THE IMPACT OF SEASONAL FLOWERING ON THE BIOLOGY OF SOME TROPICAL HUMMINGBIRDS LARRY L. WOLF Department of Zoology Syracuse University Syracuse, New York 13210 In most parts of the Neotropics the annual usually minor, differences in timing of cli- cycle of climate usually is broken into two matic events, especially rainfall, between the distirrct phases, wet and dry seasons. Char- two sites, La Pacifica has the most reliable acteristically many of the woosdy plants un- weather station in Guanacaste Province. dergo sexual reproduction during the dry The actual study area was on a hillside season while vegetative growth occurs during near the buildings of the experimemal farm the rainy season (Janzen 1967). This seasonal operated by the Ministerio de Agricultura y flowering has a marked effect on organisms Ganaderia of Costa Rica; the elevation ranges which are directly dependent on vegetative from 11-45 m. The vegetation of the hillside or reproductive phases of plant growth. One would be called tropical deciduous forest, group of tropical birds, the hummingbirds following the‘ scheme outlined for Mexico (family Trochilidae) , are thought to be closely by Leopold ( 1950). It is dominated by such tied to flowers through most or all of their tree genera as Bursera, Bombacopsis, Cochlo- life cycle. Plants are presumed to be the spermum, Quazuma, Samanea, Enterolobium, most important food source for many species, Genipa, LAea, and Tabebuiu. During the even though evidence is accumulating that height of the dry season the ground was nearly many species are to some degree insectiv- devoid of green herbaceous vegetation and the orous. Even these species may be tied to trees were almost all leafless (fig. 3). How- flowering seasons through the increased feed- ever, after the first rains in late April some ing efficiency of visiting flowers as rich, lo- ground vegetation appeared and the trees be- calized sources of insect food. gan to leaf out. The rains then stopped until If flowering is important in the economy late May, at which time there was a burst of of hummingbirds, and if this flowering is vegetative growth by both woody and herba- limited largely to a single season of the year, ceous plants (fig. 4). During the late part of it would follow that, to cope with this, there ,the dry season several small fires were started must be some major adjustments in the in the area by Ministerio personnel, but these ecology and behavior of the birds. The pres- had little effect on the important food plants ent study was undertaken to ’ discover two of the hummingbirds. things about the habits of several species of A second portion of the study area, visited hummingbirds living in such a wet-dry sea- irregularly, is along a road (“river road”) sonal climate. It was hoped, first, to discover adjacent to the stream bed at the base of the the relative importance of flowers as food study hill. This road passes through limited sources for the birds, and, second, to under- riparian evergreen forest which is much more stand the importance of seasonal flowering extensive on the other side of the river. activity in the evolution of techniques for The flowering seasons for plants observed biological exploitation of the dry lowlands of being used by the hummingbirds were scat- Middle America, and especially the influence tered throughout the study period (table 1) on the territorial systems of the humming- with relatively little overlap of important spe- birds. cies. In the dry forest area, few species flower at any one time and the flowering season for ENVIRONMENT each species is relatively synchronous and The Granja Experimental Jimenez study site short. is located approximately 14 km SW of Las Cafias, Guanacaste Province, Costa Rica (loo METHODS 20N,’ 85” OWW). The entire surrounding Visits to the area were made on the following 1967 region is tropical dry forest (Tosi 1965). dates: 19-20 January, 28 February-2 March, 24 and Figures 1 and 2 present weather data from 26 March, 19-21 April, 17-19 May, 20-22 June, and 13 July. On each of the visits, except that in July, Finca La Pacifica, located about 4 km NE at least two 2-hr. censuses were made in the early of Cafias. Although .there are definite, but morning, normally from 06:OO to 08:00, along a dirt 111 The Condor, 72 : 1-14, 1970 2 LARRY L. WOLF iitiihi;JASONO FIGURE 1. Monthly temperature data (mean maxi- mum, mean minimum, monthly mean) from Finca La Pacifica, 5 km NE Caiias, Guanacaste Province, Costa Rica. Open circles are 1964, dots are 1965. ;;Mbi;36sbkb MONTH FIGURE 2. Monthly rainfall at Finca La Pacifica, 5 road winding down the hillside from the offices of km NE Canas, Guanacaste Province, Costa Rica, 1965 the experimental farm to the Rio Higuerbn (fig. 5). ( open circles ) and 1964 ( dots ) . One census usually covered about 0.7 km. On each trip at