Influence of Agriculture on Waterbird, Wader, and Shorebird Use
Along the Lower Colorado River 1
Robert D. Ohmart 2 , Be rtin W. Anderson 3 , and
Wil liam C. Hunte r 4
Abs t ract.--Wat e r bird, wader, and shorebi rd us e of the
Col orado Riv e r was restricted to habitat s i n or i mmediately
ad j acen t to t he river prior to agric ul tural dev e lo pment. ~e
studied agri c ultural habitats syst ematicall y for t hree years
and identified those ag ricultura l s e t ti ng s t hat were mo st
impor t ant for individ ual spe ci e s a nd gro ups of wa t erbirds ,
waders , and shorebi rds .
I NTRODUCTION
The importance of ri par i an habi t ats to wild­
and t hei r r apid di s a ppearance in t he wes ter n
has receiv ed much atten t ion ov e r the pa st
Conversion of riparian habitats t o
al l and accounts fo r a major portion of
los s of ripari an habitat s . Yet, a gricultural
tats have r ece ived l i t t le atten t ion with
to wildlife value s, with the exception of es on i nt e r f aci ng agriculture-riparian sit ua­
(Ca r ot he rs e t al . 1974 , Conine et al. 1978) mo r e compr ehensiv e st udy of the influence of ure on wi ldlif e in the lo we r Colo r ado
val ley ( Anderson and Ohmart 19 82). Fr om the
study , we report he re on the f a ctors tha t
nee wat erbird, wader, and shorebird use of
icultural ar ea s of the lower Colo r a do River
ley.
man y terr es t rial ri par ian bird
s uf fer ed f r om he av y habitat loss in
conve rs io n of land to agr icul t ural production
the l owe r Colo rado Riv e r valley (a pproximately
000 ha s i nce 18 90) , sev e ral groups of species
ag ricultura l ha bita ts hav e increased.
r ds, wader s, and shorebirds are among the
lpeci es t ha t have benefit ted fr om or a t l east make
DBe of agri cultur a l fea tures.
llist waterbirds and
waders are as so ciated with open wa t e r and marsh
habitats which, l i ke agricul t ural areas, hav e
increased substa ntially since t he beginning of t he
20th ce n t ury (Ohmar t et al. 1977). Many of these
species make use of both aquatic and ag ricultural
areas for roosting and f or aging, re s pectively.
Geese, cranes, and sever al heron spe cies ar e
examples. In ad dition, shoreli ne and sandbar s are
very local in distribut i on; whereas, agricultural
areas hav e proven to be among the most prod uctive
habitats for finding and st ud ying many trans ient
and resident sho r ebird species on t he lower
Colorado River (Anderson and Ohmar t 1984 ) .
We descr i be ag r i cultura l fe at ur e s most assoc­
iated wit h occ ur r e nc e of water bi rds, wader s, and
shorebirds . ~e r epo rt ha bi tat us e by pre sence /
a bsence c r i t e r ia becaus e of t he e r r ati c oc c ur­
rence s of some agric ult ural f eat ure s , the f lo cking
tendencie s of s ome spec ie s vs. t he so l itar y natur e
o f ot her s, and t he over a l l low den sit i e s of man y
species tr eated he re. Finall y, we examine the
minimum ra nge of ag ri cultural features nec e s sar y
t o maximize use of an ar ea b y wa t e rbirds, wade r s,
a nd shorebirds in t he lowe r Color ado River valley.
METHODS
The ge nera l s t ud y area i ncl udes ag r i cul tura l
land in so ut hwe s t e rn Arizona and so ut heas te rn
Californi a . Fi ve locali ti es we r e s t ud ied; these
inc luded t he We llto n- Mo hawk and I mperi a l- Coachella
valleys, the Mohave and Colorado River Indian
Reserv a tio ns, and Cibola National Wil dl i fe Ref uge
(And erson and Ohmart 1982) . The l a tt e r thre e
s ites are directl y ad j a c e nt to t he l Ower Colorado
River.
1
Pa per pr esent ed a t t he Ri parian Ecos ys tems
Their Management: Reconciling Conflicting
COnfe r enc e . [University of Arizona, Tucson,
16-1 8 , 19 85 ].
