Habitat Specialization and Foraging Behavior by Birds of Amazonian River Ilands in Northeastern ...

17 Pages·2001·1.22 MB·English

by  Gary H. Rosenberg

The Condor9 21421-443
0 The Cooper Ornithological Society 1990
HABITAT SPECIALIZATION AND FORAGING BEHAVIOR BY
BIRDS OF AMAZONIAN RIVER ISLANDS IN
NORTHEASTERN PERU ’
GARY H. ROSENBERG~
Louisiana State UniversityM useum of Natural Science,B aton Rouge, LA 70803
Abstract. Roughly 15% of the rich Amazonian avifauna has been shown to be dependent
on riverine habitats, particularly islands that form in the channels of the Amazon River
and its largestt ributaries. In northeasternP eru and southeasternC olombia, over 230 species
have occurredo n river islands, about 19 of which appear to be restricted to habitats found
almost exclusively on islands, such as Tessaria-scruba nd large tracts of Cecropia forest.
Habitat specializationa nd foragingb ehavior were studied quantitatively for 3 1 bird species
that used only terrestrial habitats on two study islands on the Napo River in northeastern
Peru. Density estimates for residents on a small, Tessaria-dominatedi sland, exceeded 160
birds/ha, a figure an order of magnitude greater than bird densities previously reported for
forest habitats in the Neotropics. Obligate river island speciess howed a higher degree of
microhabitat specialization than did nonobligate island species. Several of the most spe-
cialized speciesw ere found mainly in the Tessarias crub or the understory of the Cecropia
forest. Most of the speciesw ere primarily insectivorous,b ut several also took advantageo f
the abundant Cecropiaf ruit and were at least partially fiugivorous. Little difference in the
degreeo f foragings pecializationw as found between obligate and nonobligatei sland species,
however, both groupsw ere specializedw ith respectt o foragingt echnique and substrateu se.
Water fluctuation of up to 20 m between seasons,c ompletely flooding the young islands
and the understory of the older islands, createst he need for speciest o be good colonizers,
as doest he natural, rapid, successiono f vegetationo n the islandsf rom Tessaria,t o Cecropia,
to an older, more complex vegetation. Conservation plans for the Amazon basin should
take into account the high dependencyo f specieso n riverine habitats, particularly islands.
Key words: Amazonia; habitat specializationr; iver islands;N apo River;foragingb ehavior;
Peru; Tessaria scrub;C ecropiaf orest.
INTRODUCTION occurs only in riverine habitats (Remsen and
Parker 1983).
Among the many factors thought to contribute
Islands that form in the Amazon River and its
to the high bird species richness in the Neotropics
larger tributaries, which are covered by early
is the high diversity of habitat and microhabitat
successionalv egetation, constitute a large pro-
types, some of which are unique to tropical re-
portion of the total riverine habitat in Amazonia.
gions (Orians 1969, Karr 1976, Terborgh 1980b).
After formation, these islands travel down-
Even more important is the increased degree of
stream, in a sense, by constant erosion of the
specialization on specific microhabitats, such as
older, upstream portions, and the constant in-
bamboo (Parker and Parker 1982, Remsen 1985)
creasei n size by the deposition of sand and silt
and foraging substrates, such as suspended dead
on the downstream ends. This constant change
leaves (Remsen and Parker 1984). Although bird
createsa n array of early successionalh abitats on
species richness reaches its peak in terra jirme
the islands. The use of and specialization on such
forest, it is the contribution by habitats other
habitats by Amazonian birds has remained poor-
than primary forest that elevates the richness to
ly known (see Remsen and Parker 1983, Hilty
such high levels in western Amazonia. For ex-
and Brown 1986).
ample, it has been shown that more than 14%
Exploratory visits to islands in the Napo River
of the nonaquatic avifauna of the Amazon basin
in northeastern Peru in 1982 with the Louisiana
State University Museum of Natural Science
(LSUMNS) revealed that the assemblageo fland-
1R eceived 4 September 1989. Final acceptance2 6
bird species on these islands was almost com-
December 1989.
* Present address: 5441 N. Swan Road, apt. 313, pletely different from that found in terra jirme
Tucson, AZ 85718. forest on the “mainland” only l-2 km away.
