Movement and Territoriality of Wintering Hermit Thrushes in Southeastern Louisiana

7 Pages·2000·3.6 MB·English

by  David R Brown

2
Wilson Bull., 1 12(3), 2000, pp. 347-353
1
MOVEMENT AND TERRITORIALITY OF WINTERING HERMIT
THRUSHES SOUTHEASTERN LOUISIANA
IN
DAVID R. BROWN,1 PHILIP C. STOUFFER, AND CHERYL M. STRONG1
—
ABSTRACT. We describe the spatial organization and social behavior of Hermit Thrushes (Catharus gut-
totus) wintering in pine plantations and an adjacent hardwood forest in Tangipahoa Parish, Louisiana over three
winters. We used point counts (n = 403) to collect data on agonistic behaviors and relative abundance within
the study area. We used mist-netting to study site fidelity on four 9-ha plots within and among years. We used
radio-telemetry to measure Hermit Thrushes’ movements and territoriality (n = 50). We found that Hermit
Thrushes saturated suitable patches within the study area. Most Hermit Thrushes actively defended small [mean
= 0.55 ± 0.03 (SE) ha], minimally overlapping (15.90 ± 3.63%) territories throughout the winter. Hermit
Thrushes established and maintained territories using the same agonistic behaviors described for breeding birds.
A few non-territorial birds (14%) moved among occupied territories, but most were faithful to a larger neigh-
borhood, apparently awaiting a territory vacancy. Territorial behavior and frequency of non-territorial birds did
not differ among age and sex classes, suggesting the absence of a sex- or age-based dominance hierarchy. The
behavior of Hermit Thrushes conformed to the emerging view that competition for spatially mediated resources
on the wintering grounds, such as food or cover, contribute to limiting populations of many species of migrant
passerines. Received 10 Dec. 1999, accepted 11 May 2000.
The winter behavioral ecology of most spe- ioral ecology of a population or species. Sev-
cies of migratory landbirds is poorly under- eral detailed investigations of territorial win-
stood. In general, we know that migrant pas- tering migrants have revealed that dominant
serines employ a variety of behavioral strate- individuals are able to maintain stable terri-
gies during the winter to deal with specific tories throughout the winter, while some sub-
environmental conditions (Rappole and Warn- ordinate birds are forced into a non-territorial
er 1980, Hagan and Johnston 1992). The spe- strategy in search of resources (Rappole et al.
cies specific behaviors used to deal with spa- 1989, Winker et al. 1990, Marra et al. 1993,
tial limitations have important implications for Wunderle 1995). Floater (non-territorial) be-
models of population regulation and the de- haviors have only recently been recognized
velopment of conservation strategies (Sherry within wintering populations (Smith 1978,
and Holmes 1996). Winker et al. 1990, Stutchbury 1991), al-
Local winter spatial strategies are highly though the behavior may be as common as on
variable among species of migrant songbirds. the breeding grounds.
The absence of breeding activities frees many Floaters are also considered indicators of
species to move with the protection of a flock habitat and/or food resource limitation, pro-
or to live solitary, vagrant lifestyles. Many viding important insight into a population's
species, including perhaps a majority of non- local distribution (Brown 1969). The poten-
flocking migrant passerine species, establish tially diverse behavioral patterns of floaters
resource-based territories that are defended for make them difficult to categorize and some-
at least part of the wintering season (Rappole times difficult to detect.
and Warner 1980, Holmes et al. 1989, Winker Among forest thrushes, only Wood Thrush-
et al. 1990). Territory defense and exclusion es (Hylocichla mustelind) and Bicknell's
based only on resource availability are likely Thrushes (Catharus bicknelli) have been stud-
indicators that habitat availability and food re- ied on their wintering grounds. Many Wood
sources are limiting factors during the winter Thrushes that wintered in Veracruz, Mexico
(Brown 1969). Before questions about habitat had stable territories, but a large subset of the
and food resource use can be answered, it is population exhibited a wandering strategy of
necessary to describe the basic winter behav- spatial use (Rappole et al. 1989, Winker et al.
1990). Similarly Bicknell’s Thrushes are ter-
ritorial on their wintering grounds and some
1 Dept, of Biological Sciences, Southeastern Loui-
siana Univ., Hammond, LA 70402-0736. may be floaters (McFarland and Rimmer, pers.
