Table Of ContentWilson Bull, 115(2), 2003, pp. 176-185
DIURNAL AND NOCTURNAL BEHAVIOR OF BREEDING
AMERICAN AVOCETS
MATTHEW JOHNSON,’ 2.4 jON P. BECKMANN,’^ AND LEWIS W. ORING’
—
ABSTRACT. We studiednocturnalanddiurnalbehaviorofbreedingAmericanAvocets(Recurvirostraamer-
icana) at the Jay Dow, Sr. Wetlands in the northwestern Great Basin, USA. Seven-day observationperiods were
centered on two full moons when ambient light was maximal and auxiliary lighting unnecessary. We recorded
avocet density, habitat use, interbird distances, and behavior three times daily (beginning at 07:00, 15:00, 23:
00 PST) for 14 days. We calculated the mean proportion ofindividuals within flocks engaged in four behavior
classes (foraging, copulating, agonistic, other). Foraging birds were further subdivided by technique (pecking,
dunking, scything). Avocetscopulatedwithsimilarfrequenciesduringthemorning,afternoon,andnight.Avocets
were more aggressive and closely spaced at night than during day. The full repertoire ofbehaviors seen during
daylight also occurred at night, though frequencies ofparticularbehaviors, flock location, and interbirddistances
varied among morning, afternoon, and nighttime observations. The role of nocturnal reproductive behavior
should be assessed in species generally perceived as being diurnal. Received 12 January 2003, accepted 20
March 2003.
Nocturnal foraging throughout the annual ically and physiologically adapted to noctur-
cycle is common among coastal shorebirds in nal activity as their eyes contain a higher pro-
North and South America, Africa, and Europe portion of rods in the retina compared to di-
(Goss-Custard 1969, McNeil and Robert urnal species, and retinal sensitivity is higher
1988, Zwarts et al. 1990, McNeil 1991, Evans among birds that forage visually at night com-
and Harris 1994, Staine and Burger 1994), yet pared to those that use tactile techniques (Tan-
there is relatively little understanding of the sley and Erichsen 1985; Rojas De Azuaje et
latitudinal and seasonal variation in nocturnal al. 1993; Rojas et al. 1999a, 1999b; McNeil
behavior within and among shorebird species. et al. 1999).
Researchers have proposed that tactile forag- Although several studies have examined
ers should be relatively less affected by dark- nocturnal foraging at coastal locations, there
ness than visual foragers (Vader 1964, Goss- are few studies of shorebird nocturnal behav-
Custard 1969, Evans 1976, Dugan 1981), yet ior on the breeding grounds and at inland lo-
plovers (Charadriidae) are primarily visual cations. During the breeding season, Red-wat-
foragers that commonly forage at night (Pien- tled Lapwings {Vanellus indicus) and Stone-
kowski 1983). The relatively large eye in re- curlews (Biirhinus oedicnemus) are vocal pri-
lation to head size of plovers may enhance marily at night (Bannerman 1959), and some
visual detection ofprey (Evans 1976), yet sev- species display or call from dusk into the
eral smaller-eyed sandpipers (Scolopacidae) night (Scolopax spp.: Sheldon 1961, Krohn
also forage visually at night, including Com- 1971, Stribling and Doerr 1985; Gallinago
mon Redshanks {Tringa totcinus\ Goss-Cus- spp.: Lemnell 1978). Breeding Common
tard 1969), Semipalmated Sandpipers (Cali- Ringed Plovers {Charadrius hiaticula), and
dris pusillcv, Evans 1979), and Whimbrels Piping Plovers (C. melodus) forage at night
{Nitmenius phaeopiis\ McNeil and Rompre
(Pienkowski 1983, Staine and Burger 1994),
1995). Nocturnal bird species are morpholog-
and the role each sex plays in nocturnal in-
cubation also has been examined for several
' Dept, of Environmental and Resource Sciences, species (Laven 1940, Mundahl 1982, Warriner
Univ. of Nevada, Reno, NV 89512, USA. et al. 1986, Staine and Burger 1994, Thibault
2Current address: Dept, of Biology, Virginia Poly- and McNeil 1995a, Warnock and Oring 1996).
technic Inst, and State Univ., Blacksburg, VA 24061, American (Recurxirostra americana) and Pied
USA.
avocets {R. avosetta) forage diurnally and
^Current address: Eastern Idaho Eield Office, Wild- nocturnally throughout the annual cycle
life Conservation Society, Rigby, ID 83442, USA.
