying my corvid attractant, and Dr. John

Marzluff for offering his unmatched insight into corvid behavior and this study design.

Without all of their help, this project would not have been possible.

iii

I am grateful to Jim Watkins, Jay Harris, Amber Transou, and Hank Seeman for

supporting this project. I thank California State Parks and Humboldt County Parks for

allowing me access to their sites and the ability to collect data on their lands.

This project was funded by California North Coast Chapter of The Wildlife

Society, U.S. Fish and Wildlife Service, and The Western Section of The Wildlife

Society.

Finally, I would like to thank my family and friends for their continued unconditional love, support, and encouragement; as well as Snowy Plovers and Common

Ravens who continue to intrigue me with their combination of determined persistence and fascinating intelligence.

iv TABLE OF CONTENTS

Page

ABSTRACT ...... ii

ACKNOWLEDGMENTS ...... iii

LIST OF TABLES ...... vi

LIST OF FIGURES ...... vii

LIST OF APPENDICES ...... viii

INTRODUCTION ...... 1

MATERIALS AND METHODS ...... 5 Study Areas ...... 5 Effigies ...... 5 Data Collection ...... 7 Data Analysis ...... 9

RESULTS ...... 11

DISCUSSION ...... 17 Management Implications ...... 21

LITERATURE CITED ...... 23

APPENDICES ...... 31

LIST OF TABLES

Table Page

1 Summary of the average response time and number of corvids occurring on Day 1 of 18 four-day trials, when I provisioned plots with food but no effigy...... 13

2 Summary of the treatment effects of average number and average incidence (proportion of time) of corvids present at varying distances from center of paired treatment and control plots on Days 2, 3, and 4 when I hung the effigy ...... 15

vi LIST OF FIGURES

Figure Page

1 Paired control (C) and treatment (T) plots were placed every 500 m (horizontal dashes), separated from each other by 25 m, with observer located at midpoint of plots (X) on Clam Beach, Humboldt County, CA ...... 6

2 Treatment effect of effigies on average number and incidence of corvids within 50 m for each trial, in sequence of trial date ...... 16

vii

LIST OF APPENDICES

Appendix Page

A Annual (March through August) variation in average (+ SD) corvid abundance and incidence (proportion of point counts with at least one corvid) on the four main breeding sites used by Snowy Plovers in Humboldt County, CA ...... 28

B Summary of plover distribution and reproductive success during the months of March through August, Clam Beach, Humboldt County, CA ...... 29

C Summary of average (+ SD) corvid abundance and incidence within 50 m of the center of the plot where food and an effigy or control pole were placed in 18 trials of paired plots ...... 30

D Summary of average (+ SD) corvid abundance and incidence within 10 m of the center of the plot where food and an effigy or control pole were placed in 18 trials of paired plots ...... 31

E Summary of average (+ SD) corvid abundance and incidence within 1 m of the center of the plot where food and an effigy or control pole were placed in 18 trials of paired plots ...... 32

F Summary of average (+ SD) corvid abundance and incidence in 500 m segments (reported south to north) of Clam Beach, 2006-2011 ...... 33

G Summary of dog (d), gull (g), and human (h) presence in control and treatment plots throughout the duration of trials. Bold letters indicate bait (fries) was consumed, and italicized letters indicate partial amount of bait was consumed ...... 34

viii

INTRODUCTION

Worldwide, corvids have had a long association with humans; they are both

revered and persecuted in many parts of the world (Moore 2002, Marzluff and Angell

2005, Londei 2010). In North America, population sizes have increased, and ranges of many corvids have spread into formerly unoccupied regions (Restani et al. 2001, Kelly et al. 2002, Kristan and Boarman 2003, Marzluff and Neatherlin 2006), primarily aided by anthropogenic resources (Liebezeit and George 2002). These increases in corvid abundance have likely contributed to declines in their prey (Lauro and Tanacredi 2002,

Kelly et al. 2005, Webb and Marzluff 2007, Klausen et al. 2010), especially threatened and endangered taxa such as Desert Tortoises (Gopherus agassizii, Kristan and Boarman

2003), Marbled Murrelets (Brachyrampus marmoratus, Peery and Henry 2010), Greater

Sage-Grouse (Centrocercus urophasianus, Coates et al. 2008), and California Least

Terns (Sterna antillarum browni, Caffrey 1995). Corvids are adept predators of eggs and young of many bird species, which is widely recognized (Ricklefs 1969, Martin 1993) as an important ecological factor that may limit population sizes in some species.

Therefore, understanding corvid behavior in response to potential deterrents can be useful for determining the most effective methods for reducing predation and increasing productivity of threatened and endangered species.

In 1993, the United States Fish and Wildlife Service (USFWS) listed the Pacific coast population of the Snowy Plover (Charadrius nivosus), hereafter plover, as threatened under the Endangered Species Act (USFWS 1993). The recovery plan for the

plover (USFWS 2007) identified several factors that were thought to limit the population.