2 Corresponding author; E-mail: [email protected] comm.). Gram and Faaborg (1997) reported
347
— n n
348 THE WILSON BULLETIN • Vol. 112, No. 3, September 2000
that wintering Hermit Thrushes in Mexico fidelity, and over-winter persistence of individual
also might be territorial. birds. We passively netted each plot for 2 days (one
We investigated patterns of spatial use and period) every 5-6 weeks from November to April for
social behavior by short-distance migrant Her- all three winters. In addition, once each year in early
winter and again in late winter we used playback re-
mit Thrushes (Catharus guttcitus) wintering in cordings and a decoy in an attempt to capture and re-
southeastern Louisiana. We hypothesized that sight every bird on the plots (Sliwa and Sherry 1992,
intraspecific competition results in a behav- Graves 1996). All captured birds were color banded
iorally mediated system of territoriality. This and aged as either hatch-year/second-year (HY/SY) or
hypothesis leads to three questions. Does the after-hatch year/after-second year (AHY/ASY) based
on criteria of Pyle (1997). Dwyer (1998) used a mo-
winter behavior of Hermit Thrushes conform
lecular technique to sex many of the Hermit Thrushes
to the standard definitions of territoriality? Do captured during the winters of 1996-1997 and 1997-
alternative classes of behavior exist within the 1998. We obtained few behavioral observations of
system of territoriality (i.e., floaters)? Finally, known age ( = 58) and known sex ( = 11) birds
does a dominance hierarchy among sex and because only captured birds could be aged and sexed.
age classes affect Hermit Thrushes’ spatial We used radio-telemetry of birds netted on study
plots to determine if individuals exhibited territorial or
distribution?
floating patterns of movement, as well as to quantify
territory size, rates and distances of movement, and
METHODS
interactions with neighbors. We put radio transmitters
The study area, located in northern Tangipahoa Par- on 50 Hermit Thrushes captured on the four study
ish, Louisiana (30° 41' N, 90 28' W), is a 30 km2 lob- plots with transmitters weighing 1.8 grams (Wildlife
lolly pine (Pinus taeda) plantation isolated by a mixed Materials Inc.), attached to a trimmed interscapular
landscape of riverine hardwood forest, agricultural feather tract with epoxy glue.
A
land, and rural housing. The pine plantation is a mo- single observer triangulated bird locations, usu-
saic of even-aged stands ranging in size from 2 to 244 ally with at least three bearings, and separated consec-
ha [mean = 53.84 ± 9.59 (SE) ha] and represents a utive locations by at least 45 min to insure indepen-
continuum of age classes from clearcut to mature for- dence (Swihart and Slade 1985, White and Garrot
We
ests (20-30 years). 1990). estimated locations to be accurate to within
m m
r1i9o9Wd7se,: ca1ro9rn9id7vua-lc1t9(e91d85,-p3o1i1n9t9O8cct-oo1ub9ne9tr9s)),fodfrualrtlihnr(eg1e-f3wi0vientNdeoirvssteim(nb1ct9e9rp6)e,- b5tryanhsamanitdt,taeradsinsdattaunskceendoowRfnA1N0lo0GcEatSiobnVass.efdWoreosnppalttoirtaitlaendagnuallloayctasitioisnonotsfo
early winter (1 December—31 January), late winter (1 territory size, distance moved between locations, and
February-14 March), and spring (15 March-30 April). interaction between neighboring individuals (Kenward
We located all point counts within the study area but and Hodder 1996). We estimated territory size with the
outside of mist-netted plots. Point count locations were minimum convex polygon (MCP) method after visual
seperated by at least 120 m. Three observers conduct- comparisons with more robust harmonic mean and
ed 422 point counts at 191 locations. Kernel methods showed that MCPs estimated biolog-
We used point counts (n = 120) conducted during ically meaningful home ranges with fewer location
the arrival and fall periods to study arrival time, rela- samples. We used incremental analysis, which plots
tive abundance, and intensity of vocalizations in the changes in territory size as sample size increases, to
study area. Arrival and fall point counts were con- determine the minimum number of locations necessary
ducted as 10 min silent surveys without any playback for a good MCP estimate of territory size. We identi-
solicitation. We repeated 40 replicate point counts fied floaters by visual inspection of the distribution of
twice during the arrival period and once during the fall MCP size among individuals. We measured movement
period at ten day intervals. rate of individuals as the straight-line distance between
We used winter and spring point counts (n = 283) consecutive locations.