Rober t D. Ohmart is an Associate Director of
:he Cente r for Environmenta l Studies at A.rizona
tat) Uni versity, Tempe , Ariz.
Bertin W. Ande rson is a Research Associate of
:he Center for Environmental Studies at Arizona
tat~ Univer si t y, Tempe, Ariz.
Ia Willi am C. Hunter is a Graduate Re search
Bistant in the Ce nte r for Env i ronmental St ud i e s
It Ari zona St a t e Univ e rsit y , Tempe, Ari z .
Birds we re censused along 23 4. 8-km transec ts
three times pe r mo nt h . Agricultural land along
each transec t wa s s ubdivided i nt o 0.2- or O.4-km
s ec tions; each se ct i on usual l y repr es ented a dis­
117 t inct unit containing only one field t ype . Sam­
pling included the area wi t hin 0 . 2 km l ater al to
each side of the transect , and t he t ot a l numbe r of
hectares censused was calculated . I n addit i on, we
me a sured the exten t of inhabi ted areas, margins,
ca nals , and feedlots.
Major crop types included cotton , al f alfa,
grass, wheat , and mil o . In ad dit i on , f ields gr ow­
ing lettuce , squash, t omatoes, onions , a nd mel ons
were grouped under the ca t egory of truck crops because they are structurally similar , and
cultivation and harvesting te chniques are simi l ar.
When a field was be i ng changed from one crop type
to a nother, it oft en was cle ared; such f ields wer e re f e r red to a s plowed. Each field t ype wa s ca te­
gor ized as to whether or not i t was irrigated and
wh ether t he crop was > or <25 cm tall.
.
,\
'i.
~
Waterbirds a nd waders were censused monthly
from March 19 78 through Octo ber 1980. The number
of times each cr op /fea ture ty pe was censused and
the numbe r of t imes each spec ies was present in
t h e fie ld type was t abulat ed . A signific ant num­
be r of occurrences i n a given si tuation was deter­
mi ned by finding t he pr opo rt ion of that fe at ure
among the total c rop/fe ature occur re nces. This
proportion was t he number of times a species would
be ex pected to ha ve been recorded in t hat situa­
tion if its occurrence was purely random. A sig­
ni ficant a s so ciat ion between the species and the
cr op /feature was defined a s t hose cases where the
number of occurre nces within a pa rticular situa­
tion was sign ificantly large r tha n expected at
PiO . 05 . The number expe ct ed was the proportion of
the crop/featur e occurrence s in the total pool of
crops and feat ures multipl ed by th e to tal number
of occur rences of the spec i e s . This approa ch
id e ntified impor tan t crop s and fea ture s fo r each
s pecies, and general t rends amo ng spe c ies gr ou ps
were def ined to maximize agricult ura l use by
bi r ds •
Sho rebird oc curr enc es were t a bula te d for
Ma rch-May (vernal migr ation) and July-Oc to be r
( aut umnal migration) duri ng 197 8, 1979 , and 1980.
These mont hs were periods of peak occur rence f or
mos t s pe c ies~ Shorebi rd as sociations wi th crop/
fea tur e ty pe s were identified f o r each spe cies as
de termined for waterbirds and waders. Addi t ional
a nal ys is was used to determ ine ove rall use of crop/ fea tures by shorebirds. We determined if the number of fiel ds with shorebirds was signi fi c antly l arger or smalle r t han would be expe cted by cha nce (PiO .05 ) for each c rop /fe ature type. Lastly , we
determined if there was a greater use of either
i rrigat ed or non i r ri ga ted f ields ac r oss all crop
types f rom a rand om distribution (PiO.05 ) for ea ch
shorebird s pecie s.
RESULTS
Crop/Featu re Types
The mo st often encount er ed crop/fea ture type s
i nclude, in order of hig hest-to-lowe st fre quency
of occurrence , margins, nonirrigat ed pl owed, non­
ir rigated alfalfa >25 em , non1rrigat ed alfalfa
<25 cm , and nonirr i gated c ot to n >25 cm ( t able 1).
Table 1.--Num be r of times each fi
(4
eld ty
ha or more ) on agricultural c pe occurr
wa ter bird s and waders ( full h ensuses f ed
h
t ree
or
s o rebirds (migration periods onl years) alld
times c r op/ fea t ure censused' P
y) . 1/ ..