Corresponding author; e-mail: (Evans and Harris 1994, Dodd and Colwell
[email protected] 1996, Hotker 1999, Johnson et al. 2002), and
176
Johnson et al. • AMERICAN AVOCET BEHAVIOR 177
TABLE I. Akaike Information Criteria (AIC) for candidate model.s used to examine variation in American
Avocet behavior during the daily cycle at Jay Dow, Sr. Wetlands, Las.sen County, California, 2001
Numberof
Model^ parameters AAIC^’ AICweighC
Overall behavior
{T, L, D, W, L X D} 5 0.0 0.865
{T, L, D, W, T X L, T X D, L X D} 7 3.7 0.135
{T, ES, L, D, EL, W, S, I, dist, inter} 23 35.1 <0.001
Habitat use
{T, L, ES, A, T X L) 5 0.0 0.932
{T, ES, L, E, A, T X FS, T X L} 8 5.9 0.050
{T, FS, L, F, A, 5, T X FS, T X L, FS X L) 9 7.8 0.019
Interbird distance
{T, FS, L, D, EL, T X L, L X D) 7 0.0 0.999
{T, FS, L, D, EL, inter) 15 15.2 0.001
{T, FS, L, D, EL, M, A, C, F, W, S, I, inter} 22 29.0 <0.001
Foraging technique
{T, FS, L, D, EL, W, 1, T X EL, L X D} 9 0.0 0.873
{T, FS, L, D, EL, W, 1, T X EL, L X D, T X D, FS X L} 11 3.9 0.127
{T, FS, L, D, EL, W, 1, inter} 17 15.5 <0.001
for^agTin=g,trMeat=menptro(pmorotrinoinngse=lf0m7a:i0n0t,eanfatnecren,ooCn== 1p3r:o0p0o,rtniiognhtco=pu2l3a:t0i0n)g,,FAS==fplroocpkorstiizeo,nLag=onilsotciact,ioWn,D==prdoaptoer,tEioLn=ineoarpleyn/lawtaetesre,asSon,=Fp=ropporrotpioorntioonn
s5,ho6r-e,10I,=11p-r2o0p,or>t2io0nmo)n,iisnltaenrds=, a1ll=twporo-pwoarytiionnteorfacbtiirodnss.<1 mapart,5 = proportionofbirds 1-5 mapart,dist = allinterbirddistanceclasses(<1, 1-
AAICistherankofeachmodelbyrescalingthemodelwithaminimumAICvaluetozero(AAIC = AlCj - AlCmin).
AICweightsarethelikelihoodofthemodelgiventhedata(Akaikeweights).
several shorebird species, including the Amer- JDW is a 540-ha constructed wetland contain-
ican Avocet, copulate at night (Johnson et al. ing 16 bodies of water at the southern end of
2002). Honey Lake, Lassen County, California. JDW
Knowledge of nocturnal behavior is essen- includes a seep draining to Honey Lake, a nat-
tial for understanding both ecology and ener- ural playa lake, and 14 constructed ponds.
getics, yet current understanding of shorebird With the exception of the seep, water was
nocturnal behavior is limited. Shorebird hab- maintained at a constant level from March to
itat use and foraging technique varies during July. Honey Lake Basin (23,000 ha) is char-
the daily cycle (Rompre and McNeil 1995, acterized by agricultural production and by
Robert et al. 1989). Nocturnality also appears Great Basin desert scrub vegetation on gently
to affect energy balance (Pienkowski 1983) sloping to nearly level alluvial fans, flood-
and, potentially, reproductive strategy (John- plain, and basin floor. Common vegetation at
son et al. 2002). Recent technological advanc- JDW included sagebrush {Artemisia tridenta-
es in night vision equipment facilitate the ta), saltbrush (Atriplex spinosa), greasewood
study ofnocturnal behavior, permitting a more {Sarcobatus vermiculatus), rabbitbrush {Chry-
thorough approach to behavioral ecology and sothamnus spp.), saltgrass {Distichlis spicata),
allowing more accurate assessment of energy rush {Juncus spp.), and cattail (Typha latifol-
budgets, space use, and social interactions. ia). Avocets arrive at JDW during late March
The aim of this study was to describe the ex- and initiate clutches from the end of April
tent of nocturnality among American Avocets through the beginning of June (Robinson and
breeding at an inland location. Oring 1997).