WI
428 GARY H. ROSENBERG
Some species encountered on the islands were from one end to the other (the down-river end
river-edge and second-growth speciest hat were being younger) and potentially have vegetation
common on the mainland, but several appeared characteristicso f all three island types.
unique to the islands. Since 1982, extensive sur- River islands are formed during the low-water
veys of river-island and mainland habitats in season (June-October), when sandbars are ex-
northeastern Peru by the LSUMNS and Phila- poseda nd enlargedb y the deposition of silt. Dur-
delphia Academy of Sciences suggestedt hat as ing low water, vegetation grows rapidly on the
many as 19 bird species may be restricted to exposeds andbars.A t first, grass( Paspalum) cov-
islands in this region ofAmazonia. These surveys ers the ground; this is later replaced by the com-
have added significantly to the growing knowl- posite shrub Tessaria integrifolia and a willow
edge of the distribution and natural history of Salix humboltiana (Remsen and Parker 1983).
these apparent obligate island species,s ome of These young islands may be completely covered
which were known from only a few museum by water during portions of the high-water season
specimens and occasional sightings. (November-May). As an island grows in age and
I returned to the Napo River in 1983 to study size, it becomesd ominated by monotypic stands
the birds found on river islands,p articularly those of Cecropia forest. Although Cecropia sp. is a
speciesa pparently restricted to the islands. My common tree of disturbed area throughout the
primary goals in this study were to: (1) enu- Neotropics, standso f suchl arge size are not char-
merate the bird species using islands in upper acteristic of disturbed areas on the mainland.
Amazonia; (2) describef or the first time the hab- During the high-water season,t he entire under-
itat use and foraging behavior of some river- story of the forest on middle-aged and old islands
island birds; (3) evaluate the degreeo f ecological may be under water. Old islands are character-
specialization among river-island birds; and (4) ized by .largers ize and vegetation that resembles
discussh ow this unique bird assemblagep ersists in complexity the varzea forest (seasonally in-
in such a dynamic, ephemeral environment. undated forest and permanently swampy forest
usually bordering rivers; as described in Parker
and Parker 1982) of the mainland.
STUDY AREAS AND METHODS
Preliminary observations indicated that most
ISLAND FORMATION AND HABITATS of the poorly known island speciesw ere found
Amazonian river islands are part of a continuous primarily in two major habitat types: Cecropia
chain of islands that extends from eastern Brazil (river-edge) forest, and Tessaria sandbar scrub,
up the Amazon River and most of its larger trib- as described by Remsen and Parker (1983). My
utaries as far as Ecuador and Peru. The islands study focusedo n one middle-aged and one young
studied here are “sandbar islands” that form in island in the Napo River that contained both
the river, as opposed to “‘oxbow islands” that habitat types. My primary study site, visited dai-
form when the river channel changesi ts course ly between 2 June and 31 July 1983, was Ron-
and isolates a portion of mainland forest. These socoI sland, located opposite the small village of
two island types differ in vegetation complexity, Libertad, approximately 1 km east of the village
but a very old sandbar island will eventually vi- of Negro Urco, 80 km north of Iquitos. Accord-
sually resemblea n oxbow island in terms of vege- ing to local residents,R onsocow as about 16 years
tation complexity. old. Approximately 1 km by 0.25 km in size, this
Water levels on the Amazon and Napo rivers island was about equidistant between the two
in northeastern Peru fluctuate as much as 20 m banks of the Napo (which was about 1.5 km wide
between seasonsd ue to runoff from the Andes at that point). Monotypic stands of Cecropia
(Gentry and Lopez-Parodi 1980; Peter Jenson, ranging from 5 to 13 m tall were prevalent on
pers. comm.), and as a result of the dynamic the island, with an occasional emergent Mimosa
nature of this system, river islands vary substan- sp. A distinct middle-story layer consisted of a
tially in age and size. Three age classeso f islands “broad-leafed” tree (identification unknown) that
may be recognized( young, middle-aged, and old, ranged from 4 to 7 m in height. The understory
as describedb elow). Some islands are even-aged was dominated by Heliconia sp. thickets and a
throughout and easyt o classifya s one ofthe three vine (Zpomea sp.). Shorter Cecropia forest was
types, whereas others vary considerably in age also present on Ronsoco and was characterized
BIRDS OF AMAZONIAN RIVER ISLANDS 429
high rater
Tess8ri8 Short Cecrupi8 T a 11 Cmzropi8
Sandbar Scrub - River Edge Forest 2
RIVER - ISLAND HABITATS
FIGURE 1. A diagrammatic sketcho f river-island microhabitats:( 1) canopy of tall Cecropiaf orest; (2) canopy
of short Cecropia forest; (3) middle story of tall Cecropiu forest; (4) understory of tall Cecropia forest; (5)
understoryo f short Cecropiuf orest; (6) early successionaTl essurius crub. Note the high-water line that indicates f
which habitats are completely covered during high water.
by even-aged trees 5 to 7 m tall, the lack of a was visited on 10 days, on five of which complete
distinct middle-story, and an understory of vines censuseso f speciesa nd individuals were taken.
and grass,w ith few Heliconia. Basedo n the structure of the vegetation on the
The second study site was Tuhuayo Island, two islands, I divided the two macrohabitats
only 130 m by 30 m, which was equidistant be- (river-edge and sandbar scrub) into six micro-
tween Ronsoco Island and Negro Urco on the habitats that might be recognized by birds (Fig.