' r op. '" pr °por_
tion.
Waders and
Wa ter bi rds
Crop/
feature
Plowed
Alfalfa
>25 cm
>25 cm
<25 cm
<2 5 cm
Cot ton
>2 5 cm
>25 cm
<25 cm
<25 cm
Milo
>2 5 cm
>25 cm
<25 cm
<25 cm
Whea t
>25 cm
>2 5 cm
<25 cm
<25 cm
Truc k crop
>2 5 cm
>25 cm
<25 cm
<2 5 cm
Grass
>25 cm
>25 cm
<25 cm
<25 cm
Canal
Margins
Inhabited
Irr iga ted
N
Prop.
of t otal
----­
Shorebirds
-:------.
N
Prop.
of tOtal
--­
Yes
No
73
46 8
0 .017
0 .1 09
54
308
Yes
No
Yes
No
55
432
49
39 3
0 . 013
0.0 11
0 . 092
34
162
21
95
0.020
0.097
O.Ol l
0.057
Ye s
No
Ye s
No
75
388
23
177
0 .018
0 . 091
0 . 005
0 .041
57
220
12
99
0.034
0.132
0. 007
0.059
Ye s
No
Ye s
No
11
48
0
16
0.003
0 . 011
0 . 00 0
0.004
Yes
No
Ye s
No
22
163
15
148
0 . 00 5
0 . 038
0.004
0.035
25
121
18
51
0.015
0.072
0.010
O.Oll
Ye s
No
Ye s
No
6
51
9
82
0.001
0.012
0 . 002
0 . 0 19
3
29
2
23
0.002
0.017
0.001
0.015
Yes
No
Ye s
No
10
107
14
132
165
496
88
0 . 002
0 . 025
0.003
0.031
0.03 9
0.116
0.021
7
70
14
62
182
0.004
0.041
0.008
0.037
0.109
0 ~101
0.032
0. 185
Perc e ntag e of irrigated field types for the entire
census period was 8.4% , and during the period of
s ho rebird censusing the percentage was 14.6% .
Milo, marg ins, and inha bi ted are as were not used
by a ny s hore bi rds so were not included in analysis
for thi s species grou p .
Wading Birds
Th e Gre at Egre t (Ca smerodius al bus) , Great
Blue Hero n (Ardea he rodias), Gree~ked Heron
(Bu torides st r ia tus) , and Snowy Egret (~
thul a ) were as s oc i a t ed with canals far more tha nOi
wi th a ny other agricultural fea ture (table 2). 1
t he se spe c ies, the Green-backed Heron was the o~,;
heron to be largely restricted to canals with t 1
1
2.--Field s i n wh ic h use by wadi ng birds was signi f ican t ly ( P<0.0 5) gre a te r tha n expe cted wi th a
rand om di st r i but ion . IR = i r r igat ed ; NI = no t irrigated ; ) = height gr e ater than 25 cm; < =
heig ht l es s t ha n 25 cm; + i nd i c ate s pr e s ence in crop /feature but not significant l y associ ated
with i t . Not al l fi e l d types are r ep resented, only those are shown t hat had at l eas t one
specie s s i gni f ic a ntly associated with i t.
Percent of total occurrences
Alfalf a
Pl owed
Milo
Gra ss
To t al
<IR
sign.
a ssoc.
%
Total
oc currences
<IR
<NI
)IR
+
+
15
+
17
+
26
55
56
81
19
8
+
4
+
109
+
+
4
6
+
15
+
33
109
64
37
Egret
Eg ret
cus ibi s)
\~:.:.:--....,. Ibis
\~;><-_ ch i hi)
74 Canals
)NI
IR
NI
)NI
<IR
)IR
<NI
70
+
+
73
81 +
+
+
20 7
20
6
24
+ 19
8
4
+
4
4
6
+
7
+
35 15
41
67
75
f o r 81% of its t otal occurrence t ural are a s . Snowy and Cattle egret i ons were closest to a random distr i bu­ ac ros s available ag ricultural fe atures among impor tant to puddle ducks and ge e s e , given the
num ber of occurre n c e s compared with other species
within th is group.