—
Observations. During 2001, we centered
METHODS
7-day observation periods on two full moons
—
Study area. We studied American Avocet (3-10 May and 2-8 June) when ambient light
behavior at the Jay Dow, Sr. Wetlands (JDW; was maximal and auxiliary lighting unneces-
40° 07' N, 120° 14' W, elevation 1,220 m). sary. We used a hand-held generation III night
1
178 THE WILSON BULLETIN • Vol 115, No. 2, June 2003
TABLE 2. Multivariate analysis of variance for time ofday (07:00, 13:00, 23:00) and covariate effects on
American Avocet behavior at Jay Dow, Sr. Wetlands, Lassen County, California, 2001.
Degreesoffreedom
Wilk’s
Factor Numerator Denominator lambda F p
Overall behavior
Time ofday 8 210.00 0.278 23.52 <0.001
Date 4 105.00 0.679 12.40 <0.001
Location 28 380.01 0.434 3.54 <0.001
Proportion birds in open water 4 105.00 0.770 7.84 <0.001
Date X location 24 367.51 0.663 1.92 0.007
Habitat use
Time ofday 6 200.00 0.691 6.77 <0.001
Location 21 287.70 0.199 10.37 <0.001
Flock size 3 100.00 0.934 1.62 0.189
Proportion birds agonistic 3 100.00 0.954 1.61 0.192
Time ofday X location 36 296.19 0.568 1.73 0.008
Interbird distance
Time ofday 10 182.00 0.328 13.59 <0.001
Date 5 91.00 0.719 7.12 <0.001
Month 5 91.00 0.744 6.25 <0.001
Location 35 385.23 0.630 1.28 0.139
Flock size 5 91.00 0.728 6.79 <0.001
Location X time ofday 60 429.90 0.644 0.70 0.952
Location X date 30 366.00 0.570 1.84 0.005
Foraging technique
Time ofday 6 202.00 0.749 5.24 <0.001
Date 3 101.00 0.799 8.47 <0.001
Month 3 101.00 0.789 8.98 <0.001
Location 21 290.57 0.430 4.73 <0.001
Flock size 3 101.00 0.891 4.14 0.008
Proportion birds in open water 3 101.00 0.645 18.54 <0.001
Proportion birds <1 m apart 3 101.00 0.892 4.10 0.009
Time ofday X month 6 202.00 0.865 2.54 0.022
Location X date 18 286.16 0.717 1.98 0.01
vision scope with a 100-mm zoom lens to intraspecific encounters, and behavior class
(NiteMate NAV-3, Litton) to observe avocet “other” included selfmaintenance, nestbuild-
behavior at night. We established 43 obser- ing, and incubation. We further subdivided
vation points that enabled total coverage of foraging birds by technique (pecking, dunk-
JDW
during diurnal and nocturnal surveys. ing, scything). Foraging classes corresponded
We recorded avocet density, habitat use, and to the visual (pecking) and tactile (dabbling
behavior three times daily (morning, after- scythe and multiple scythe) foraging tech-
noon, and night: 07:00, 15:00, and 23:00 PST, niques described by Hamilton (1975). After
respectively) for 14 days at each observation we completed each survey of JDW (approxi-
point. We recorded avocet density for each mately 1 h), we returned to the three locations
pond and the entire study site. We recorded containing the largest avocet flocks and
the distance from each individual to the near- scanned flocks, recording each bird’s behavior.
est next individual as one offive interbird dis- We scanned each flock five times with >3 min
tance classes (<1 m, 1-5 m, 6-10 m, 11-20 between successive samples. Time required
m, >20 m), and we categorized occupied hab- per scan sample (1-3 min) varied with flock
itats as shoreline, open water, or island. We size. The five scans represented subsamples of
categorized behavior as foraging, copulating, each flock, which we used as ourexperimental
agonistic, or other. Agonistic behavior refers unit (mean proportion of total number of av-
Johnson et al. • AMERICAN AVOCET BEHAVIOR 179
ocets present at JDW for which individual be- tributed. When we performed Huber’s itera-
havior was recorded after each census was tive robust ANOVA to reduce the impact of
0.74 ± 0.07 SD, n = 42 censuses, n = 126 highly influential observations, the data as-
flocks). sumed a normal distribution (D’Agostino-
—
Analysis. From each set of scans, we cal- Pearson Omnibus P = 0.62; Huber 1981). We
culated the mean proportion of individuals used Tukey’s test for comparison ofall means,
within each flock engaged in the fourbehavior and we report nontransformed values.
