Yellow-breasted Chat and Gray Catbird Productivity in a Fragmented Western Riparian System

2007·3.5 MB·English

by  Tawna C Morgan

494 THE WIl.SON JOURNAL OF ORNITHOLOGY • Vol. 119, No. 3, September2007
The Wilson Journal ofOrnithology 119(3):494^98, 2007
Yellow-breasted Chat and Gray Catbird Productivity in a Fragmented
Western Riparian System
Tawna C. Morgan, Christine A. Bishop, and Tony D. Williams*
—
ABSTRACT We studied the effects of habitat some conditions, habitat fragmentation may
fragmentation on productivity ofYellow-breastedChat lead to a decline in population numbers (Sher-
(Icteria virens) and Gray Catbird (Dumetella caroli- ry and Holmes 1992, 1993).
npeanrsiiasn)eicnossyosuttehme.rnNeBsrtiitinsghdCaotelsumwbeirae ilnatearwienstiseorlnatreid- Robinson et al. (1995, 2000) documented a
habitat patches than in continuous habitat patches for relationship between lower productivity and
both species. We found no direct evidence that habitat increased habitat fragmentation in eastern for-
fragmentation decreased productivity in eitherspecies. ests. Western riparian ecosystems are frag-
Average fecundity did not significantly differbetween mented by nature because of frequent flood-
continuous (2.54 fledglings forGray Catbird; 1.78 and ing, ephemeral water sources, and mountain-
1.67 fledglings for Yellow-breasted Chat in 2002 and ous topography (Hejl 1992, 1994; Ohmart
2003, respectively) and isolated sites (1.33 fledglings
forGray Catbird; 1.78 and 0.87 fledglings for Yellow- 1994). The few studies that have examined the
breasted Chat in 2002 and 2003, respectively). Terri- effects offragmentation in western landscapes
tory size, as measured by mapping perch locations for have generally found a less direct relationship
breeding adults, was smallerforYellow-breastedChats between habitat fragmentation and productiv-
breeding in the Okanagan Valley (0.25 ha) than for ity (Tewksbury et al. 1998).
chats in mid and high-density southern populations. We tested the hypothesis that fragmented
However, overall fecundity and nest success were sim-
ilar to more southerly populations. The.se results sug- habitat negatively affects reproductive success
gest that both .species can persist in a relatively frag- of Yellow-breasted Chat {Icteria virens) and
mented ecosystem. Received 30 June 2005. Accepted Gray Catbird {Dumetella carolinensis) in
17 October 2006. western riparian habitats throughout the Okan-
agan Valley, British Columbia, Canada. Both
species predominantly use wild rose {Rosa
Riparian areas provide critical breeding and spp.) as a nesting substrate and forage in close
stopover habitat for a large diversity of birds. proximity to one another (Cannings et al.
Despite its importance to avian communities, 1987; TCM, pers. obs.). Yellow-breasted Chat
there has been considerable loss of riparian and Gray Catbird nests are commonly within
habitat throughout North America (Croonquist 5 m ofone another. The Okanagan Valley rep-
and Brooks 1993). Thus, there is a need to resents the northwest fringe of the Yellow-
understand avian population trends in relation breasted Chat’s breeding range (Eckerle and
to habitat alteration. Several studies have doc- Thompson 2001), while Gray Catbird popu-
umented a decrease in productivity and an in- lations extend farther north into northern Brit-
crease in brood parasitism in association with ish Columbia (Cimprich and Moore 1995).
increased habitat fragmentation (Wilcove and We compared reproductive parameters for
Robinson 1990, Donovan et al. 1995). Under Yellow-breasted Chat with more southerly
populations.