Principal among these factors was predation of eggs and chicks by native vertebrate predators, especially Common Ravens (Corvus corax) and American Crows (C.

brachyrhynchos). In an effort to ameliorate the negative effects imposed by corvids on

the breeding productivity of plovers, managers have protected nests with predator

exclosures (Hardy and Colwell 2008), restored nesting habitat by removing invasive

European beach grass (Ammophila arenaria) (Muir and Colwell 2010), spread oyster

shells to increase egg and chick crypsis (United States Department of the Interior Bureau

of Land Management 2010), and killed predators (Neuman et al. 2004). Despite these

efforts, Snowy Plover population size remains below the region-wide recovery goal of

3,000 breeding adults (USFWS 2007, Eberhart-Phillips 2012).

In northern California, where extensive monitoring began in 2001, plovers have

been observed breeding at approximately 20 locations (Colwell et al. 2010a). The

leading cause of reproductive failure across breeding sites in northern California is

predation of eggs by corvids, as evidenced by a negative correlation between raven

activity and per capita reproductive success of plovers (Burrell and Colwell 2012, Burrell

2010). Two adjacent sites (Clam Beach and Mad River Beach) stand out among the 20

breeding sites because most plovers breed there and nest predation rates are high (Hardy

and Colwell 2012), presumably because of high corvid activity (Burrell and Colwell

2012, Appendix A). In 2008 and 2009, video cameras confirmed that ravens caused 70%

of plover nest failures at Clam Beach (Colwell et al. 2009, Burrell and Colwell 2012).

This situation is exemplar of a threatened native species in conflict with an abundant

native species.

To remedy the negative impacts of corvids on plovers, a variety of lethal and non- lethal methods have been used with varying degrees of success. Lethal predator control is often controversial and lacks public support (Messmer et al. 1999). Moreover, lethal

control may not be effective at minimizing negative impacts of corvids on reproductive

success if control does not reduce predator population size or fails to remove predators that are directly responsible for losses of plover eggs and chicks. For example, lethal removal of individuals may be a temporary solution if conspecifics replace corvids on vacant territories. In either case, lethal control may not be sufficient to significantly increase reproductive success (Donehower et al. 2007). Additionally, the residual effects of removing predators are often unknown and, as with other predator control methods, may only provide a short term solution without the desired effect of ultimately increasing breeding bird population sizes (Côte and Sutherland 1997). Before the final decision to kill predators is made, other options of predator management should be explored.

Although lethal methods have been used in an effort to increase reproductive success of

Snowy Plovers elsewhere along the Pacific coast (e.g., Lauten et al. 2009, Robinson-

Nilsen et al. 2009), results are mixed with regard to their effectiveness (Neuman et al.

2004); lethal methods have not been used in Humboldt County.

A variety of non-lethal control methods have been used by wildlife managers to deter avian predators, including scare tactics, repellents, and nest exclosures. Each of these methods have shortcomings and often only provide short term benefits

(Schmelzeisen et al. 2004, Hardy and Colwell 2008, Pauliny et al. 2008), whereas other

methods such as conditioned taste aversion have proven unsuccessful (Catry and

Granadeiro 2006). Effigies are another method of non-lethal predator control. Effigies are carcasses, taxadermic preparations, or artificial bird models hung in unnatural positions to resemble a dead organism; they are used to scare birds and deter their use of specific areas. Effigies have been used with gulls (Larus spp.), turkey vultures

(Cathartes aura), ravens, and crows to deter roosting in undesirable locations such as

airports and urban areas (Avery et al. 2008, Ball 2009, Caffrey 1995, Seamans 2004).

Effigies have also been reported as successful corvid deterrents at California Least Tern

breeding colonies (Caffrey 1995).

I investigated the effects of the presence of effigies on the activity of Common

Ravens and American Crows at a Snowy Plover breeding site where predation of eggs

and chicks by corvids is especially problematic (Burrell and Colwell 2012). I conducted

my study at Clam Beach, Humboldt County, CA, which hosts the majority of the Snowy

Plover population in Recovery Unit 2 (i.e., Del Norte, Humboldt, and Mendocino

Counties, USFWS 2007; Appendix B) year round (Colwell et al. 2010a). If the presence

of effigies was effective, I predicted that there would be a lower abundance of corvids in

treatment plots with effigies than in control plots.

MATERIALS AND METHODS

Study Area

I studied corvid response to effigies at Clam Beach, Humboldt County (Figure 1),

which hosts one of the highest abundances of corvids among all sites where plovers breed

in Recovery Unit 2 (Appendices A and B). Clam Beach also has the lowest plover

productivity in Recovery Unit 2. Clam Beach extends for approximately 7 km between

the mouths of Little River and Mad River. Vegetation at the site is dominated by

invasive European beach grass (Ammophila arenaria); native plants such as searocket

(Cakile maritima), sand verbena (Ambronia umbellate), and dune grass (Leymus mollis)

are also common. Unvegetated substrates consist mostly of homogeneous sand littered

with shells, woody debris, eel grass bundles, and brown algae (Hardy and Colwell 2012).

Of all plover breeding sites, Clam Beach has the highest level of human use (Burrell

2010), including recreational activities such as jogging, clamming, dog walking, and

horseback riding. As a result, the site has a comparatively large amount of anthropogenic

garbage (Colwell M.A., unpublished data).

Effigies

I acquired Common Raven carcasses from the Humboldt Wildlife Care Center.

These birds either died at the center, or were euthanized due to severe trauma. I prepared

effigies of ravens, with wings and tail feathers spread to portray an unnatural position of a dead bird.