classes, as well as the proportion of foraging
birds utilizing each foraging technique. We RESULTS
also calculated the proportion ofbirds in each Avocet behavior varied within the daily cy-
habitat, and the proportion of birds in each of cle, and among days, locations, and months.
five interbird distance classes. These methods The mean number of avocets was similar at
produced four multidimensional response vec- all times of day (morning: 148 ± 14 SE, af-
tors for the behavior of each flock. The re- ternoon: 145 ± 12 SE, night: 157 ± 8 SE; P
sponse vectors (behavior, foraging technique, = 0.92), but more avocets were present early
habitat use, interbird distance) served as in- i3n) the breeding season than later (May: 170 ±
dependent data points for analysis. Because 6 SE, June: 130 ± 10 SE; P = 0.015).
—
percentages within vectors summed to 100 Behavior. Behavior (foraging, copulating,
and were thus correlated, it was necessary to agonistic, other) varied as a function of time
use multivariate analysis techniques. We arc- ofday, date, proportion ofbirds in open water
sine transformed data and set alpha = 0.05 a habitat, and location (Tables 1, 2). There also
priori, then tested for behavioral variation was a significant interaction between date and
during the daily cycle using a multivariate location (Tables 1, 2). We found no significant
analysis of covariance model (MANCOVA) difference in the proportion of avocets copu-
for each response vector, with time of day lating or foraging throughout the day (Table
(morning, afternoon, night) as the main effect However, birds were five times more ag-
.
and month (May, June), date (day one through onistic at night compared to morning and af-
seven during May and June sampling periods), ternoon (Table 3), and birds engaged in other
location (pond), and flock size as covariates behavior less at night than during morning
(Seber 1984, Johnson and Wichem 1988). Be- and afternoon (Table 3).—
havioral responses not modeled by the re- 4)Foraging technique. Eoraging technique
sponse vector also were included as covariates varied as a function oftime ofday, flock size,
in each model. We compared potential models location, date, month (Table 4), open water
for each behavioral response beginning with habitat, and proportion of birds < Im from
the saturated two-way interactive model using another avocet (Tables 1, 2). There also was
information-theoretic methods to direct model an interaction between time ofday and month
selection (Burnham and Anderson 1998). For and between location and date (Tables 1,2).
each model we calculated Akaike information Avocets foraged by pecking more in the
criteria (AIC), which were used to compare morning compared to afternoon and night (Ta-
candidate models to achieve the most parsi- ble 3). The proportion of birds dunking in-
monious model that accurately represented the creased from morning to afternoon and again
data (Burnham and Anderson 1998, Anderson from afternoon to night (Table 3), and higher
et al. 2000). proportions of avocets foraged by dunking
We used three-way ANOVA to examine av- early in the breeding season compared to later
ocet density at ponds during the daily cycle (Table 4). Avocets scythed less at night than
(morning, afternoon, night) and throughout during day (Table 3), and lower proportions
the study (May, June). We used SAS macro of avocets used the scythe technique to forage
FXQLQLQL
(Fernandez 1999) to check nor- early in the breeding season than later (Table
mality and equal variance assumptions ofAN-
OVA . —
and examined student residuals for the Habitat use. Habitat use by avocets varied
presence of significant outliers and influential as a function of time of day and location, and
observations. Data differed significantly from there was a significant interaction between
normality and variances were not equally dis- these two variables (Tables 1, 2). Avocets fre-
180 THE WILSON BULLETIN • Vol. 115, No. 2, June 2003
TABLE 3. Mean (± SE) proportion ofAmerican Avocets conducting various behaviors in differenthabitats
during three daily sampling periods (morning = 07:00, afternoon = 13:00, night = 23:00) in early (May; n =
21 flocks) and late (June; n = 21 flocks) breeding season and combined (May-June; n = 42) at Jay Dow, Sr.
Wetlands, Lassen County, California, 2001. Different superscripts within a row denotes statistically different
means (Tukey’s test, P < 0.05).