to quantify relative abundance and agonistic behaviors. Amount ofterritory overlap provides information on
Winter and spring point counts always began with a level of saturation and potential for interaction be-
10 min quiet period, followed by 5 min of playback tween neighbors. Measuring territory overlap requires
recordings of Hermit Thrush songs and calls. For all simultaneously radio-tracked groups of individuals and
point counts, we counted Hermit Thrushes and record- biologically meaningful estimates of territory bound-
ed the time, distance, and method by which each Her- aries. We used harmonic mean estimates of territory
mit Thrush was originally detected, and the occurrence boundaries to compute overlap because, they visually
of agonistic displays. We tested for changes in the lev- reflected the areas used most intensely by radio-tagged
MCP
el of vocalizations through fall and winter using con- birds better than or Kernel methods. Harmonic
tingency table analysis of detections with and without mean home range estimates are extremely biased to
playback. grid cell spacing (White and Garrot 1990). We used
We used four 9-ha study plots, each with 28 mist- the program TRACKER (Angerbjorn 1994) instead of
nets arranged in a grid, to study the movements, site RANGES V for this analysis because it suggests a grid
n
Brown HERMIT THRUSH TERRITORIALLY 349
et al. •
cell spacing based on the density of locations. Because
harmonic mean estimates of home range are more ro-
bust when built from large sample sizes, we limited
the analysis of territory overlap to birds with at least
30 locations.
Dynamic interaction is a measure of the attraction,
repulsion, or indifference between neighboring indi-
viduals. It uses temporally coinciding locations to
compute the distance between actual locations of an
individual and randomly selected potential locations of
its neighbor. Because there was some overlap in the
neighborhood of nine simultaneously tracked birds, we
conducted a dynamic interaction analysis to determine
if pairs of birds were actively avoiding each other. Dy- Arrival Fall Early Late Spring
namic interaction analysis is based on Jacob’s Index; winter winter
calculated with Ranges V; Kenward and Hodder 1996). FIG. 1. Point count detections of Hermit Thrushes
Jacob’s values approaching 1.0 indicate that simulta- in southeastern Louisiana with and without playback
neous locations of a pair are closer to each other than of Hermit Thrush vocalizations. Detections without
the distance between their territory centers given equal playback are dependent on vocalizations by Hermit
likelihood that they could be anywhere on their terri- Thrushes, while total detections with and without play-
tory at that time. Values approaching —1.0 indicate back more accurately reflect actual density. Arrival =
batiilcroadnlse.aanrWaeleyfsaiursst,ehdesrtaSatYiwsSatiyTcsAfTarroe8m.r0eeapfcoohrrtoealtdlheanrsontm-hseapanantbiya±lcSshtEaa.nticse- 3M11a5r-.3Ja-1n3.0O,ctAl.ap,tre.fawllin=ter1-=301NoFve.b,.-e1ar4lyMawri.n,tesrpr=ing1 D=ec.15-
RESULTS
Fall point count surveys showed that Her- notes. Other detections were made by a high
mit Thrushes began to arrive on their winter- pitched wheez note (10%), a catbird-like rank
ing grounds in southeastern Louisiana during vocalization (3%), and wing winnows (1%;
the second week of October. Density and see description ofbehaviors in Jones and Don-
abundance increased rapidly and stabilized by ovan 1996). We heard singing in the form of
the end of the third week in October. Terri- soft, whispering song several times both with
torial vocalizations as measured by point and without playback, usually soon after fall
Common
count detections without playback increased migration. agonistic visual displays
with population density and peaked in the fall included tail- and wing-flicking, crest raising,
before declining early in winter (Fig. 1). Den- foot quivering, bill snapping, puffing, and
sity, as measured by number of detections stretching. We recorded agonistic displays
with playback, remained high through the ear- during almost every visual observation that
ly winter period, although the frequency of followed playback recording of Hermit
vocalizations declined as indicated by fewer Thrush vocalizations.