mainland. Tuhuayo, only 2 years old according 1): (1) canopy of tall Cecropia forest; (2) canopy
to local residents (in 1982 it was a grass-covered of short Cecropia forest; (3) middle-story of tall
sandbar), was typical of a young island in an early Cecropia forest; (4) understory of tall Cecropia
successionasl tage.V egetation structure was sim- forest; (5) understory of short Cecropia forest;
ple on this island compared with that of older and (6) Tessaria scrub. Ronsoco was a mosaic
islands, with the dominant plant speciesT essaria of tall and short Cecropia forest, as well as small
integrijidia, which attained a maximum height patcheso f Tessaria scrub. Tuhuayo was covered
of 3 m. A lesser amount of Salix humboltiana entirely by Tessaria scrub.
was common along the edge of the riverbank,
AVIAN HABITAT USE AND FORAGING
with some individuals reaching a height of 5 m.
Bacharis sp. was also prevalent on the down- To establish the status of river-island speciesi n
stream end of the island, as were many sapling mainland habitats, and to determine which
Cecropia scattereda mong the Tessaria. Tuhuayo speciesw ere restricted to islands in this region
430 GARY H. ROSENBERG
TABLE 1. List of obligate river-island speciesf ound study sitesa nd other river islands on the Amazon
in western Amazonia. Some of the speciesl isted here River were deposited at the LSUMNS.
were encounteredt oo infrequently to be included in
To evaluate specialization by obligate and
the quantitative portion of this paper.
nonobligate island speciesw ith respectt o habitat
and foraging parameters, I first examined fre-
Blue-rumped Parrotlet Forpusx anthopterygius
quency distributions of observations for each
Olive-spotted Hum- Leucippusc hlorocercus
mingbird variable. I then arbitrarily chosea value of 75%
Pale-billed Homer0 Furnarius torridus or greater use of an ecological category to rep-
Lesser Homer0 F. minor resent “specialization” on that category. In ad-
White-bellied Spinetail Synallaxis propinqua
dition, niche breadths were calculated using the
Red-backed Spinetail Cranioleuca vulpina
formula B = l/[np,2] where pi is the proportion
Red-and-white Spinetail Certhaixis mustelina
Castlenau’sA ntshrike Thamnophilusc ryptoleu- in category i, and n is the total number of cate-
goriesf or the variable under consideration (Lev-
Leaden Antwren M~~motherula assimilis ins 1968) for each of the following variables:
Ash-breasted Antbird Myrmoborus lugubris
plant species; foraging height; foraging tech-
Black-and-white Ant- Myrmochanesh emileucus
bird nique; and foraging substrate.
Brown Elaenia Elaenia pelzelni
River Tyrannulet Serpophagah ypoleuca
RESULTS
Lesser Wagtail-Tyrant Stigmatura napensis
FuscousF ’lvcatcher Cnemotriccusfu scatus COMPOSITION OF RIVER-ISLAND AVIFAUNA
Riverside Tyrant Knipoleguso kocensis
In this part of Amazonia, at least 18 species
Bicolored Conebill Conirostrumb icolor
Pearly-breastedC onebill C. margaritae seemed to be restricted to river islands (hereafter
obligate island species; see Table 1). Of these, 14
were included in the quantitative portion of my
study. During more than 8 months of fieldwork
of Amazonia, I used data from three field ex- along the Napo and Amazon rivers in north-
peditions to northeastern Peru by the LSUMNS, eastern Peru by the LSUMNS that included ex-
an unpublished speciesl ist for a tourist lodge on tensive collecting in all habitats, not one indi-
the Napo River (Explomapo Lodge) compiled vidual ofany obligate island speciesw as recorded
by Theodore A. Parker, III, unpublished field off islands.
notes of J. V. Remsen, Jr. from Monkey Island About 23 1 speciesh ave been recordedo n river
and adjacent vurzea forest, near Leticia Colom- islands in northeastern Peru and southern Co-
bia, from 1974-1975, and my own experience lombia (Appendix). Of these, only 3% have been
from fieldwork on the Napo and Amazon rivers recorded in terra jirme forest on the mainland
spanning 4 years. This included visits to more (Table 2). A far greaterp roportion of river-island
than 10 islands representing all age classes.A ny speciesw as found in other mainland habitats;
speciesr ecorded solely from river islands during 33% of the species also occurred in secondary
these periods of observation is considered here woodland or second-growth, 25% occurred in
to be an obligate island species. varzea (seasonally flooded forest), 40% occurred
During my own visits to the two main study in water-dependent (riverine) habitats that in-
islands, at the initial point of observation of each eluded river, lake, and stream margins, sandbars,
bird, I recorded the following: bird species;t ree and marsh; and at least 14% were seen as aerial
species;t ree height; distance from ground; and transients, that included both foraging and non-
distance from canopy. If an individual was seen foragingb irds that flew over the islands. Virtually
foraging,I alsor ecordedf oragingt echnique( glean, all of the speciesr ecorded on river islands be-
sally, hover, lunge, hang, or hammer) and for- tween May and August were residents in this
aging substrate (leaf, bark, fruit, flower, grass, region of Amazonia, with the exception of the
ground, or air). Data were not (knowingly) col- occurrence of several “austral” migrants (i.e.,
lected on the same individual during the same three specieso f Elaenia; see Appendix). Between
day. For each observation, I placed the individ- August and April, severalN orth American species
ual into one of the six microhabitats base upon may use river islands, particularly shorebirdst hat
its location within the vegetation strata. Voucher probably use the rivers as migratory pathways
specimens for all study speciesc ollected on my (Appendix).