' Centre for Wildlife Ecology, Simon Fraser Uni- METHODS
versity, 8888 University Dr., Burnaby, BC V3J 1S6,
Canada. Aerial photographs ofriparian habitat in the
2Environment Canada, 5421 Robertson Rd., Delta, Okanagan Valley, British Columbia and Yel-
BC V4K 2N2, Canada. low-breasted Chat surveys in 2001 were used
’Current address; Department ofBiology andWild- to categorize four large riparian patches (15-
life, Institute of Arctic Biology, University of Alaska,
Fairbanks, AK 99775, USA. 70 ha) as continuous and five smaller patches
Corresponding author; e-mail: as isolated (<2 ha). Continuous sites were
[email protected] tracts of unbroken riparian habitat that con-
SHORT COMMUNICATIONS 495
tained >15 ha of unbroken habitat with flow- the mean number of fledglings per female per
ing water. Isolated sites were small fragment- season. All females were single brooded. A
ed patches of riparian habitat, primarily along second method of calculating fecundity, in
roadsides and drainage ditches. Isolated sites which only successful nests were considered,
were separated from other riparian habitats by was used for comparison with other studies.
>5 km and had ephemeral water sources. All Percent parasitism was calculated as the per-
sites were <600 m in elevation and were be- centage of all monitored nests that contained
tween the International Boundary (49° 02' N, >1 Brown-headed Cowbird {Molothrus ater)
1 19° 27' W) and the city ofPenticton (49° 30' egg or young at any point during the nesting
N, 119° 35' W). All study sites were used by cycle. Gray Catbirds were omitted from this
chats in 2002. Three of the isolated sites and portion of the study because they reject cow-
one continuous site were unused by Yellow- bird eggs.
breasted Chats in 2003. Gray Catbird nests Spot mapping was conducted for 20 Yel-
were monitored at the same sites as Yellow- low-breasted Chat territories (5 observations
breasted Chats in 2003. All sites were moni- per territory) in 2003. Territory visits were at
tored for breeding activity between 13 May least 3 days apart and observations were con-
and 15 August 2002 and 2003. ducted for one-half hr during which all Yel-
Surveys for breeding Yellow-breasted low-breasted Chat sightings were recorded.
Chats and Gray Catbirds were conducted at Observations began 15 min after entering a
all sites every 3 weeks to detectbreeding pairs territory. If a focal bird disappeared for more
and locate nests. We checked nests using mir- than 5 min, observations were excluded from
ror-poles or direct observation every 3 days the analysis. Time of observation, bird activ-
during the nesting cycle to examine clutch ity, location, perch height, and any other com-
size, cowbird parasitism status, hatch date, ments pertaining to bird behavior or surround-
and nest success or failure (Martin et al. ing ecology were recorded for all sightings.
UTM
1997). locations were taken after the observa-
Nestlings were measured and banded with tion period based on maps and descriptive in-
a single Canadian Wildlife Service band and formation recorded during the observation pe-
three-band color combination at 6 (Yellow- riod. Territory size was calculated using
breasted Chat) or 7 (Gray Catbird) days of Arcview GIS 3.1 area calculator.
age. All nestlings were banded between 0700 We used STATISTICA (StatSoft 2000) for
and 0930 hrs PDT. Adults were captured using all statistical analysis. One-way analysis of
passive and target netting techniques. We variance (Sanders 1995) was used to test for
banded adults with a three-band color com- differences in fecundity, clutch size, territory
bination and an aluminum Canadian Wildlife size, nest initiation, and hatching and fledging
Service band. All fieldwork was in accordance dates between years and patch type. Student’s
with Simon Fraser University Animal Care r-test was used to test for differences in nest
permit 663B-03, bird banding permit 10365 success between years. There were between
CN, and Canadian Wildlife Service permit 59- year differences in nest success rate, and the
03-0426. We used mist nets for up to one-half data are presented and analyzed by year. Re-
hr within territories at least 6 m from nests. sults were considered significant at P <0.05
Locally-recorded species-specific male songs and are presented as mean ± SD unless oth-
were played for a maximum of 10 min to at- erwise stated.