birds detected during the silent period of point Intraspecific agonistic interactions between
counts (x2 = 109.83, n = 260, P < 0.001). two or more Hermit Thrushes, such as chasing
We detected 263 Hermit Thrushes at 283 or perch displacement, were observed in about
winter points counts with at least one detec- 21% of playback events. We recorded behav-
tion at 52% of locations. Only 11% of the iors of 1 1 individuals of known sex and 58
Hermit Thrushes we detected at winter point individuals of known age. Females ( = 9)
counts were detected before the playback of and males (n = 2) displayed most of the com-
vocalizations. mon aggressive behaviors. Similarly, all be-
Wintering Hermit Thrushes displayed a va- haviors were observed in both HY/SY (n =
riety of agonistic behaviors, most of which are 43) and AHY/ASY (n = 15) age groups.
commonly used on the breeding grounds (Dil- We captured 206 individual (newly banded)
ger 1956, Jones and Donavon 1996). At win- Hermit Thrushes in 24,416 hours of passive
ter point counts with playback, 82% of Hermit mist-netting. Thirty-seven percent of all cap-
Thrushes were first detected by chup notes. tured individuals were AHY/ASY and 74% of
Only 2% were first detected visually. Almost 82 sexed individuals were female (Dwyer
all non-playback detections were by chup 1998). Capture rate of Hermit Thrushes in-
350 THE WILSON BULLETIN • Vol. 112, No. 3, September 2000
eluding recaptures (within the same year), re-
turns (between years), repeats (same day or
period), and unbanded birds was 1.4 per 100
We
net-h. captured 65 additional individuals
and 24 recaptures using playback and decoys,
for a total of 271 individuals captured. We re-
captured 95 (35%) of these within a year (but
different period). Twenty-five (18% of indi-
viduals banded in the first and second years)
returned in subsequent years to winter on the
same study plot. The age ratio of returning
individuals in their first year of capture did not
Territory size (ha)
differ from the age ratio of all individuals cap-
tured (x2 = 0.21, n = 323, P > 0.05). Simi- FIG. 2. Minimim convex polygon territory size for
larly, the sex ratio of returning Hermit Thrush- 41 Hermit Thrushes. The large gap in the distribution
es (75% of 12 were female) did not differ between 34 individuals with territories smaller than 1
from the overall ratio in the study area (Fisher ha and 7 individuals with larger territories was used to
Exact Test: x2 = 0.03, n = 94, P > 0.05). Of characterize floaters.
the 25 returns only two were captured more
than 30 m from the net where they were orig- floaters moved more than 1 km between lo-
We
inally captured. never captured or ob- cations (Fig. 3A). Floaters sometimes dis-
served any marked Hermit Thrush at more played aggressive behavior similar to that of
than one 9 ha plot or at more than one point territorial birds. One floater was originally
count location. captured using playback vocalizations and an-
We
radio-tracked 50 Hermit Thrushes. Ra- other responded to playback after its radio
dios fell off after an average of 21.76 days failed. Dwyer (1998) sexed two floaters; one
(±1.98, range = 2-53). During the period was a male and one was female. Age of float-
when radios were attached, we collected an ers (5 HY/SY, 2 AHY/ASY) did not differ
average of 26.44 (± 2.5, range = 3-63) lo- from the age ratio of all other captured Hermit
cations per bird. Incremental analysis of 12 Thrushes (65% HY/SY).
home ranges showed that MCPs begin to pla- Of nine simultaneously radio-tracked
teau after 10-20 locations. Seven radio neighboring birds, eight had distinct, abutting
equipped Hermit Thrushes were categorized territories. The analysis revealed little territory
as non-territorial based on their large home overlap except for one bird that we catego-
range sizes. Of the remaining birds, we had rized as a floater. We simultaneously radio-
enough locations (>10) to make territory size tracked four additional pairs and one triad
comparisons for 34 Hermit Thrushes. Nine with overlapping or abutting home ranges.
birds with transmitters were not included in Overlap between territorial birds averaged
the analysis because of small location sample 15.90% ± 3.63%, n = 21) and ranged from
sizes. Territory sizes averaged 0.55 ± 0.03 ha, 0-79% of territory area (Fig. 3B). Every com-
=
(range 0.22-0.97 ha). Territory sizes did not bination of overlapping pairs scored near zero
differ between sexes (F, = 0. 164, P > 0.05), on Jacob’s Index, indicating that birds were
14
or between age groups (F, = 1.212, P > indifferent to each other (mean = 0.005 ±
32
We
0.05). located nine birds at night, all roost- 0.005).
ing within their territories.