BIRDS OF AMAZONIAN RIVER ISLANDS 431
TABLE 2. Bird speciesc omposition among three age represented taxa were flycatchers (Tyrannidae;
classeso f Amazonian river islands.N umbers represent 38 of 72 species) and icterids (Icterinae; 11 of
the percentageo f speciesf ound on a particular classo f
15 species).
island (let?h and column) and also found on the other
Bird species composition on young islands was
two island classes( across the top). Note that some
speciesa re found on more than one age classo f island. more similar to middle-aged islands than to old
islands (Table 2). A higher percentageo f species
Island age found on middle-aged islands were also found
NO. Middle- on old islands, as opposed to young islands. A
Island age species YOlUg aged Old
similarly.high percentageo f speciesf ound on old
Young 62 - 83% 37% islands were also found on middle-aged islands,
Middle-aged 143 31% - 63%
with very few having been recorded on young
Old 166 13% 59% -
islands (Table 2).
More striking is the comparison between the
number of bird speciesf ound on the three age
Most of the obligate island species are repre- classeso f islands and the various mainland hab-
sentatives of genera typically found in open hab- itats (Table 3). Of the 62 speciesr ecorded from
itats. Those found in open shrubby habitats in- young islands, most were found either in sec-
cluded: Leucippus (Trochillidae), Furnarius and ondary habitats or riverine habitats on the main-
Certhiaxis (Furnariidae), and Elaenia, Stigma- land, whereasn one was found in terrafirme for-
tura, and Serpophaga (Tyrannidae), whereas, est, and only one was found in varzea. As on
others that were found in a mixture of forest and young islands, a high percentageo f speciest hat
open habitats included Synallaxis and Crani- occurredo n middle-aged islands were also found
oleuca (Fumariidae), Thamnophilus (Formica- in secondaryh abitats on the mainland, but more
riidae), Cnemotriccus (Tyrannidae), and Coni- speciesw ere also found in varzea. The forest on
rostrum (Coerebidae). Only Myrmotherula and old islands visually resembled mainland varzea
Myrmoborus (both Formicariidae) can be con- forest, and a much higher percentageo f the old
sidered mostly forest genera. island speciesa lso occurredi n varzea. In general,
Several families or subfamilies of forest birds the bird-species composition on islands resem-
were absent or poorly represented on river is- bled the composition in the riverine habitats
lands including quail (Odontophorinae), trum- (river, lake, and stream margins, sandbars, etc.)
peters (Psophiidae), motmots (Momotidae), tro- on the mainland with at least 40% of the island
gons (Trogonidae), tapaculos (Rhinocryptidae), specieso ccurring there.
and gnatcatchers (Polioptilinae). Equally striking During June and July 1983, I found 110 species
is that only one of seven toucans (Ramphasti- on the two study islands. Ofthese, sufficient sam-
dae), three of 16 woodcreepers (Dendrocolapti- ple sizes of observations of 31 speciesw ere ob-
dae), seven of 56 antbirds (Formicariidae), one tained for quantitative analysis (Table 4). Many
of 11 manakins (Pipridae), and nine of 27 tan- additional nonpasserinest hat visited the islands
agers (Thraupidae), found in this region of Ama- were eliminated from the study becauset hey in-
zonia were found on islands. Among the better- frequently used the terrestrial habitats, or were
TABLE 3. Number of bird speciesf ound on three agec lasseso f river islandsa nd also found in various mainland
habitats. Terrufirme refers to upland forest that is never flooded. Vurzeu refers to seasonallyf looded forest.
Secondaryh abitatsa re influencedb y man, sucha s second-growtha nd older secondaryw oodland.R ivetine habitats
include river, lake, and stream margins, open river, sandbars,a nd marsh. Aerial refers to speciest hat were seen
flying over islands( foraginga nd nonforaging).N umbers in parenthesesa re percentageso f specieso n that particular
age classo f island. Rows add up to greater than 100% becauses ome specieso ccupy more than one habitat.