Morning Afternoon
May June May-June May June
Behavior
Agonistic 0.019 -1- 0.007 0.026 H- 0.009 0.023 -1- 0.006^ 0.040 -F 0.009 0.033 ± 0.008
Copulate 0.017 ± 0.005 0.005 H- 0.002 0.011 ± 0.003^ 0.027 -F 0.005 0.002 -h 0.001
Other 0.449 ± 0.060 0.350 -h 0.033 0.400 -h 0.035^ 0.421 -F 0.042 0.425 -F 0.035
Foraging 0.515 -1- 0.057 0.620 -h 0.034 0.567 -h 0.034^ 0.512 -F 0.038 0.540 -F 0.033
Dunking 0.591 ± 0.075 0.466 -h 0.063 0.529 -+- O.O49A 0.777 -F 0.056 0.446 -F 0.068
Pecking 0.395 -H 0.077 0.213 -h 0.047 0.304 -h 0.047A 0.186 -F 0.053 0.231 -F 0.040
Scything 0.014 ± 0.005 0.321 -h 0.058 0.168 ± 0.037^ 0.037 -F 0.027 0.323 ± 0.063
Habitat
Water 0.510 ± 0.081 0.847 -h 0.038 0.679 -1- 0.05U 0.705 -F 0.054 0.733 -F 0.038
Island 0.261 ± 0.073 0.042 0.020 0.151 ± 0.04U 0.130 -F 0.050 0.039 -F 0.019
Shore 0.229 0.053 0.111 -h 0.028 0.170 -1- 0.03U 0.164 -F 0.027 0.227 -h 0.033
Interbird distance
<1 m 0.160 0.032 0.062 -h 0.016 0.111 0.020^ 0.222 -F 0.032 0.113 -F 0.024
1-5 m 0.230 ± 0.029 0.180 -h 0.025 0.205 -+- 0.019A 0.280 -F 0.021 0.243 -h 0.027
6-10 m 0.209 ± 0.024 0.221 ± 0.024 0.215 ± 0.017^ 0.175 ± 0.025 0.184 -F 0.024
11-20 m 0.193 0.031 0.380 -1- 0.036 0.287 ± 0.028^ 0.144 -h 0.018 0.292 ± 0.045
>20 m 0.207 0.029 0.157 ± 0.025 0.182 ± 0.019^ 0.179 -F 0.016 0.169 -F 0.027
quented certain ponds at specific times of the an interaction between date and location (Ta-
day and some ponds were used predominately bles 1, 2). Mean proportion of avocets < Im
during either the early or late portion of the apart was higher at night than during day (Ta-
breeding season (Fig. 1; ANOVA; = ble 3). Similarly, the proportion of birds 1-5
36.39, P < 0.0001; location F,o4,6 = 61.65, P m apart was higher in the afternoon compared
< 0.0001; month = 13.44, P = 0.0003; to morning but greatest at night (Table 3). The
time of day F24K, = 0.49, P = 0.61; time of proportion of birds in all larger interbird dis-
day X location F20416 = 0.3.47, P < 0.0001; tance classes was least at night (Table 3).
mmoonntthh XXlotciamtieonoFf|o,d4ai6y=F1274.K,84,=P9<.0,0.0P001=; hMiegahnerperaorployrtiinonthoefsaevaoscoent,sa<ndIamlaarpgaerrt pwraos-
0.0001). Island habitat use decreased from portion ofbirds were spaced farther apart later
morning through night (Table 3), and a greater in the season (Table 4).
proportion ofavocets used islands early in the
DISCUSSION
breeding season compared to later (Table 4).
Mean proportion ofavocets utilizing open wa- All behaviors performed by breeding
ter did not differ significantly among the three American Avocets during the day also oc-
sampling periods (Table 3), but avocets used curred at night, though frequencies of partic-
open water more often late in the breeding ular behaviors, flock location, and interbird
season (Table 4). The proportion of avocets distances varied among morning, afternoon,
utilizing shoreline habitat did not differ sig- and night. Nocturnal social behavior was suf-
nificantly among the three daily sampling pe- ficiently frequent that further study of avocet
riods (Table 3) or between early and late sea- behavior must eonsider nocturnal habits.
son (Table 4). While observations in this study were limited
—
Interhird distance. Interbird distance var- to the week around the full moon, earlier ob-
ied as a function of time of day, flock size, servations at the study site confirm that sev-
date, and month (Tables 1, 2). There also was eral shorebird species are socially active at
8
Johnson et al. • AMERICAN AVOCET BEHAVIOR 181
TABLE 3. Extended.