DISCUSSION
Most Hermit Thrushes (85%) maintained
territories with a MCP less than 1 ha. We clas- Banding data indicated that many wintering
sified seven birds with large MCPs as floaters Hermit Thrushes were site faithful from No-
(5.83 ± 1.06 ha, range = 2.53-11.3; Fig. 2). vember to March. Ninety percent of within
The average distance between consecutive lo- year recaptures occurred at the same net
cations for territorial birds was 55.30 ± 1.59 (45%) or a net within 80 m (the average di-
m. In contrast, floaters moved an average of ameter of a territory). Hermit Thrushes did not
121.28 ± 25.36 m between locations, two move out of their territories to roost elsewhere
Brown et al. • HERMIT THRUSH TERRITORIALLY 351
FIG. 3. Movements of Hermit Thrushes wearing radio telemetry transmitters. A. Movement patterns of two
floating and three territorial Hermit Thrushes in relation to 9 ha mist-net plot (box). Home range contours are
also shown for territorial birds. Lines indicate movements between successive locations. B. Territory contours
(95% hamonic mean method) of eight simultaneously radio-tracked Hermit Thrushes showing variation in ter-
ritory size and overlap among neighbors. A floater that moved among territories of these individuals is not
shown.
at night. Although only 18% of Hermit territorial birds and experience lower survival
Thrushes returned between winters, the indi- relative to territory holders (Brown 1969).
viduals that returned used the same territory Collectively, the floaters we monitored do
as in previous winters. Additional Hermit not fit into either definition but showed a di-
Thrushes may have returned to the area, but verse array of spatial strategies. For instance,
relocated outside of our plots. Telemetry data one radio-tagged bird had a very small core
indicated that most wintering Hermit Thrushes home range but otherwise wandered widely
maintained distinct, minimally overlapping among neighboring territories and surround-
territories. High vocalization rates during the ing habitats. We suspect this bird may have
initial settlement indicated aggressive territory been looking for a new, less constrained ter-
establishment. The lack of spatial interaction ritory but was encountering spatial pressure
between neighboring individuals and the low wherever it wandered. Another individual
m
level of vocalizations following settlement crossed a 60 wide river channel more than
suggests that once territory boundaries were 10 times during 2 months, moving back and
established, boundary maintenance was min- forth between two habitats. Two other Hermit
imal. Thrushes moved long distances (0.6 km and
Hermit Thrushes that did not establish a sta- 1 km) before radio contact was lost. One pat-
ble territory in a suitable habitat were lumped tern was consistent among all the floaters in
into the broad category of floaters. The term this study: they chose alternatives to territo-
floater has been used to describe two different riality instead of establishing territories in un-
patterns of behavior for non-breeding birds. suitable habitats (i.e., clearcuts).
First, floaters have been characterized as in- We found no evidence that winter territorial
discriminate wanderers, perhaps faithful to behavior of Hermit Thrushes resulted in dom-
particular habitats, but not defending specific inance of males over females or older over
territories. Usually such floaters are recog- younger age groups, although we had small
nized only as individuals not recaptured or re- sample sizes for these comparisons. Females
sighted in repeated mist-netting and spot-map- used agonistic calls and displays as did males,
ping periods (Bates 1992). The second expla- and females maintained territories of the same
nation characterizes non-breeding season size as males. Males as well as females be-
floaters as secretive but intent subordinates, haved as floaters, and females returned to the
stalking selected territories in search of va- same territory between winters as frequently
cancies (Smith 1978). In general, floaters are as males. Similarly, younger Hermit Thrushes
thought to be at a behavioral disadvantage to used agonistic behaviors to successfully de-
352 THE WILSON BULLETIN • Vol. 112, No. 3, September 2000
fend territories that did not differ in size from etry, estimated that 50% of the Wood Thrush-
those of older individuals. Finally, younger es in Veracruz, Mexico were floaters, com-
Hermit Thrushes were no more likely to be pared to 14% of Hermit Thrushes in our study.