Mainland habitats
Island age No. species Terra jirme VUrZL?U Secondary habitats Riverim habitats A&?1
Young 62 0 (0%) 1 (lo/o) 23 (29%) 48 (60%) 10 (12%)
Middle-aged 143 4 (3%) 25 (17%) 63 (44%) 47 (33%) 11 (8%)
Old 166 7 (4%) 51 (31%) 53 (32%) 77 (46%) 17 (10%)
Total 231 8 (3%) 58 (25%) 76 (33%) 92 (40%) 33 (14%)
432 GARY H. ROSENBERG
TABLE 4. Frequency of observations within microhabitats: (1) canopy of tall Cecropiaf orest; (2) canopy of
short Cecropiaf orest; (3) middle story of tall Cecropiuf orest; (4) understoryo f tall Cecropiaf orest; (5) understory
of short Cecropiaf orest; and (6) Tessarias crub. Also, frequency of observationsi n various plant species:C P =
Cecropia;M = Mimosa; BL = broad-leafed; T = Tessaria;S = Sal& P = Paspalum; V = vines (Ippomoea); 0
= other. n = sample size.
Microhabitat Plant species
Bird species n I 2 3 4 5 6 CP M BL T S P V 0
Obligate island species
Leucippusc hlorocercus 36 6 17 14 25 8 31 6 3 14 19 31 28
Furnarius minor 30 47 10 43 17 7 30 3 10 33
Synallaxis propinqua 34 9 91 3 56 9 24 9
Cranioleucav ulpina 41 7 17 7 10 12 46 17 10 20 20 20 13
Thamnophilusc ryptoleucus 47 4 15 21 23 21 15 26 9 43 4 15 3
Myrmotherula assimilis 12 17 33 50 75 17 8
Myrmoborus lugubris 22 86 14 55 23 22
Myrmochanesh emileucus 3 7 31 17 43 1 9 13 5 63 8
Elaenia pelzelni :z 81 14 5 61 36 3
Stigmatura napensis 19 11 5 84 11 79 10
Serpophagah ypoleuca 15 100 20 80
Cnemotriccusfi catus 45 7 4 11 33 29 16 9 7 27 13 2 20 23
Conirostrumb icolor 44 86 14 61 36 2
Conirostrumm argaritae 10 90 10 100
Nonobligate island species
Chrysoptilusp unctigula 21 29 19 24 5 10 14 33 43 24
Veniliornisp asserinus 14 21 7 36 21 14 43 36 7 14
Synallaxis gujanensis 22 68 32 36 5 45 14
Synallaxis albigularis 29 3 3 17 24 52 10 10 45 38 7
Thamnophilusd oliatus 11 18 9 9 27 36 27 36 18 19
Todirostrumm aculatum 47 36 21 15 2 6 19 42 28 7 21 2
Myiarchusf erox 13 62 15 23 46 31 15 8
Myiodynastesm aculatus 23 43 35 13 9 70 17 13
Tyrannus melancholicus 19 42 42 21 37 5 32
Tyrannus albogularis 21 52 24 :: 29 48 14 ;
Turdus ignobilis 39 69 18 5 8 82 8 5 5
Thraupis episcopus 41 78 20 2 71 22 7
Thlypopsiss ordida 36 22 28 8 9 3 28 39 8 11 17 8 8 9
Ramphocelusc arbo 14 29 64 7 79 14 7
Sporophilac astaneiventris 39 23 31 3 44 54 3 18 5 15 5
Saltator coerulescens 39 54 15 10 13 8 59 10 8 3 13 7
Ammodramus aurtfrons 9 22 79 22 33 22 22
difficult to observe (e.g., speciesi n the Ardeidae, idents” included several species that represented
Cathartidae, Accipitridae, Rallidae, Charadri- obvious wanderers from older islands (e.g., Pale-
idae, Columbidae, Psittacidae, Caprimulgidae, vented Pigeon, Columba cayennensis,S wallow-
and Alcedinidae). Fourteen of these 110 species winged Puffbird, Chelidoptera tenebrosa,G reat
were determined to be obligate island species. Kiskadee, Pitangussulphuratus,B icolored Cone-
Five complete censuseso n Tuhuayo yielded a bill, Conirostrum bicolor, and Blue-gray Tana-
total of 44 species( see Appendix for complete ger, Thraupis episcopus).C ounts of individuals
river-island speciesl ist), averaging 22.5 species averaged 108 (SD = 5.7) per visit. The 16 species
(SD = 1.8) per visit. That only half the total presentd uring all five censusest ogethere xceeded
number of speciesw ere detected per censuss ug- 160 birds/ha. This amazingly high density is an
gest a high day to day turnover of migrating or order of magnitude greater than any published
roosting birds; this is consistent with other Tes- density figures for mature forest or secondary
saria-dominated habitats in southeastern Peru habitats in the tropics (Karr 1976), and illustrates
(Scott Robinson, pers. comm.). These “nonres- how abundant birds on river islands can be. The
BIRDS OF AMAZONIAN RIVER ISLANDS 433
FIGURE 2. Niche breadthsf or microhabitata nd plant FIGURE 3. Mean foraging heights of river-island
speciesu sageo n the two study islands. The six cate- birds and niche breadths for foraging heights. Niche
goriesf or microhabitat are presentedi n Figure 1. The breadths are presented as a proportion of the total
eight categoriesf or plant speciesa re listed in Table 4. number of foraging heights (categories) available.