Figure 1. Paired control (C) and treatment (T) plots were placed every 500 m (horizontal dashes), separated from each other by 25 m, with observer located at midpoint of plots (X) on Clam Beach, Humboldt County, CA.

Data Collection

Before effigies are used during the plover breeding season, the behavioral

response of corvids to effigies should be studied. I conducted this study from September

2011, after the plover breeding season, through February 2012 before the earliest nests

were initiated by plovers (Colwell et al. 2010b). To investigate the effectiveness of

effigies, I used a before-after/control-impact study design with paired control and

treatment plots (Figure 1). During a 4 day interval (i.e., a trial), I observed corvids on paired plots for 4 hours beginning at sunrise. The trial consisted of one day in which I baited both plots with food (i.e., before), followed by three days of treatment during which I hung the effigy from a pole and played raven distress calls on a randomly chosen experimental plot (i.e., after). I placed informative signs for the public at beach access points and at the north and south boundaries of my study plots every day of observation.

I used ArcGIS v.9.3 to subdivide the 7 km length of Clam Beach into 14 500-m segments

(Figure 1), and randomly assigned each segment to a trial date. I established paired control and treatment plots in the foredunes north and south of the center of each 500 m section, with the plot edges located 25 m from each other. I placed small pieces of woody debris already present at the plots 1 m, 10 m, and 50 m from the center of each plot as visual markers for observation. I documented corvid activity while hidden at the midpoint between these two plots (Figure 1).

To attract corvids, I placed a white paper bag, a large container of french fries, and a large soft drink cup in the center of the plots each day before I started observations.

I hung the effigy from a 0.5 m plastic pipe attached horizontally to a 2.5 m metal pole

8 placed in the center of the plot. An identical pole was also placed in the control plot.

Birds may learn that an object is harmless and subsequently ignore it (Godin 1994,

Schmelzeisen et al. 2004, personal communication, J. Marzluff 2011. Box 352100,

School of Environmental and Forest Sciences, College of The Environment, University of Washington, Seattle, WA 98195). It is important to test if corvids continue to utilize an area in the presence of a risk factor (personal communication, J. Marzluff 2011. Box

352100, School of Environmental and Forest Sciences, College of The Environment,

University of Washington, Seattle, WA 98195); therefore I observed corvid activity for 3 consecutive days after the effigy was hung. Since the effectiveness of scaring devices can be enhanced with the use of loud noises to reduce the potential for habituation (Godin

1994), I supplemented the effigies with scare tactics. On the first treatment day (i.e., Day

2) for each plot before the effigy was hung, an assistant wearing a mask to eliminate facial recognition by corvids, acted out a theatrical death scene at the center of the plot.

During this scene the assistant yelled while shaking the effigy, threw the effigy on the ground, and pretended to shoot it while a speaker broadcasted loud gunshots. After the scene, the assistant hung the effigy and started playing a continuous loop of raven distress calls (Cornell Lab of Ornithology Macaulay Library), which was broadcast for the duration of the observation period. After each morning observation period, I removed all items from the plots.

I observed the plots from a vantage point that maximized observation of both plots and minimized the likelihood that corvids were aware of my presence (Marzluff and

Angell 2005) by sitting in a blind hidden in the dune grass. I used an instantaneous

sampling method (Martin and Bateson 2007) to record corvid presence within the paired

plots. I alternated observations between control and treatment plots every minute, recording the number of corvids within 1 m, 10 m, and 50 m from the bait/effigy. I

summarized both the number of corvids present within each radius (1, 10, and 50) and the proportion of time that a corvid was present (incidence) within each radius for both the

control and treatment plots. I conducted research under state (Department of Parks and

Recreation #11-635-020), county (Humboldt County Parks), and university (IACUC

#10/11.W.53-A) permits.

Data Analysis

To evaluate the effectiveness of the attractant (i.e., garbage and food) on Day 1, I

examined the average time (min) required for the first corvid to respond (response time)

to the attractant and enter the 1 m, 10 m, and 50 m radius plots. I also calculated the

average number of corvids initially present in plots at each of the 3 spatial scales. I

calculated average number and average incidence (proportion of n = 120 of observations

with at least one corvid in a plot) of corvids within a 1 m, 10 m, and 50 m radius of the plot center during each trial for both treatment and control plots (Appendices C-E) for my analyses. The response variables were average number of corvids and average incidence of corvids; the predictor variable was the presence of effigies.

Following the methods of Tarr et al. (2010), I used the formula Di=(XIAi – XIBi) –

(XCAi – XCBi) to calculate the difference between the change in corvid activity at the

control plot and the treatment plot. Within this formula, Di denotes the treatment effect,

XIA and XIB denote the mean responses at treatment plot before and after treatment, and

XCA and XCB denote the mean responses at the control plot before and after treatment. To

evaluate these data I used the paired differences in paired t-tests. Using a Spearman’s rank correlation test, I performed a post-hoc analysis to detect a difference in treatment effect over time (trial sequence). I also used an ANOVA to detect a difference among

treatment Days 2, 3, and 4 for average number and average incidence of corvids present

at each spatial scale.

RESULTS

I conducted trials on 14 paired control and treatment plots between 17 September

2011 and 28 February 2012 (Appendices D-F). To increase my sample size, I conducted

a second trial on 4 sections of beach, resulting in a total of 18 trials. I observed only

ravens in the majority (83%) of trials; both ravens and crows occurred in 17% of trials.