Cattl e Eg r et, Wh it e- fa ced Ibis, and Sa nd­
Crane were not signi fi c a n tly associat ed wit h (ta ble 2 ). The Ca t tl e Egr e t was signifi­ plowed, al fa lfa, and gra s s matter wh e t her the fields In contra st, t he win t e ring Cr ane oc curred almost exclusively in non­ alfalf a , plowed, a nd milo f i elds; where­ the other extreme, t he migratory Whi te­
s occur red only in i rr igated alfa lfa, I and grass fie lds. Fif tee n of 17 species of s hore bi rds were
signific a nt ly a ssociated wi th irrigated plowed
f ields (tabl e 4) . In addi t i on, four spec ies e ac h
were signific antly assoc iated wi t h non irriga t ed
plowed, i rr igated and noni r rigated grass <25 cm;
t hree spe cie s with i rr igated whe at )25 cm; two
specie s each wi th nonirrigated alfalfa <25 cm,
i rr igated a nd no ni r rigated grass )25 cm, and
ca nals. Significant as sociations wit h f i e ld types
we r e r estricted to plowed fie l ds fo r ni ne s pecie s
with al l but t wo of these only wi t h ir r i gated
plowed fi e lds ( t he ex c ep tions were Bl a ck- belli ed
Plover and Mount ai n Plover). Common Snipe and
Spotted Sandpi per were associat ed mos t sign ifi­
cant ly with cana l s over all other c rop/f e ature s.
Wat erbird s
bes , cormorant s , coots, and divi ng du cks
rds and mergans ers ) wer e overwhelmingl y wit h canals (table 3). Gee se were exclus ively with plowed, alfa lfa, milo, and gr a ss fi e ld s , wi th all but the Snow Occurring l arge l y in non irrigat ed fields. ducks we re as soc iated both with canals and ted cr op s wi t h t wo s pecies (Ame rican Wigeon tn Pinta i l) occuring only in irrigated 1i1neteen of the 25 species t re ated under t hi s
have 100% of their occur rences associat ed
y wi th one or more agr icultur al crop/
types. Ho wever, t hree species (Snow
60~l1a rd , a nd Nort he rn Pintail) have less
of their occurrences associated signi fi­
h ~t h some a gr icult ural crop/ f eatur e and
~ve a mo r e r a nd om dist ri bu t ion in ag ricul­
abi ta t s. Ag ricultural si tuations were mos t
Shorebi rd s
Eig h t speci e s oc curred signi fic antly more
of t en in irriga ted than in nonirr igat ed fields
(table 4). Four s pe cies occ ur r ed sign i f i cant l y
mo re often i n noni rr ig ated than in i rr i gated
fields. Ove r a l l, the r e were 26 signif i cant
as sociat i ons betwe e n shore bi rd s a nd irr i gated
f i elds and 12 with nonirrigated fiel d s, wi th e i ght
species signi f icantl y assoc iat ed wi t h at least one
each i rr iga ted and noni r r igated c ro p/feature type.
All s ho r e bi r d oc cur rences t05 ethe r ind i cated
that they were f ound more of t e n t ha n expected by
ch ance al one i n irrigated and noni rrigated plowed,
irrigat ed gra ss )25 cm and <25 cm, irr i gated wheat
)25 cm, and ca nals (t able 5) . Shorebirds occurred
less of ten t han expec t ed by cha nce alone i n non­
irr iga te d alfa l fa )2 5 cm, i rrig a ted and nonirri­
ga te d cot ton )25 cm, and non i r ri gated cotton
1 19 -Tabh 3 .--Agricu ltur a l c rops and f eat ures wit h ~hich gr ebes, cormo r ants, coots, duc ks . a nd geese 'Were ass ocia t ed ~ith a
frequency g r eater t han ex pec ted with a random dis t r ibut ion . Abbr e viat ions, symbols, and not es as i n t a hIe 2.