Breadth values are presented as a proportion of the Shaded circles represent obligate island species and
total number of categoriesf or each variable (e.g., [ac- open circles represent nonobligate island species.S ee
tual breadth value/61 = value presentedf or breadth of Table 5 for speciesa bbreviations.
microhabitat as there are six microhabitat categories).
Shaded circles represent obligate island species and
open circles represent nonobligate island species.S ee
Table 5 for speciesa bbreviations. The Black-and-white Antbird was largely re-
stricted to vine tangles, where it occasionally as-
cended along tree trunks into the middle story.
most common resident on Tuhuayo was the The Leaden Antwren foragedm ostly in the lower
White-bellied Spinetail (Synalluxis propinqua) branches of the “broad-leafed” trees that grew
with density estimates averaging about 20 indi- primarily in the tall Cecropia forest. The Lesser
viduals/visit (or about 47 birds/ha!). Homer0 foraged primarily on the ground in any
habitat, and would occasionally be found off the
HABITAT AND MICROHABITAT USE
ground on exposed roots or low branches.
Seven of the 14 obligate island species that I In contrast, only two of the 17 nonobligate
studied were specialized on a particular micro- island species studied here were microhabitat
habitat (> 75% of observations, Table 4). Of these specialists.B oth are widespread tropical species;
Brown Elaenia, Elaenia pelzelni, and the two the Blue-gray Tanager, a canopy species,a nd the
conebills, Conirostrum bicolor and C. margari- Yellow-browed Sparrow, Ammodramus auri-
tae, were restricted to the canopy of tall Cecropia frons, a specieso f open, grassy,h abitats both on
forest. Pearly-breasted Conebill, C. margaritae, and off islands. Four flycatchers,t he Short-crest-
perhaps the least-known species in my study, was ed Flycatcher, Myiarchusferox, the StreakedF ly-
observed only near the tops of Cecropia,w hereas catcher, Myiodynastes maculatus, the Tropical
the other two canopy species used Mimosa fre- Kingbird, Tyrannus melancholicus, and the
quently. Three other specialists were restricted White-throated Kingbird, T. albogularis, as well
largely to sandbar scrub. White-bellied Spinetails as the Black-billed Thrush, Turdus ignobilis,a nd
and Lesser Wagtail-Tyrants, Stigmatura napen- the Silver-beaked Tanager, Ramphocelusc arbo,
sis, used Tessaria most heavily within the hab- were all canopy specialists( when microhabitats
itat, but the River Tyrannulet, Serpophaguh y- 1 and 2 were combined); the two kingbird species
poleucu,s eemed to prefer the taller willows (Sulix) also perched in the tops of willows in sandbar
when it foragedi n the Tessaria-dominated scrub scrub habitat and occasionally flew between the
habitat. The Ash-breastedA ntbird, Myrmoborus two islands.
lugubris, was restricted to the understory of tall To summarize, a majority of the most spe-
Cecropia forest. cialized species,a s illustrated by the breadth of
The remaining obligate island speciesw ere not microhabitat and tree-speciesu se (Fig. 2) were
specialists;t hey occurred in more than one mi- obligate island species.M ost of thesew ere found
crohabitat. The Lesser Hornero, Furnarius mi- almost exclusively in either Cecropiao r Tessaria
nor, Leaden Antwren, Myrmotherula assimilis, trees, and were either sandbar scrub or Cecropia
and Black-and-white Antbird, Myrmochanes forest specialists. However, some of the most
hemileucus,w ere primarily understory species. generalized speciesw ith regard to these habitat
434 GARY H. ROSENBERG
TABLE 5. Frequencyo f observationsin variousf oragingt echniquea nd substratec ategorieus sedb y 3 1 bird
specieso n two river islands.n = samples ize.S ubstrateisn clude:L F = leaf;B K = bark, FR = fruit; GR = grass;
GD = ground;F L = flower;a nd AR = air. Techniquesin clude:C L = glean;S L = sally;H M = hammer;H V
= hover;L G = lunge;a nd HG = hangingd own. Speciesc odesa re includeda fter the name.