Afternoon Night
May-June May June May-June
0.036 ± 0.006^ 0.186 ± 0.017 0.210 ± 0.021 0.198 ± 0.014«
0.014 ± 0.003^ 0.025 ± 0.008 0.008 ± 0.003 0.016 ± 0.004^
0.423 ± 0.027^ 0.240 ± 0.028 0.160 ± 0.019 0.200 ± 001 »
.
0.526 ± 0.025^ 0.549 ± 0.027 0.623 ± 0.026 0.586 ± 0.019^
0.612 ± 0.051^ 0.816 ± 0.037 0.685 ± 0.055 0.750 ± 0.034c
0.209 ± 0.033« 0.142 ± 0.032 0.204 ± 0.045 0.173 ± 0.028^
0.180 ± 0.041^ 0.042 ±0.017 0.111 ± 0.028 0.076 ± 0.017B
0.719 ± 0.033^ 0.776 ± 0.035 0.768 ± 0.027 0.772 ± 0.022^
0.085 ± 0.027^* 0.053 ± 0.016 0.002 ± 0.001 0.027 ± 0.009c
0.196 ± 0.022^ 0.171 ± 0.029 0.231 ± 0.027 0.201 ± 0.020^
0.167 ± 0.021^ 0.438 ± 0.024 0.305 ± 0.023 0.372 ± 0.019^'
0.261 ± 0.017'^ 0.312 ± 0.017 0.419 ± 0.016 0.365 ± 0.014c
0.179 ± 0.017A 0.063 ± 0.009 0.138 ± 0.010 0.101 ± 0.009^
0.218 ± 0.027^ 0.086 ± 0.010 0.079 ± 0.015 0.082 ± 0.009c
0.174 ± 0.015^ 0.101 ± 0.012 0.059 ± 0.010 0.080 ± 0.008'"
—
night throughout the lunar cycle (Johnson et ior. Avocets were more aggressive and more
al. 2002). Further, Pied Avocets are active at closely spaced at night than during the day,
night, regardless oflunar illumination (Hotker and the mean proportion of agonistic birds
1999). was five times higher at night. Avocets ap-
—
Copulations. Nocturnal copulation rarely parently increased nocturnal agonistic behav-
has been reported, but is not necessarily un- ior at the expense of self maintenance or in-
common among shorebirds (Lemnell 1978, cubation, as the proportion of birds engaged
Johnson et al. 2002). We found avocets cop- in other behaviors was 50% less at night. Av-
ulating with similar frequency during morn- ocets and other shorebirds potentially are bet-
ing, afternoon, and night, and previous obser- ter able to reduce incubation duties at night
vation at JDW found several shorebird spe- when the risk of desiccation to uncovered
cies, including avocets, copulating at night re- eggs due to overheating is minimal (Thibault
gardless of lunar phase (Johnson et al. 2002). and McNeil 1995a). Shorebird eggs are robust
Theory on mating strategy has failed to ad- to cool temperatures, but highly susceptible to
dress the possibility of nocturnal reproductive extreme heat. Thus, avocets may be more
tactics among most shorebirds (e.g., Oring prone to leave the nest at night when ambient
1982). Considering that Ring-billed Gull {La- temperature is low. Conversely, it is possible
rus delawarensis) copulations are interrupted that we failed to detect incubating birds during
less often at night (Hebert and McNeil 1999), nocturnal observations. However, this is not
and ourobservation that nocturnal copulations likely as there was no significant difference in
are likely to play substantial roles in avocet the number of birds observed on the entire
reproduction, study of shorebird nocturnality wetland during morning, afternoon, and noc-
is critical for accurate assessment of repro- turnal surveys.
ductive tactics among temperate breeding Avocets also may be tightly spaced at night
shorebirds. to reduce the threat of predation. However,
Interbird distance and agonistic behav- nocturnal agonistic encounters generally were
182 THE WILSON BULLETIN • Vol. 115, No. 2, June 2003
TABLE 4. Mean (± SE) proportion of American Pond2 Pond3
Avocetsperformingvariousbehaviorsindifferenthab-
itats early (May; n = 63 flocks) and late (June; n =
63 flocks) in the breeding season at Jay Dow, Sr. Wet-
lands, Lassen County, California, 2001. Different su-
perscripts within a row denotes statistically different
means (Tukey’s test, P < 0.05).