floaters and returned between winters at the The higher proportion in Mexico might be
same frequency as older Hermit Thrushes. caused by more pronounced habitat limita-
Territorial wintering species with obvious tions than occur in southeastern Louisiana.
morphological differences between age and Site fidelity between winters is an important
sex classes, like American Redstarts Setopha- indicator of breeding season survivorship and
(
ga ruticilla) and Hooded Warblers (Wilsonia winter habitat quality. The proportion of Her-
citrina), quickly sort into distinct dominance mit Thrushes returning between winters in our
hierarchies after arriving on their wintering study is lower than for all other studied spe-
grounds (Holmes et al. 1989, Stutchbury cies of migratory passerines. Other research-
1994). In contrast, the subtle morphological ers on wintering passerines report overwinter
differences between age and sex classes of returns near 50% (Nisbet and Medway 1972,
Hermit Thrushes may be related to our failure Price 1981, Winker et al. 1990). Differences
to detect a dominance hierarchy. The absence may be due to species-specific survivorship
of dominance hierarchies during the winter and dispersal patterns. Other studies of win-
might be expected in other monomorphic mi- tering Hermit Thrushes report return rates
grant species (Bates 1992). similar to ours. In South Carolina, 13% of 81
We
have provided four types of evidence birds returned between winters (Bent 1964),
that suggest Hermit Thrushes saturate suitable and in California, 18% of 306 returned be-
patches within the local landscape. First, Her- tween winters (reported in Jones and Donovan
mit Thrushes aggressively defended resources 1996). Estimates for Hermit Thrushes, includ-
with agonistic vocalizations and displays that ing our own, might be biased low by sampling
are also used on breeding grounds for territory methods or species-specific behaviors. Long-
defense. Second, Hermit Thrushes occupied term studies would allow for more robust
distinct home ranges that abut or slightly over- mark-recapture analyses and increase the ac-
lap with neighboring conspecifics. Third, curacy of these estimates.
some individuals behaved as floaters, moving The behaviors of wintering Hermit Thrush-
within and among territories of other individ- es in southeastern Louisiana clearly conform
uals. Finally, vacated territories were quickly to Nice’s definition of Type-E winter territo-
occupied by either expanding or shifting ries (Nice 1941). Territoriality in non-breeding
neighbors, or floaters. Two territories vacated migrants is now a widely recognized behavior,
by Sharp-shinned Hawk (Accipiter striatus) but its function in population regulation is not
predation were reoccupied within one week fully understood (Rappole and Warner 1980).
by vocalizing Hermit Thrushes. We do not The evidence for overlapping territories and
know if replacements were floaters or territo- the presence of a floater class confirms that
rial neighbors that occupied the open space. the distribution of wintering Hermit Thrushes
In either case, rapid replacements indicate sat- in southeastern Louisiana agrees with Brown’s
uration. (1969) Critical Density Level 3. Most birds
Wintering Hermit Thrushes defended small- actively defend distinct, minimally overlap-
er territories at higher densities than eastern ping territories, probably in response to some
populations of breeding Hermit Thrushes.
limiting resource (i.e., food or space).
Martin (1960) estimated territories in Ontario,
Canada averaged 0.72 ha. Jones and Donovan ACKNOWLEDGMENTS
(1996) reported mean sizes of 2.5 ha (range
= 0.5-14.3) from 12 breeding season studies. We thank G. Dwyer, D. Barringer, J. Long, and A.
In comparison, we estimate 0.56 ha based on Bassett for field assistance. B. McFarland; M. Hartley,
telemetry data from all plots. Our estimates of and an anonymous reviewer provided constructive
comments on an earlier version of the manuscript. Fi-
territory size are similar to those found for
nancial support was provided by a grant from the Lou-
Wood Thrushes wintering in Veracruz, Mexi-
isiana Educational Quality Support Fund to P. Stouffer.
co (Winker et al. 1990). Weyerhauser, Inc. also provided financial and logistical
Winker and coworkers (1990) using telem- support. The staff at the Girl Scouts of America’s
Brown et al. • HERMIT THRUSH TERRITORIALLY 353
Camp Whispering Pines generously allowed access to in relation to forest succession in Algonquin Pro-
their land. vincial Park, Ontario. Ecology 41:126-240.
Nice, M. M. 1941. The role of territory in bird life.
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