Crows occurred in the most northern plots near Little River, and the plots located west of

the two parking lots managed by Humboldt County Parks and Recreation. These

observations are consistent with data collected on Clam Beach during the plover breeding

season, which shows higher crow abundance and incidence at these locations (Appendix

F).

Most trials (97%) were affected by non-corvids that sometimes consumed bait

(Appendix G). Control and treatment plots were visited by dogs (78% and 78%,

respectively), humans (78% and 83%), and gulls (83% and 67%). The bait was

completely consumed or removed in control and treatment plots by dogs (44% and 17%,

respectively), humans (6% and 6%), and gulls (72% and 67%).

Since there was no significant difference at each spatial scale (all P-values >0.38)

for the amount of time it took for a corvid to appear in a plot, I pooled all trials for

control and treatment plots accordingly to increase sample size (n = 36) (Table 1). On

Day 1, corvids approached to within 50 m of the food on average in 53 + 2 minutes,

within 10 m of the food in 111 + 3 minutes, and within 1 m of the food in 136 + 4

12 minutes. On average, corvids took 2-2.5 times longer to approach within 1 and 10 m of the bait than 50 m of the bait.

Table 1. Summary of the average response time and number of corvids occurring on Day 1 of 18 four-day trials, when I provisioned plots with food but no effigy.

Corvid Response

Distance Avg. # of minutesa SE Avg. # of corvidsb SE

50 m 52.94 1.47 1.39 0.04

10 m 111.28 3.09 0.83 0.02

1 m 135.72 3.77 0.61 0.02

a Average time it took for at least one corvid to arrive in study plot. b Average number of corvids present at initial response to attractant.

After the effigy was hung, (i.e., Days 2, 3, and 4) the average number and

incidence of corvids on treatment plots (for the three treatment days combined) decreased

by 81% and 70%, respectively, within 50 m of an effigy (Appendix C). The average

number and incidence of corvids present within 10 m was reduced by 96% and 93%,

respectively (Appendix D). The average number and incidence of corvids present within

1 m was reduced by 96% and 94%, respectively (Appendix E). However, there was only

a significant treatment effect within a 50 m radius of the effigy (Table 2).

The effectiveness of effigies within 50 m did not change over the progression of

this study (i.e., trial sequence). There was no significant treatment effect in average

number (rs = 0.27, P = 0.87) or incidence (rs = 0.27, P = 0.86) of corvids with respect to

trial sequence (i.e., #1-18). There was a noticeable treatment effect (difference between

control and treatment plots) during the first four trials, conducted in September and early

October (Figure 2). However, with each subsequent trial this value was reduced. In

January and February, there was again a noticeable treatment effect; however these

paired plots were located in the northern most section near Little River and adjacent to a parking lot, locations that commonly have a large number of crows present (Appendix F).

With crow activity removed from trials, there was still no significant treatment effect in

average number (rs = 0.27, P = 0.87) or incidence (rs = 0.27, P = 0.86) of ravens with respect to trial sequence. There was also no significant difference in treatment effect

among trial Days 2, 3, and 4 for average corvid abundance and incidence within 1m,

10 m, and 50 m of an effigy.

Table 2. Summary of the treatment effects of average number and average incidence (proportion of time) of corvids present at varying distances from center of paired treatment and control plots on Days 2, 3, and 4 when I hung the effigy.

Abundance Treatment Effect Incidence Treatment Effect

a Distance Average Di SE t.s. df P Average Di SE t.s. df P

50 m -0.134 0.042 -3.16 17 0.006 -0.112 0.043 -2.58 17 0.019

10 m -0.031 0.028 -1.10 17 0.287 -0.073 0.038 -1.90 17 0.074

1 m -0.016 0.021 -0.75 17 0.461 -0.029 0.018 -1.66 17 0.115

a Di=(XIAi – XIBi) – (XCAi – XCBi), the difference between the change in corvid activity at the control plot and the treatment plot.

Figure 2. Treatment effect of effigies on average number and incidence of corvids within 50 m for each trial, in sequence of trial date. 16

DISCUSSION

Two main results emerge from my study. First, although effigies significantly reduced corvid abundance and incidence within 50 m, the biological significance and wildlife management application of these results are questionable given that corvids still visited plots to scavenge food. Second, the garbage and food I used as bait readily attracted corvids to my study plots, suggesting that they regularly scavenge beaches.

This finding can be especially problematic and should be considered when managing for breeding plovers at high human-use beaches that also have high abundances of corvids.

Although there were several limitations to my study and effigies are not likely to be effective at Clam Beach, there is a potential for effigy use as a nonlethal predator control method in other areas.

My study is the first to experimentally test the effectiveness of effigies as a non- lethal method of reducing predator activity within the breeding habitat of a threatened species. Effigies significantly reduced corvid abundance and incidence, but this effect was confined to the 50 m radius plot. Corvids still visited control and treatment plots at the 1 m and 10 m spatial scale, with no significant treatment effect. My observations at these smaller spatial scales are dominated by many zeroes, which are likely due to corvids taking longer to enter plots closer to the effigy. In addition, the treatment effect appeared to diminish over time, although this effect was not statistically significant.