May June
Behavior
Agonistic 0.082 0.012^ 0.089 -+- 0.013^
Copulate 0.023 -E 0.004^ 0.005 0.001A
Other 0.370 0.028^ 0.312 0.022^
Foraging 0.526 0.024^ 0.594 ± 0.018^ Time ofday
Dunking 0.728 -+- 0.035^ 0.532 -1- 0.038«
Pecking 0.241 ± 0.035^ 0.216 ± O.O25A PIG. 1. Variation in mean number of American
Scything 0.031 0.011^ 0.252 ± 0.032^ Avocets at four locations at Jay Dow, Sr. Wetlands,
Lassen County, California during morning (07:00), af-
Habitat use
ternoon (15:00), and night (23:00) observations, 2001.
Water 0.664 ± 0.037^ 0.783 -E 0.02P White bars represent May observations (n = 7), black
Island 0.148 ± 0.031^ 0.027 -E 0.009« bars represent June observations {n = 7), and error
Shore 0.188 ± 0.022^ 0.190 ± 0.018^ bars provide standard error (ANOVA: all locations, n
Inter<b1irdmdistance0.273 0.023^ 0.160 -E 0.018B 6=1.6151:, PE46<.4160.=000316;.3m9o,nPth<^,.04.,0^0=01;13l.o4c4a,tiPon=F0,o.040i603=;
>210611---m51200mmm 0000....111246471394 ±±±- 0000....000011114443^^^^ 0000....211288251180 ±--EE 00O0....O00021115428«^^B ^ttl2ioi.ocm4na1et6Fio,o=f0n.49d1F.6a02y,0=.4P1E6127.=~.4186400,..=03P.0400<7.1,4)9.0,P.0P0<01=0;.00m0.o06n11t;;htmiXomnettihomfeXdoaflyodcaaXy-
loud and created a commotion easily detected 1974, 1976; Evans 1976; McNeil and Robert
by predators. Hawk and owl predation is a key 1988; Robert and McNeil 1989, McNeil and
factor in shorebird mortality during the non- Rompe 1995, Rompre and McNeil 1995). Eye
breeding season (Page and Whitacre 1975, morphology and physiology are correlated
Townshend 1984, Whitfield 1985, Whitfield et with nocturnal foraging behavior in many bird
al. 1988); however, the extent of diurnal ver- species (Rojas De Azuaje et al. 1993; Rojas
sus nocturnal raptor predation on shorebirds, et al. 1999a, 1999b; McNeil et al. 1999).
especially during the breeding season, has not Shorebirds that used tactile foraging tech-
been thoroughly investigated (Townshend niques, including Short-billed Dowitchers
1984). (Limnodromus griseus), American Woodcocks
—
Foraging. The proportion of birds en- (Scolopa.x minor), and Willets exhibited lower
gaged in foraging behavior varied from 0.50- retinal sensitivity and had lower rodicone ra-
0.60 throughout the daily cycle and the entire tios compared to those that used visual for-
season; however, foraging technique was less aging techniques at night, such as Black-
consistent. Change in prey visibility or avail- winged Stilts and Wilson’s Plovers (Charad-
ability may account for daily and seasonal rius w'ilsonio', Rojas et al. 1999a, 1999b). Var-
variation in foraging technique. A shift to tac- iation in foraging technique between day and
tile foraging techniques at night may be more night also may be a consequence of variation
efficient for avocets if prey is difficult to de- in prey behavior during the daily cycle, re-
tect visually at lower light intensities. Eur- sulting in changes in prey availability. Similar
asian Oystercatchers {Haematopus ostrale- or distinct prey communities may be available
gus), yellowlegs {Tringa flavipes, T. melano- to shorebirds in common or discrete habitats
leuca). Black-winged Stilts {Himantopus hi- between day and night (Thibault and McNeil
mantopus), and Willets {Catoptrophoriis 1995b, Rompre and McNeil 1995). Overall,
semipabnatus) showed less visual foraging shorebird prey were more abundant at night at
and more tactile foraging at night (Hulscher a Venezuelan lagoon (McNeil et al. 1995), and
Johnson et al. • AMERICAN AVOCET BEHAVIOR 183
two annelid prey species commonly taken by McNeil, B. Sandercock, J. Sedinger, L. Smith, and T.
shorebirds in Europe {Nereis diversicolor, Sordahl for manuscript reviews.
Lumbricus terrestris) also were most available LITERATURE CITED
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JDHWabiwtearte uussee.d—cAolnstihsotuengthlystohmreougphoonudtsthaet Andesrosno.n,20D0.0.R.,NuKl.lRhBypuortnhheasims, atensdtinWg.: Lp.rTobhloemmps-,
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