Perce n t of to t al occurren c es
Alfa lfa
Pl owed
To ta l
occur rence s
Spe c ies
Pied-billed Grebe
(Podil ym bu s podiceps)
Eared Grebe
(Pod1ce ps nl gr i co llis )
Double- c res ted Cormo r ant
(Pha la c ro c o rax a ur l t us)
Amer i can Co o t
( Fu lic a ame ric ana )
Common Me rgan s e r
(Mer gus mer ganser )
12
Red-brea sted Me rga nse r
3
(M. se rrat od
Bufflehea d (B ucepha la albeola ) 2
Commo n Gol deneye (!. c langu la ) 15
Total Me rg i ni
32
Gr ea t er Scaup eAy thya marU a)
I
4
Les ser Sc a up ( ~. ~)
Redhead (A . america na)
4
Ring-ne cked Duck (~.' c011aris) 3
To t al po chards
12
Wood Duck ( Aix sponsa)
I
Cinnamon Teal
( Ana s cyanopte ra )
63
Blue -winged Teal (A . disc ors)
5
Gree n-winged Teal (A.~) I S
Nor the r n Shov e le r
clypeata) .3
Ma ll a rd (.! . pla t yrhynchos)
12
Ame ri ca n Wige o n (A . amer ica na) 6
Nor t he r n Pint a il TA .~ 27
Tot a l pud d le du cks­ - 131
Canada Goos e
(Bra n t a canadens is)
30
Bran t (8. bern1cla )
I
G re B te r-Wh~t ed Goose
( Anse r al b1 fr on s)
Ros~se (Chen ross11 )
Snow Go ose C.£. . cae ru le s c e ns )
9
Tota l ge e se
43
Milo
Whe at
Grass
To t al
%
lR
Cana ls
Nl
>IR
<IR
>Nl
<Nl
>IR
>Nl
(IR
( N1
) IR
<lR
sign.
BSSoe .
100
100
100
100
100
100
100
100
100
100
100
100
80
91
100
100
25
100
7S
100
100
100
80
91
100
100
100
100
100
100
50
S6
60
47
33
25
(:!.
25
17
5
20
16
40
67
33
83
48
31
38
+
+
+
30
23
27
100
+
+
+
24
22
+
90
100
100
100
II
II
P:
86
80
100
100
58
100
S9
78
17
+
Cr
fe
+
+
36
4
100
100
22
88
+
,~
Ta ble 4 . - - Agr 1c ult ura l cr ops a nd fe a t ures with wh ich shorebirds were associated wit h a f req uency g r eat e r tha n expected
with a r andom distributi o n on transects . Ab brev iations I s ymbols I a nd notes as 1n t able 2 .
Per cent of t otal oc curr e nces
All c r opl
Plowed
Spe c ies
TotCll
occurren ces
1R
N1
Black-bel lied Pl over
(Pluv1al1s s quataro la)
Killdee r
(Chsradr ius vociferus)
Mount a in Plove-r - - ­
(C . montanus)
Black-~tUt
(Himant opus mexi ca nus )
American Avoce t
(Re curvi ros t ra americana)
Great e r Yellowl eg- . - - ­
(T ringa melanoleuca)
Le sse r Yellowlegs
(!. fla v1pes )
Wil l et ( Ca tOEtroEhorus
semi e almatus)
Spotted Sandpiper
(Aetltis macu la ria )
Whimhel
- - -­
(.Numen 1us phaeopus )
Long -bill ed Curlew
( N. amer icanus )
Western Sand piper
(Calidris mau!"i)
Lea st Sandpi~
(C . minu t illa )
Long-b~it che r
(L imn od r omus sco l opac eus)
Common Sni pe
(Ga lUnago 8al11n8go)
Wi! son Pha l a rope
( Pha lar oEu s t!" icolor )
Ring-bil led Gun- - ­
(~ del awsre nsis)
19
42
37
287
II
+
16
25
63
28
2S
+
Al fal fa
(N1
Grass
)IR
) N1
+
+
to
+
+
+
II
18
39
+
29
+
+
+
+
45
36
+
+
53
II
27
13
+
+
10
40
+
30
+
+
+
27
41
+
+
+
+
19
+
+
+
120 12
+
+
+
+
Pe rc en t
accounted
f o r by
c rop/fe atur es
feature s
1R
Nl
79
+
+
25
+
68
88
+
69
54
+
+
67
78
+
53
+
+
50
+
+
+
39
+
83
100
75
75
+
78
+
68
+
40
70
+
+
48
64
+
41
63
+
71
82
+
38
88
+
76
+
65
+
+
46
+
+
IS
+
+
38
+
22
61
30
+
Canals
10
30
+
37
+
) 1R
+
50
17
<IR <NI
11
67
36
Whe a t
59
__overal l s ho rebi rd use of agric ultur e by
5~bers of s pec ie s a nd al l s horebi rds com­
• d
Irr . - i rr igated; Pres . = present ;
bine •asso c. = s igni fic a ntl y a ssociated; Obs .