Corvids are intelligent (Emery 2006), highly social birds that transmit knowledge culturally via interactions with conspecifics (Marzluff and Angell 2005). Effigies were 17

likely effective at the beginning of this study as a novel item, but as the season

progressed, information of the effigy presence was likely passed on to other corvids in

the area potentially explaining the appearance of a diminishing treatment effect with

subsequent trials. This study potentially could have studied the effectiveness of effigies

with a larger sample size by using only one treatment day per trial, since there was no

significant difference in treatment effect among trial Days 2, 3, and 4 at each spatial

scale.

My results conflict with reports that effigies are effective at keeping corvids out

of undesirable locations (Avery et al. 2008) such as nesting areas (Caffrey 1994). The

discrepancies in my results with published literature are likely due to differences in

location and data collection methods. For example, my study took place at a corvid

foraging site, whereas reports of successful effigy use took place at corvid roosting sites

(Avery et al. 2008). Corvid behavior, specifically willingness to enter an area, may be

different at a foraging site than a roosting site because there is a potential food reward

located at a foraging site. In addition, I collected data with a scientific study design,

whereas the report of successful effigy use as a non-lethal predator control method was merely based on verbal accounts of reduced corvid activity (Caffrey et al. 1994). Thus, my data are a stronger indication of effigy effectiveness as a nonlethal predator control method.

My examination of bait effectiveness further supports findings that corvids respond to anthropogenic resources such as food and trash. Corvids responded to the presence of food and trash within 1-2 hours on Day 1 of the trial and they continued to

19 visit control and treatment plots in similar abundance on subsequent days of each trial.

Corvids were attracted to my study plots despite disturbance and bait consumption and/or removal (food removed but garbage remained) by dogs, humans, and gulls. A positive correlation between human presence and corvids has also been evidenced at Clam Beach by an association of corvid tracks with human tracks (Colwell, unpubl. data). Similar to data collected during the breeding season (Appendix F), crows visited my study plots that were located adjacent to parking lots and camping sites. Corvid abundance has been shown to be positively correlated with human settlements and campgrounds, presumably because food is readily available (Kristan and Boarman 2003, Marzluff and Neatherlin

2006, Withey and Marzluff 2009). The rapid response of corvids to anthropogenic resources evidenced in my study may place plover nests at a greater risk of predation, since the presence of human trash and food in Clam Beach nesting areas can potentially attract corvids to the immediate vicinity of a plover nest.

In northwestern California, ravens and crows are abundant, and raven populations have continued to increase (George 2009). Data collected over six years indicate that corvid abundance differed at plover breeding site locations. Ravens and crows are 2.5 to

4 times more abundant at Clam Beach and Mad River Beach (two adjacent sites) than at the other two beaches (Centerville Beach and Eel River Wildlife Area) where plovers have bred occasionally at low densities (Colwell et al. 2011). The high prevalence of corvids coupled with the highest proportion of breeding plovers at Clam Beach emphasizes the need for effective predator management at this site. The high abundance of corvids poses threats to plovers, as evidenced by a positive correlation between

predation of plover eggs and chicks and corvid abundance (Burrell and Colwell 2012). If

Recovery Unit 2 is to reach the target population size of 150 breeding adult plovers

(USFWS 2007), predators, specifically corvids, should be managed at sites that host large

numbers of breeding plovers. Since Clam Beach hosts both a high number of corvids and

the highest number of breeding plovers, corvid attractants should be looked at more

closely and rigorously removed.

Several unforeseen problems may have affected experimental results of the effigy experiment. First, although I used signage to inform the public and reduce impacts on data collection, humans and their dogs still entered plots and ate or removed food that I had placed there as an attractant. As a result, on most days, the effectiveness of the attractant may have been compromised, leading to lower corvid abundances. There was no detectable difference, however, in loss of bait from experimental and control plots.

Testing the effectiveness of effigies would be performed best at a secluded site without the interference of the public. Second, the choice of attractant may have also affected the outcome of this study. On roughly half of trials, the french fries were consumed by gulls and dogs in control (54%) and treatment plots (43%), leaving only trash to attract corvids.

Corvids visiting plots may not have remained at the site or returned on subsequent days if

there was no initial reward. Since single or paired ravens often recruit groups of ravens

to food sources (Heinrich 1988, Wright et al. 2003), the information of no reward may have been passed on to other individuals of the population. This information transfer may have affected the corvid abundance and incidence I recorded on subsequent observation days. Using a carcass, which would have persisted as an attractant for the

entire observation period, may have been a better option. A third limitation of this study

is that I collected data from September through February, which is the nonbreeding

season for plovers. Corvid population dynamics and distribution including foraging

behavior may be different during the winter (Heinrich 1988, Preston 2005) than at other

times of the year (Kristan and Boarman 2003, Roth et al. 2004). Therefore, with respect

to using effigies during the plover breeding season, the responses observed during this

study may not be representative of corvid behavioral responses during the plover

breeding season. If I were to fairly conclude that effigies were ineffective, I would need

to study the use of effigies during the plover breeding season. However, due to the

paucity of information regarding corvid behavior associated with effigies, an initial study

needed to be conducted before the use of effigies in the vicinity of a breeding and

threatened species could be warranted.