tract territorial males and females. We also
RESULTS
placed passive nets within territories near
known flight paths, but did not play tapes to Twenty-five Yellow-breasted Chat nests
capture some birds. were monitored in 2002 while 32 Yellow-
Daily nest success probability was estimat- breasted Chat nests and 34 Gray Catbird nests
ed using maximum likelihood (Hensler and were monitored in 2003. Initiation of incuba-
Nichols 1981, Rotella et al. 2000). A nest or tion began the third week of May and contin-
female was considered successful if >1 Yel- ued through 12 July in 2002-2003 for Yellow-
low-breasted Chat or Gray Catbird nestling breasted Chats and 4 June to 12 July 2003 for
fledged. Female fecundity was calculated as Gray Catbirds. Nest incubation, hatching, and
—
5
496 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 119, No. 3, September2007
fledging dates were later in all years at isolat-
ed sites for Yellow-breasted Chats and Gray m 00 lo 8.72 11.41
Catbirds (Table 1).
There were no significant differences in
clutch size (3-6 eggs) or number offledglings
Jul Jul
between years for Yellow-breasted Chats (Ta- 00 CO oc 'It 23 9
ble 1). Percent parasitism ranged from 0 to ^ E 04
44% for Yellow-breasted Chat nests during
2002-2003. There was no difference in fecun- "XTOTi. 0o—0i 0io0in 00O40' i^oinit 9.74 11.34
dity between parasitized and non-parasitized UJ
females (2.26 ± 1.53 and 1.71 ± 1.74 fledg-
0li.n2g8s2,).reFsipfetcyt-isveevleyn,pter=cen1t.0o1f,Ydefllo=w-5b1r,eaPste=d 3 1£) “UC3ji _"35 I033—0 33 9Jul Jun
CO 00 CO 28
Chat nests monitored fledged at least one nest- CO 04
ling in 2002-2003. Similarly, there was no
csligunticfhicsainzte adinfdfefreecnucneditiny oefitYheerlloywe-abrreawshteend o —OO'; 'i—Int 00 d’00 7.94 10.96
Chat females breeding in isolated sites were
compared with females in continuous tracts of
habGirtaaty(CTaatbblierd1).clutch size ranged from two £I•Fs CFI (N (N — 2Ju7n 1Ju6n
to five and did not differ between isolated
(3.67 ± 1.37) and continuous sites (3.76 ± — 0d0 c (OdNN 1.75 1.65
0.72). Female fecundity of Gray Catbirds in ' ri
isolated (1.33 ± 1.75) and continuous (2.54 ±
1.65) sites did not differ {t = 0.23, df = 29, -
P = 0.814; t = 1.84, df = 32, P = 0.075; '3o^£,5 1.33 2.54
=
Table 1). There was no difference in nest suc- li!
cess between habitats for Gray Catbird (Table
1). 1.37 0.72
Yellow-breasted Chat nest success rates dif-
fered between years and between continuous
and isolated sites. In 2002, nest success was iTi
higher in continuous habitats than isolated d din dTf (dn 3.67 3.76
habitats while in 2003 there was no difference
bhiegthweereninhiasboiltaattesd (siTtaebslein12)0.0N3estthasnuicnceisssolawtaesd Tt O (N 00 0 0
sites in 2002 (Table 1).
Mean Yellow-breasted Chat territory size in
2003 was 0.25 ± 0.24 ha {n = 20). No sig-
tniwfeiecannthadbiiftfaetretnycpeein(0t.e2r2rit±ory0.s2i8zefeoxrisitseodlabtee-d ^Ov in do 0.916 0.914
sites, 0.29 ± 0.16 for continuous sites; t =
-0.56, df = 18, P = 0.584). Territory size O' 00 —sC ^ 6 28
(N
was not measured in 2002.