substrate Technique
Bird species n LF BK FR GR GD FL AR GL SL HM HV LG HG
Obligatei slands pecies
Leucippusc hlorocercusL C 16 19 6 6 6 ;: 6 100
Furnarius minor FM 17 59 41
Synallaxis propinqua SP 25 20 72 4 4 100
Cranioleucav ulpina CV 30 37 63 93 7
Thamnophilusc ryptoleucus
TC 21 52 48 76 10 14
Myrmotherula assimilisM A 9 67 33 89 11
Myrmoborus lugubrisM L 8 25 62 13 88 12’
Myrmochanesh emileucusM H 35 54 37 9 100
Elaenia pelzelni EP 24 58 17 25 13 87
Stigmatura napensisS N 16 94 6 38 50 12
Serpophagah ypoleucaS H 11 82 18 18 82
Cnemotriccusfu scatusC F 1984 5 11 100
Conirostrum& color CB 32 75 25 84 9 7
Conirostrumm argaritae CM 8 88 12 87 13
Nonobligatei slands pecies
Chrysoptilusp unctigula CP 16 100 19 81
Veniliornisp asserinksV P 11 100 100
Synallaxis gujanensisS G 7 71 14 15 100
Synallaxis albigularisS A 8 25 63 12 100
Thamnonhilusd oliatus TD 5 80 20 100
Todirosthm maculatum TM 39 97 3 5 95
Myiarchusf erox MF 7 86 14 100
Myiodynastesm aculatusM M 14 79 7 14 100
Tyrannus melancholicusT Z 12 8 :: 100
Tyrannus albogularisT A 15 13 100
Turdus ignobilisT I 19 5 9:
Thraupis episcopusT E 21 33 10 52 5 :; :4 5
Thlypopsiss ordida TS 26 92 4 4 92 8
Ramphocelusc arbo RC 7 29 29 42 57 29 14
Sporophilac astaneiventrisS Z 10 10 30 10 50 100
Saltator caerulescenSs C 17 12 12 71 59 35 6
Ammodramus aurtfions AA 5 20 20 6; 40 60
variables were also obligate island species.T hese mediate, with only four speciesf ound primarily
were all found in almost all the available micro- in the understory vegetation. In addition to this
habitat on the islands. Most nonobligate island difference in mean foraging height, obligate is-
speciess tudied had intermediate breadth values land speciesa ppeared to use a narrower range of
for these measures. heights than nonobligate island speciesa s illus-
trated by their smaller niche breadths (Fig. 3).
FORAGING BEHAVIOR
In general, most river-island speciesw ere in-
Little was known about the foraging behavior of sectivorous,a lthough some were at least partially
many speciess tudied here, particularly those re- frugivorous, taking advantage of the abundant
stricted to river-island habitats. Among the 14 supply of Cecropia fruit (Table 5). Among the
obligate island species, 11 fed primarily in the obligate island species,o nly the Brown Elaenia
understory within 3 m of the ground (Fig. 3) fed to any extent on Cecropia fruit (4/4 stomachs
whereas the other three fed exclusively in the contained Cecropia fruit). In contrast, seven of
upper canopy. The mean foraging heights for the the nonobligate island speciesw ere considered
nonobligate island speciesw ere generally inter- specialized frugivores; the Black-billed Thrush
BIRDS OF AMAZONIAN RIVER ISLANDS 435
mainly sallied out to pluck a piece of fruit, and
then returned to a perch to eat it (l/l stomach
contained Cecropia fruit), whereas the Grayish
Saltator, Saltator caerulescensu, sually clung to
a cluster of fruit from which it fed (no stomachs
examined). In addition to the fruit specialists
mentioned above (based upon foraging behav-
ior), examination of stomachs from specimens
collectedo n islands (LSUMNS tag data) revealed
that the following speciesf ed at least partially on
Cecropia fruit: the Short-tailed Parrot, Graydi- FIGURE 4. Niche breadth values based on foraging
dasculusb rachyurus( 818 stomachs);t he Canary- techniquea nd foraging substrate.T he six categorieso f
winged Parakeet, Brotogeris versicolorus (4/8 techniquesa nd seven categorieso f substratesa re listed
in Table 5. Breadth values are presented as a propor-
stomachs); the Tui Parakeet, B. sanctithomae,
tion of the total number of categoriesf or each variable.
(518 stomachs);t he Blue-winged Parrotlet, For- Shaded circles represent obligate island species and
pus xanthopterygius (4/4 stomachs); the Large open circles represent nonobligate island species.S ee
Elaenia, Elaenia spectabilis( 2/4 stomachs); the Table 5 for speciesa bbreviations.