Management Implications

Corvids continued to enter plots at all three spatial scales after the effigy was

hung, which suggests effigies may not protect plover nests from predation. Although

there was a significant treatment effect at the 50 m scale, which suggests effigies may be

effective in preventing corvids from entering a large area, this effect was relatively small compared to mean corvid abundance and corvids still entered treatment plots after the effigy was hung. Corvids entering areas at any of the three spatial scales poses threats to

plover nests; therefore if effigies are to be used as a predator management tool they need

to be effective at reducing corvid activity all spatial scales. In addition, the high occurrence of human activity at Clam Beach creates an environment unsuitable for effigy

22 use since the high level of human interaction with the effigies in this study (n = 23) could place plover nests at risk by potentially attracting more humans and their dogs to plover nesting areas during the breeding season. Effigy use and/or studies of their effectiveness may be more suitable for plover nesting sites with little to no human use such as certain plover breeding sites located in Washington (Pearson et al. 2010).

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Appendix A. Annual (March through August) averages (+ SD) of corvid abundance and incidence (proportion of point counts with at least one corvid) on the four main breeding sites used by Snowy Plovers in Humboldt County, CA.

Year 2007 2008 2009 2010 2011 Total Site Avg. # Inc. Avg. # Inc. Avg. # Inc. Avg. # Inc. Avg. # Inc. Avg. # Inc.

Clam Beach 2.0+4.4a 0.5+0.5 1.5+2.6 0.5+0.5 1.5+3.3 0.5+0.5 1.4+2.4 0.5+0.5 2.1+3.5 0.5+0.5 1.7+3.4 0.5+0.5

Mad River 2.9+4.3 0.7+0.5 2.4+3.5 0.7+0.5 2.1+2.7 0.6+0.5 1.6+3.1 0.5+0.5 1.8+2.7 0.6+0.5 2.1+3.3 0.6+0.5

Eel River 0.2+1.0 0.1+0.3 0.4+1.3 0.2+0.4 0.8+2.0 0.2+0.4 0.8+1.5 0.4+0.5 0.4+1.2 0.2+0.4 0.5+1.5 0.2+0.4 Wildlife Area

Centerville 0.9+1.7 0.4+0.5 0.2+0.6 0.2+0.4 0.3+1.0 0.1+0.3 0.5+1.4 0.2+0.4 0.5+2.4 0.2+0.4 0.5+1.5 0.2+0.4

a Data collected during point counts performed every 20 minutes during which total number of ravens and crows present within a 500 m radius of observer were recorded. See Colwell et al. (2010), Burrell and Colwell (2012), and Hardy and Colwell (2012) for details on field methods.

Appendix B. Summary of plover distribution and reproductive success during the months of March through August, Humboldt County, CA.

Year

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

Total Breeding Plovers 60a 63 54 72 63 57 30 30 19 31 36 36

% Breeding at Clam Beach 16 29 38 40 49 53 56 68 63 52 56 67

Number of Nests at Clam Beach 10 35 31 35 36 39 31 39 25 24 18 29

Apparent Nest Success (%) at Clam Beach 64 26 35 72 96 57 15 52 16 8 77 28

% Nest Failures Caused by Corvids 7 16 23 26 12 19 27 28 31 19 13 21

Per capita male fledging successb 1.4 0.22 0.5 1.25 1.25 0.38 0.17 0 0 0 0.36 0.9

a Total breeding plovers in Recovery Unit 2. b Average number of young fledged per male at Clam Beach. See Colwell et al. (2010) for details on field methods.

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Appendix C. Summary of average (+ SD) corvid abundance and incidence within 50 m of the center of the plot where food and an effigy or control pole were placed in 18 trials of paired plots.

Avg. corvid abundance Avg. corvid abundance Avg. corvid incidence Avg. corvid incidence Before After Before After Trial Treatment Control Treatment Control Treatment Control Treatment Control 1 0.86+1.18a 0.39+0.95 0.24+0.56 0.03+0.17 0.48+0.50 0.20+0.40 0.21+0.41 0.04+0.17 2 0.47+0.63 0.20+0.51 0.18+0.53 0.36+0.78 0.39+0.49 0.15+0.36 0.12+0.33 0.61+0.41 3 0.25+0.44 0.03+0.16 0.04+0.21 0.18+0.38 0.25+0.44 0.03+0.16 0.03+0.18 0.18+0.38 4 0.98+1.87 0.77+1.35 0.19+0.42 0.17+0.45 0.33+0.47 0.34+0.48 0.18+0.39 0.41+0.36 5 0.20+0.53 0.28+0.48 0.06+0.30 0.11+0.42 0.16+0.37 0.26+0.44 0.05+0.22 0.14+0.27 6 0.64+2.01 0.48+1.20 0.11+0.38 0.03+0.19 0.20+0.40 0.20+0.40 0.08+0.28 0.04+0.17 7 0.08+0.28 0.02+0.13 0.03+0.19 0.04+0.27 0.08+0.28 0.02+0.13 0.02+0.14 0.03+0.16 8 0.16+0.53 0.08+0.33 0.01+0.09 0.02+0.15 0.11+0.31 0.07+0.25 0.01+0.09 0.09+0.15 9 0.19+0.64 0.18+0.81 0.03+0.22 0.03+0.31 0.10+0.30 0.06+0.24 0.03+0.16 0.12+0.11 10 0.03+0.26 0.04+0.20 0.01+0.11 0.00+0.05 0.02+0.13 0.04+0.20 0.01+0.11 0.00+0.05 11 0.03+0.37 0.00+0.00 0.00+0.00 0.02+0.14 0.01+0.09 0.00+0.00 0.00+0.00 0.02+0.14 12 0.02+0.13 0.01+0.09 0.02+0.15 0.00+0.00 0.02+0.13 0.01+0.09 0.01+0.12 0.00+0.00 13 0.03+0.18 0.03+0.18 0.01+0.14 0.01+0.09 0.03+0.18 0.03+0.18 0.01+0.11 0.01+0.09 14 0.03+0.16 0.05+0.31 0.00+0.00 0.15+0.88 0.03+0.16 0.03+0.18 0.00+0.00 0.03+0.18 15 0.17+0.56 0.10+0.44 0.00+0.05 0.06+0.31 0.12+0.32 0.06+0.24 0.00+0.05 0.04+0.19 16 0.63+2.15 0.25+0.82 0.06+0.44 0.30+0.75 0.13+0.33 0.13+0.33 0.03+0.17 0.17+0.37 17 0.17+0.77 0.25+0.95 0.01+0.07 0.04+0.23 0.09+0.29 0.08+0.28 0.01+0.07 0.03+0.17 18 0.28+0.16 0.18+0.24 0.02+0.17 0.01+0.16 0.14+0.16 0.10+0.24 0.01+0.11 0.01+0.09