^ ^
DISCUSSION 2003 2003
04 (N (N 0^
We found no evidence that fragmentation or
isolation ofriparian habitats in a western land-
scape reduced reproductive success or in- Catbird
creased brood parasitism of Yellow-breasted Continuous
Isolated
Chats or Gray Catbirds. Percent parasitism,
Gray
clutch size, and fecundity did not differ be-
SHORT COMMUNICATIONS 497
tween isolated and continuous habitat patches birds can successfully reproduce in a highly
for either species. However, delayed incuba- fragmented environment. Settlement patterns
tion at isolated sites by both species suggests in this study suggest that, although continuous
continuous sites may be preferred habitat, fill- and isolated sites may be comparable in qual-
ing up sooner and causing late arrivals to ity, continuous tracts may be preferred breed-
breed elsewhere (Fretwell and Lucas 1970). ing locations and have the capability to sup-
Differences between years may reflect local port higher breeding densities. Alternatively,
environmental fluctuations. More breeding isolated patches may lag behind continuous
pairs were detected in 2003 but decreased nest patches in resource availability for reproduc-
success in 2002 may be indicative of overall tion. It is also possible that continuous patches
decreased habitat quality in that year. may cater to the semi-colonial nature thought
Both Gray Catbirds and Yellow-breasted to be a characteristic ofYellow-breastedChats
Chats had similar patterns of productivity and (Griscom 1923).
nest success between habitats in 2003. When The current level of riparian habitat frag-
compared to other Yellow-breasted Chat pop- mentation does not appear to be a limiting fac-
ulations breeding at the core of their range tor for either Yellow-breasted Chat or Gray
(Thompson and Nolan 1973, Schadd and Catbird populations in the Okanagan Valley,
Ritchison 1998, Rickets and Ritchison 2000), British Columbia. Parameters for Yellow-
the Okanagan population had similar levels of breasted Chat breeding at the northwest extent
productivity. of their range are comparable to more south-
Yellow-breasted Chat territory size (0.25 erly populations, suggesting that conditions at
ha) in the Okanagan Valley was smaller than the edge of the species’ range do not reduce
reported for mid and high-density southern productivity. The smaller territory size ofYel-
Yellow-breasted Chat populations (0.5-1.0 ha, low-breasted Chats in the Okanagan Valley
Dennis 1958; 1.24 ha, Thompson and Nolan may suggest a higher concentration of breed-
1973). Clutch size in the Okanagan (3.5 eggs/ ing resources.
nest) was the same as that of more southerly
ACKNOWLEDGMENTS
populations (3.5 eggs/nest; Thompson and
Nolan 1973). Overall fecundity (2.86 young/ We thank Environment Canada and WWF Endan-
successful nest) was within the range of that gered Species Recovery Eund forfunding support. We
reported for southeastern populations (3.25 also thank those who offered research expertise and
young/successful nest, Schadd and Ritchison assistance in the field forthis project. In particular, we
a1n9d98;non2-.p3a-r2a.s9itiyzoeudngn/essutcs,cesWshfiutlehpeaardasiettizaeld. tDKh.iabnbJ.kinEalmhleetrlhype,eddeRcdooinlclaetHcaetldlt,fhieeDsleadvdaisadstias.NtiaWenl.tds,;E.Ma.EnadsAt.RoenBn,eezeKn.MecrL-,.
2000). The percentage of nests that were suc- DeGroot, J. M. N. Smith, and Barry Lancasterprovid-
cessful (57%) was midrange when compared ed editorial help. We thank Audra Moir and the Oso-
to other studies (22.5%, Thompson and Nolan yoos Indian Band for providing access to their land,
1973; 84.2%, Schadd and Ritchison 1998; and all members of the community who provided in-
45%, Rickets and Ritchison 2000). The con- formationoncurrentandhistoricYellow-breastedChat
sistency of fecundity values between the saingdhtRi.ngsA,.iCnanpnairtnigcsu.laWreStaelvseoOtghlaen,kLtahurreeieanRoocnkywmeoluls,
Okanagan Valley and southern populations in- reviewers for their suggestions.
dicates conditions at the fringe of this species
range are not negatively affecting productivi- LITERATURE CITED
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