least some observations were made along the river road. An attempt was made to identify each visit by prolonged observations in various localities hummingbird encountered and to record the activity at other times of the day. of the bird at the time of observation. No attempt Following the censuses, several places on the hill- side were selected for more intensive observations was made to follow individuals for extended periods for the rest of the study period. Where possible, during these censuses. By moving fairly rapidly along these were areas of bird concentrations, usually around the census route it was hoped to avoid much dupli- a feeding source. Observations generally were made cation of individuals in the census figures. Unavoid- until 12:00 and then later for another 2-3 hr. until ably, there were varying numbers of birds that I was 18:O0. This eliminated the hot part of the day from unable to identify on these censuses, so the results the study time but should not affect the results re- have been presented only as relative values for each ported here. species (table 2). All species regularly using the Records were kept of the major tree species in hillside during the study period probably were re- bloom and of all plants at which hummers were corded. The species composition was checked each observed foraging (table 3). FIGURE 3. View on study hillside during dry season, April 1967. Note bare trees and absence of ground vegetation. IMPACT OF SEASONAL FLOWERING ON HUMMINGBIRDS 3 FIGURE 4. View on I ground vegetation has SPECIES OF HUB RECORDED General characteri: season, degree of habitat at Granja J bird species observ listed in table 4. S of the nine species There are roughly tl Archilochus, Phueti are relatively sma TABLE 1. Flowering Species of plant Eombacopsis Fendleri Combreturn farinosum - - - - side and only single, quietly foraging birds Tabebuia crysantha “;“x___ were encountered during the census. Samanea saman - x x x x - - Scaly-breasted Hummingbird. Phueochroa Vine ( Bignoniaceae ) cuvierii. This is an uncommon species which (possibly several sibling species ) - - x x - - x may be migratory or nomadic in the Granja Cenipa caruto -x-xxxx Jimenez region since it was not recorded on Inga vera - - x x - - - the hillside until May. It was first recorded - - - - Bromelia pinguin x - - in the general area during the study period Manihot utilissima -----xx in an evergreen forest approximately 5 km Muntingia calubura x x x x x x x from Granja JimCnez on 29 March (D. R. a X indicates relatively more flowering than x; Xs’ account for >50 per cent of flowering in an observation period. Paulson, pers. comm.). In the study area it 4 LARRY L. WOLF FIGURE 5. Portion of census road from buildings of experimental farm looking toward Rio Higuerbn at base of hill, June 1967. was prominently associated with two abun- forest localities nearby (G. H. Orians and dant flowering plants, Bromelia and Genipa. D. R. Paulson, pers. comm.). Its appearance on the study hillside coincided The Fork-tailed Emerald. Chlorostilbon with the leafing of the trees and the growth canivetii. It is generally an uncommon species of herbaceous vegetation. of the dry forest area, and seldom encountered The Green-breasted Mango. Anthraco- in riparian forest, although it may visit the thorax prevostii. Although recorded in the forest edge. It is probably more common at study area from January-April, it was slightly higher elevations (Slud 1964). It was not found after April in several visits to recorded on all but two visits to the area. evergreen riparian forest and to other dry Blue-vented Hummingbird. Amazilia sau- cerottei. This is one of the two common resident hummingbirds of the tropical dry TABLE 2. Relative numbers of hummingbird spe- cies observed on monthly censuses at Granja JimBnez.” forest region of Costa Rica. However, it is more common than A. rut&z on the central Species Jan. Feb. Mar. Apr. May June plateau of Costa Rica in the vicinity of San Amazilia rutila 221144 Jose. It also makes “a limited entry to the Amazilia saucerottei 3 3 1 4 3 4 Caribbean slope” (Slud 1964) and in general Phueochroacuvierii 0 0 0 0 2 2 seems to inhabit slightly moister conditions Anthracothuraxprevostii 2 1 1 1 0 0 than A. rutila. It probably is slightly more Heliomuster constantii 1 1 Ob 0 1 0 Archilochus colubrk 231000 common than A. rutilu in the study area. Amuziliu tzacatl 000010 None of the aduhs collected in May and Chlorostilboncanivetii 0 1 0” 3 1 2 June were in breeding condition. Phaethornis longuemarew 0 0 0 1 0 3 Cinnamon Hummingbird. Amazilia rutila. 64+25 6 5 All species 5 Although common on the dry-forested hill- a Numbers per visit refer to the following categories: 0 = sides in the Granja Jimenez region, it is much no birds recorded on that visit; 1 = l-2 birds recorded; 2 = 3-5; 3 = 6-10; 4 = 10 or more.