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Appendix D. Summary of average (+ SD) corvid abundance and incidence within 10 m of the center of the plot where food and an effigy or control pole were placed in 18 trials of paired plots.

Avg. corvid abundance Avg. corvid abundance Avg. corvid incidence Avg. corvid incidence Before After Before After Trial Treatment Control Treatment Control Treatment Control Treatment Control 1 0.44+0.87 0.14+0.58 0.01+0.12 0.00+0.05 0.29+0.46 0.08+0.26 0.01+0.12 0.03+0.05 2 0.24+0.57 0.10+0.30 0.01+0.09 0.08+0.38 0.18+0.38 0.10+0.30 0.01+0.09 0.47+0.22 3 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 4 0.52+1.17 0.33+0.91 0.01+0.07 0.08+0.27 0.26+0.44 0.15+0.36 0.01+0.07 0.32+0.27 5 0.01+0.09 0.08+0.31 0.00+0.00 0.01+0.11 0.01+0.09 0.08+0.26 0.00+0.00 0.12+0.11 6 0.03+0.16 0.03+0.22 0.00+0.00 0.00+0.05 0.03+0.16 0.03+0.16 0.00+0.00 0.04+0.05 7 0.03+0.16 0.00+0.00 0.00+0.00 0.00+0.00 0.03+0.16 0.00+0.00 0.00+0.00 0.00+0.00 8 0.10+0.49 0.06+0.27 0.01+0.09 0.02+0.15 0.05+0.22 0.05+0.22 0.01+0.09 0.06+0.15 9 0.11+0.46 0.14+0.73 0.03+0.22 0.03+0.31 0.07+0.25 0.05+0.22 0.03+0.16 0.09+0.11 10 0.00+0.00 0.04+0.20 0.01+0.11 0.00+0.05 0.00+0.00 0.04+0.20 0.01+0.11 0.00+0.05 11 0.01+0.09 0.00+0.00 0.00+0.00 0.00+0.00 0.01+0.09 0.00+0.00 0.00+0.00 0.00+0.00 12 0.01+0.09 0.00+0.00 0.00+0.00 0.00+0.00 0.01+0.09 0.00+0.00 0.00+0.00 0.00+0.00 13 0.00+0.00 0.03+0.16 0.00+0.00 0.00+0.00 0.00+0.00 0.03+0.16 0.00+0.00 0.00+0.00 14 0.01+0.09 0.03+0.18 0.00+0.00 0.02+0.26 0.01+0.09 0.03+0.18 0.00+0.00 0.01+0.09 15 0.04+0.20 0.04+0.30 0.00+0.00 0.00+0.05 0.04+0.20 0.03+0.16 0.00+0.00 0.00+0.05 16 0.16+0.94 0.12+0.47 0.00+0.00 0.00+0.07 0.05+0.22 0.08+0.26 0.00+0.00 0.00+0.07 17 0.00+0.00 0.20+0.85 0.00+0.00 0.00+0.00 0.00+0.00 0.07+0.25 0.00+0.00 0.00+0.00 18 0.09+0.09 0.08+0.00 0.00+0.00 0.00+0.00 0.06+0.09 0.04+0.00 0.00+0.00 0.00+0.00

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Appendix E. Summary of average (+ SD) corvid abundance and incidence within 1 m of the center of the plot where food and an effigy or control pole were placed in 18 trials of paired plots.