S ig~:erved·, Exp . = expe c t ed ; NS = no t sign i f ­
. o ~
Number of
specie s
Fields wit h
shorebi rds
I rr.
Pres.
Sign .
as s oc.
Obs .
Exp.
Binomial
P
Yes
No
17
13
15
4
32
130
12 .5
72 .3
+<0.001
+<0.001
Yes
No
Yes
No
5
7
8
8
0
0
0
2
8
18
9
23
7.8
37.9
5.1
22.3
NS
-<0 .001
NS
NS
Yes
No
Ye s
No
5
4
4
2
0
0
0
0
7
9
3
6
13 .3
51.6
2.7
23 .1
-<0.05
-<0.001
NS
- <0.001
Yes
No
Yes
No
9
9
8
2
2
4
4
3
12
10
21
1.2
16 . 8
2 .4
16. 8
NS
NS
+<0.001
+<0.025
Ye s
No
Yes
No
1
1
1
3
0
0
0
0
2
6
1
4
0
7.4
0
5. 9
NS
NS
NS
NS
Yes
No
Ye s
No
6
7
4
2
3
0
0
0
12
12
2
7
5. 9
28 . 2
3.9
12.1
+<0 . 0 1
- <0.0 01
NS
NS
7
2
59
42 .6
+<0.01
11
t ion of the environment by humans . Clearing of
la nd for ag ri cu l tural pur pos e s c reates new habi tat
f o r spec ie s t ha t previo usly may not have occurred
i n t he are a while , a t the same t ime, de stroyi ng
ha bi ta t that may cause t he or ig i nal faun a to dis­
a ppea r unl e ss thes e latte r spe ci e s are a ble to take
ad va ntage of t he new s i tuati on. All of the s pecies
treated he re hav e i ncrea sed i n t he ir occurrenc e in
the val ley since the 1800' s due l a rgely to t he
a ctivities of man, including agricultural prac­
tices, s torage of wa t e r i n large reservoi r s ( pri­
marily for irrig a tion), and creation of stable
marshlands.
Gri nnel l (1914 ) described t he lower Colorado
Ri ver's a qua tic a nd semiaquatic avifauna as depaup­
e r at e; he re co rde d few s pe cies and low numbers of
herons, waterfowl, and shorebirds nort h of the
Gila-Colorado river conf luence. Presently, all of
these species occur in rather stable numbe rs re­
sponding to the food resources prov ided by agric ul­
tural a re as, roosting and foraging si tes of open
water areas, and/or a comtinat i on of the se . The
vast majority of species treated here are tr a nsie nt
and winter visitors and do not de pend on agric ul­
tural areas for nesting. Exc e pt i ons are Killdeer
and a few Cinnamon Teal.
Cranes and geese are almost tot ally dependent
on agricultural areas for forag ing . On t he three
lower Colorad o River national wi ldli f e refuges,
recent increa s es i n abundance for both cranes and
gees e a re a t tributed to managing aquatic or semi­
aquatic roosting si tes with adjac ent agric ul t ural
f oragi ng si tes ( mo stly al f a lfa and milo f ields).
Shore bird , wa terbird, and wa ding species associated
with irrigated fields mus t t r a ck this resource as
f looded f ie lds a re infrequent a nd ephemeral.
Flooding of fi elds attracts not only spe c ies
t r eated here, but also swallows, wate r pipit s, and
blackbirds, which flo ck to f e ed on inve r-t ebr ates
d ispla ced by irrigati on wate r. Wa terbird and some
wading s pe ci es (especially Gree n- backed He r ons)
find more sta ble foraging sites in the exte nsive
canal systems, e spec i ally along large r d irt-ba nked
c anals that us ua lly hav e a constan t flow of wa t e r.