Small-billed Elaenia, E. parvirostris (2/2 stom-
achs); the Short-crested Flycatcher (l/4 stom-
achs); the Great Kiskadee (3/4 stomachs); the technique. The Spotted Tody-Flycatcher, Todi-
Streaked Flycatcher (l/l stomach); the White- rostrum maculatum, fed by sallying out to pick
throated Kingbird (4/8 stomachs); the Hooded insects off the undersides of leaves, as do many
Tanager, Nemosia pileata (l/4 stomachs); and of its congeners( Fitzpatrick 1976). Of the glean-
the Blue-gray Tanager (2/2 stomachs). Cecropia ers, the Dark-breasted Spinetail, Synallaxis al-
trees do not appear to be seasonalw ith regard to bigularis, and the Plain-crowned Spinetail pri-
fruit production, with fruit present during visits marily searcht wigs and small branches, whereas
in July, as well as January. the BarredA ntshrike, Thamnophilusd oliatus,a nd
Among the obligate island species, most fed the Orange-headed Tanager, Thlypopsiss ordida,
by either gleaning or sallying for insectso n leaves searched mostly leaves. The Chestnut-bellied
or branches, with gleanerso utnumbering salliers Seedeater,S porophilac astaneiventrisf,e d mostly
nine speciest o three. Of the understory species, on grasss eeds,a nd the Yellow-browed Sparrow
only the LesserH omer0 and Ash-breasted Ant- was seen “leaping” up into the air (a seemingly
bird were seen to feed on the ground. The hor- uncharacteristic behavior for an emberizid) for
nero often foragedi n clearingso r in exposeds andy flying insects. The two woodpeckers both spe-
areasa way from vegetation, whereast he antbird cialized by hammering for insects on trunks and
(along with the Plain-crowned Spinetail, Synal- branches.
laxis gujanensis) often fed by moving about on Overall, there appeared to be no difference in
exposedC ecropiar oots occasionally dropping to the degreeo f foraging specialization between ob-
the ground to searchf or insects.T he Olive-spot- ligate and nonobligate island species, as illus-
ted Hummingbird, Leucippusc hlorocercusw, as trated by niche breadths for technique and sub-
frequently seen hover-gleaning for insects at a strate (Fig. 4). All species, however, were
variety of substrates.T his is consistent with re- specialized in foraging technique and substrate.
cent findings that most (if not all) tropical hum-
DISCUSSION
mingbirds feed extensively on insects (Remsen
et al. 1986). No obligate island speciesw ere ex- The extreme geographicr estriction exhibited by
clusively bark-foragers, or sallied to any extent island birds in northeastern Peru represents a
for aerial prey, except perhapst he Brown Elaenia situation unique to the Amazon River and its
(25% of the observations). tributaries. In no other tropical or temperate riv-
Nonobligate island speciesa lso mostly gleaned er basin in the world is such a high percentage
or sallied for insects, but, in contrast to obligate of the avifauna dependent upon river-created
island speciest, here were as many sallying species habitats (Remsen and Parker 1983). If such a
as gleaners. Six speciess allied for flying insects, degreeo f river-created specialization were wide-
with the two kingbird speciess pecializing on that spread, then, for example, an island in the Mis-
436 GARY H. ROSENBERG
FIGURE 5. Distribution of four river-island bird species.O pen circles represent all the collection localities
for all river-island speciesc ombined basedu pon an extensivem useum search.S hadedc irclesr epresentc ollecting
localities at which specimenso f that speciesw ere attained.
sissippi River would have 15-20 speciesp resent small (e.g., Fuscous Flycatcher, Cnemotriccus
that were absent from the adjacent mainland de- fuscatus,R ed-backed Spinetail, Cranioleuca vul-
ciduous forest (and absent from nonforested re- pina, and Leaden Antwren); and (3) speciesr e-
gions outside of the Mississippi drainage). What stricted to western Amazonia along the Amazon,
are the primary factors causing the habitat and Napo, and Ucayali rivers (e.g., Black-and-white
geographicr estrictionf ound in northeasternP eru? Antbird and Olive-spotted Hummingbird).
Are bird speciest hat are restrictedt o river islands Away from northeastern Peru, several obligate
in northeastern Peru restricted to islands island speciesh ave been found in similarly struc-
throughout their range, or are they attracted to tured habitats away from islands. For example,
specific habitats that are found only on islands in southeasternP eru, Terborgh and Weske (1969)
in this region of Amazonia? found the River Tyrannulet in “matorral” hab-
Most obligate island speciesh ave widespread itat. Also, the Ash-breasted Antbird is known
but linear distributions that fall into three cate- from river-edge forest on the mainland of Co-
gories (Fig. 5): (1) species found throughout lombia, near Letecia, although the record rep-
Amazonia but restricted to the Amazon River resentso nly one sightinga way from a river island
and its largest tributaries, such as the Ucayali in more than 10 months of fieldwork in varzea
and Napo rivers (e.g., Ash-breasted Antbird, forest (J. V. Remsen, Jr., pers. comm.). Another
Yellow-hooded Blackbird, Agelaius icterocepha- pair was also recordedf rom varzea on the “main-
lus, White-bellied Spinetail, and others); (2) like land near Letecia in January 1984” (R. S. Ridge-
(1) but also found on most tributaries, large or ly, pers. comm.). The Fuscous Flycatcher occurs