Avg. corvid abundance Avg. corvid abundance Avg. corvid incidence Avg. corvid incidence Before After Before After Trial Treatment Control Treatment Control Treatment Control Treatment Control 1 0.24+0.64 0.02+0.13 0.00+0.00 0.00+0.00 0.18+0.38 0.02+0.13 0.00+0.00 0.01+0.00 2 0.17+0.49 0.08+0.28 0.00+0.00 0.03+0.20 0.12+0.32 0.08+0.28 0.00+0.00 0.17+0.17 3 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 4 0.33+1.01 0.25+0.78 0.00+0.00 0.07+0.26 0.14+0.35 0.13+0.34 0.00+0.00 0.16+0.26 5 0.00+0.00 0.06+0.24 0.00+0.00 0.00+0.05 0.00+0.00 0.06+0.24 0.00+0.00 0.02+0.05 6 0.03+0.16 0.01+0.09 0.00+0.00 0.00+0.05 0.03+0.16 0.01+0.09 0.00+0.00 0.01+0.05 7 0.03+0.16 0.00+0.00 0.00+0.00 0.00+0.00 0.03+0.16 0.00+0.00 0.00+0.00 0.00+0.00 8 0.10+0.49 0.04+0.24 0.01+0.09 0.02+0.14 0.05+0.22 0.03+0.18 0.01+0.09 0.04+0.14 9 0.08+0.39 0.08+0.54 0.03+0.21 0.03+0.25 0.04+0.20 0.03+0.18 0.02+0.15 0.06+0.11 10 0.00+0.00 0.04+0.20 0.01+0.11 0.00+0.00 0.00+0.00 0.04+0.20 0.01+0.11 0.00+0.00 11 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 12 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 13 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 0.00+0.00 14 0.00+0.00 0.01+0.09 0.00+0.00 0.01+0.12 0.00+0.00 0.01+0.09 0.00+0.00 0.01+0.07 15 0.03+0.18 0.01+0.09 0.00+0.00 0.00+0.00 0.03+0.18 0.01+0.09 0.00+0.00 0.00+0.00 16 0.07+0.36 0.11+0.45 0.00+0.00 0.00+0.00 0.04+0.20 0.08+0.26 0.00+0.00 0.00+0.00 17 0.00+0.00 0.19+0.84 0.00+0.00 0.00+0.00 0.00+0.00 0.06+0.24 0.00+0.00 0.00+0.00 18 0.06+0.09 0.05+0.00 0.00+0.00 0.00+0.00 0.04+0.09 0.03+0.00 0.00+0.00 0.00+0.00

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Appendix F. Summary of average (+ SD) crow and raven abundance and incidence present in 500 m segments (reported south to north) of Clam Beach, 2006-2011.

Crows Ravens

Section Avg. # Avg. Incidence Avg. # Avg. Incidence

1 0.020 + 0.172a 0.015 + 0.122 1.575 + 2.870 0.490 + 0.501

2 0.004 + 0.060 0.004 + 0.060 1.333 + 1.958 0.495 + 0.501

3 0.012 + 0.144 0.008 + 0.091 1.141 + 1.783 0.440 + 0.497

4 0.018 + 0.163 0.013 + 0.115 1.084 + 2.104 0.409 + 0.493

5 0.043 + 0.433 0.014 + 0.120 0.928 + 1.716 0.384 + 0.488

6 0.304 + 0.849 0.156 + 0.364 2.000 + 4.908 0.422 + 0.496

7 0.705 + 1.515 0.261 + 0.440 1.342 + 2.621 0.410 + 0.493

8 0.596 + 1.706 0.202 + 0.403 1.606 + 2.779 0.465 + 0.500

9 0.208 + 0.774 0.098 + 0.298 1.588 + 2.915 0.475 + 0.472

10 0.150 + 0.573 0.083 + 0.277 2.042 + 3.831 0.479 + 0.501

11 0.038 + 0.245 0.027 + 0.162 2.103 + 4.882 0.553 + 0.498

12 0.018 + 0.156 0.015 + 0.123 2.021 + 4.434 0.466 + 0.500

13 0.008 + 0.089 0.008 + 0.089 1.544 + 3.896 0.393 + 0.489

14 0.176 + 0.671 0.091 + 0.288 0.997 + 2.434 0.347 + 0.477

a See Colwell et al. (2010) for details on field methods.

Appendix G. Summary of dog (d), gull (g), and human (h) presence in control and treatment plots throughout the duration of trials. Bold letters indicate all bait (fries) was consumed, and italicized letters indicate partial amount of bait was consumed.

Control Treatment

Day

Trial 1 2 3 4 1 2 3 4

1 d, h d, h d, h d, h d, h d, h d, h d, h

2 d d, h 3 d 4 d, g, h d, h d, g, h d, g, h d, h d, h d, h d, g, h

5 g d, h d, g d, g, h g d d, g, h 6 d, g, h d, g, h d, g, h d, g, h d, g, h g, h g 7 d, g, h d, g, h g g, h g 8 d, g, h d, g, h d, g, h d, g, h d, g, h d, g, h d, g, h g, h 9 d, g, h d, g, h d, g, h d, g, h d, g, h d, g, h d, g, h d, g, h 10 d, g, h d, g, h d, g, h g d, g, h d, g, h d, g, h g 11 d, g, h d, h g, h d, g, h d, h h 12 d, g, h d, g, h g, h d, g, h d, g, h d, g, h g, h d, g, h 13 d, g, h d, g, h d, g, h g d, g, h d, g, h d, g, h g 14 d, g d, g, h d, g, h g, h d, g d, g, h d, g, h g, h 15 d, g, h d, g, h d, g, h d, g, h d, g, h d, g, h d, g, h d, g, h 16 d, g, h d, h d, g, h d, g, h d, g, h d, g, h 17 g g g d, h g g g h 18 g g d, h g g