Spotted Sandpipers and some wa terbird s use the more
sterile cement- l ined canals to a grea te r extent.
The most abundant f i eld type s were
no t the places where s ho r ebirds occur red
equentl y. Although cotton f ield s were
la r gely by shorebi r ds , these f i e ld s
"'--" "'C.l ~ enhanced shorebird habi t a t because t hey
~ri odically pl owed and attracted significant
of shorebirds . Overal l, s horebird
,~curr.n,'~ i n agr icul t ur a l area s wa s related to
icultural pra ctic es (plowi ng,
and wi th t he presence of canals.
, 12 of the 17 species pre f erred either
ted or non i rrig a t ed fiel ds and did not ran­
occur in both situations. In order to maxi­
diversity of s horebirds, a mosaic of irri­
and nonirr iga ted fie ld s are des irable.
Severa l species trea ted here have undoubtedly
undergone range ext e nsi ons direc tly associated with
the expansion of agricult ural prac tic es. Most dra­
matic has bee n t he expansi on of the Cattle Egret,
orig i nal ly i n troduced fr om the Old Worl d into t he
New World Tr opic s . Thi s s pe cies became e stabl i s hed
in the sout heastern Uni ted States in the 1950's and
has s pread no rt h and wes t into a re as of ext e nsive
a gr i c ultural and hor ticultural pr oduction. Cattle
Egrets we re f irst found on t he lowe r Colorado River
i n 1970 a n d are on the verge of es ta blishi ng sever­
al breeding c olonies . Mo un t ain Plover s and Whim­
breI s wer e cons idered casual in Arizona be for e the
1940's (Monson and Phillips 1981), bu t both species
a r e now regula r, and hundreds of i nd ividuals occa­
siona l l y occur in agricult ural si t uations .
DISCUSS ION
As Orians (1975 ) has stated, t here is a need to
organism s t ha t have be e n expos ed to pertur ­
resulting from the unprec ede nted modif ic a­
12 1
Most , if no t all, s ho rebirds have benefitted
from var i ous agric ult ura l fe atures on t he lower
Colorado River, al though thi s r egion is not a major
migration route for t his group . However, shorebird
use of agr icul ture in re gions where sho r elines and
estuaries have been extensively devel oped , such as
in Florida and Califo rnia, i s in ne ed of i mmediate
at t ention , given t he de sperate situa t ion ou t l i ned
by Myers (1983) f or thi s gr oup. Our pape r outli ne s
important agricultural featur es for shorebird use
t ha t may be compared with data col lect e d in othe r
reg i ons.
Of all the ha bi ta t changes exper ienced in the
10wer ' Col orado River vall ey , as well a s many other
majo r southwestern r iparia n sys tems, the conversion
of va s t areas to agricultural production is cer­
t ai nly the most drama t ic. The abundant food s pro­
vided by agr i cultural habitats benefit a wide
varie t y of bi r ds that can opportun istically use
them . Si nce ne st ing s ite s a r e not a concern for
ma ny of these s pe ci es on the lowe r Colorado Rive r ,
t hey may a ttain relative ly hi gh densities. If s ur­
v ival ha s been high i n these ove r wintering or
mi gra to r y ind i viduals, then the population biology
of these s pecies may be ch ang ing, with effects per­
hap s ev i de nt on their nor therly breeding grounds.
Although a numce r of speci e s are benefitting from
the development of agriculture, t he fut ur e of many
ripa rian spe cies in agricultural areas is not opti­
mistic unle s s a mosaic of native riparian habitats
and deve l ope d lands is managed for conserving both
the original a nd new avifaunas.
ACKNOWLEDGMENTS
,~"
.
The developmen t of this work and t he agr i cultural
study a s a whole was aided by George F . Dr ake and
Kat hleen Conine. ¥~r k Kaspryzk and Camil le Romano
organized and acc om plished much of the or ig inal
data analys is . We thank the many dedicated biolo­
gis t s ~n o as sisted in data collection and ana lysi s .
Edi t or ial a s sistance was provided by J a ne R. Durham
and Cindy D. Zi s ner . Cindy D. Zisner also t yped
t he manuscri pt. Th is s tudy was supported by U.S.
Bureau of Re clamat ion Contract No. 8-07-30-X0027.
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1