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Roosting by Pelagic Seabirds; Energetic, Populational, and Social

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Roosting by Pelagic Seabirds; Energetic, Populational, and Social The Condor88:487492 0 The CooperOrnithological Society 1986 ROOSTING BY PELAGIC SEABIRDS: ENERGETIC, POPULATIONAL, AND SOCIAL CONSIDERATIONS RALPH W. SCHREIBER AND JUDITH L. CHOVAN Los AngelesCounty Museum of Natural History, 900 ExpositionBoulevard, Los Angeles,CA 90036 Abstract. Great Frigatebirds (Fregata minor) and Red-footed Boobies (Sula sula) roost in large numbers on the guy wires of the LORAN-C tower on Sand Island, Johnston Atoll, in the central Pacific Ocean. We quantified the diurnal pattern of movement to and from the atoll by the roosting, but nonnestingbirds. The total number of birds using the atoll is difficult to determine but must be consideredwhen making population estimates. By counting the roosting birds at 10 min after sunset a reliable estimate can be made of the maximum number of birds which will roost that evening. The number of roosting birds increasessignificantly when the trade winds decreasein velocity. Utilizing energy from winds and thermals is critical to these speciesfor efficient flight, and energetic considerations may determine roosting patterns. Social interactions probably are secondaryand result from the scarcity of suitable roost sites in the pelagic zone. Key words: Red-footedBooby; Sula sula; Great Frigatebird;Fregata minor; roosting;flighten- ergetics;population -dynamics; seabirds. INTRODUCTION mitting tower with 24 external guy wires ra- Seabirdsare among the most aerial of all birds. diating from the top of the tower to bases in Many spend virtually their entire lives at sea the lagoon forming a ca. 300 m diameter circle. and return to land only to nest, and in some To the northeast, southeast,and west a series species, to roost. Accurate population esti- of inner guy wires support the tower. mates of seabirdsare difficult to obtain (Crox- POBSP personnel made hourly counts of all et al. 1984; Nettleship and Birkhead 1985), Great Frigatebirds (Fregata minor) and Red- but are critical to studies of population dy- footed Boobies (S&Z s&z) once a week from namics and establishment of valid conserva- 0700 to 1900, from September 1964 through tion policies. Censusesof roosting populations March 1965. We use those data to illustrate on central Pacific islands or other oceanic lo- the diurnal cycle in the numbers of birds pres- cales are rare. Our data have broad implica- ent. tions for obtaining and interpreting such pop- The counts used to analyze evening arrivals ulation estimates. Further, the sociological were made by RWS from 3 to 29 January 1967, implications of communal roosting by birds at 1200, and 60, 30, and 15 min before sunset, have received considerable attention (Morri- at sunset,10 min after sunset,and at midnight. son and Caccamise 198 5, and referencesthere- The counts used to correlate the number of in). Our data provide another hypothesis for birds present at midnight, dusk (10 min after why birds form communal roosts. sunset),and noon were made by RWS from 22 December 1966 through 3 1 January 1967. The STUDY SITE AND METHODS midnight data were collected only when suf- Johnston Atoll consistsof a fringing reef with ficient moonlight was present to ensure accu- two small (lessthan 23 1 and 8 ha) natural, but rate counts. As the birds were difficult to cen- highly modified islands,and two small entirely sus in low light, and flushed readily when man-made islands (9.7 and 6.9 ha), all with artificial light was directed on them, we esti- low vertical profile (Amerson and Shelton mate that figures presented for midnight for 1976). The atoll lies at 16”45’N, 169”3O’W. Red-footed Boobiesare accurateto f 20%, and The nearest landfall is French Frigate Shoals for Great Frigatebirds to ? 10%. Data used for in the Leeward Hawaiian chain, 450 nautical correlating the numbers of birds present and miles to the north northwest. Sand Island (ca. environmental parameters,were taken by RWS 8 ha) is the major seabird nesting area, and from 22 December 1966 to 10 February 1967 through the effort of the Smithsonian Insti- at 10 min after sunset. tution’s Pacific Ocean Biological Survey Pro- Daily mean wind speed, percent possible gram (POBSP), ranks among the best studied sunshine, sky cover from sunrise to sunset, colonies in the world. Sand Island is domi- mean temperature, and precipitation were ob- nated by a 196.6 m high LORAN-C trans- tained from the U.S. Department of Com- merce Weather Bureau on JohnstonIsland, ca. 2 km from the LORAN tower. Rainfall of less I Received 28 January 1986. Final acceptance11 June than 0.25 mm was listed in the records as 1986. “trace”; we substituted the value of 0.01 mm 488 RALPH W. SCHREIBER AND JUDITH L. CHOVAN 0 - 0 Great Frigatebird * - -+ Red-footed Booby 100 +---+ Light intensity 1 + , 0 NOON +60 30 15 -10 25 MID- SUNSET NIGH1 TIME OF DAY FIGURE 2. Time of arrival, based on a percent of the daily maximum population, in relation to time of day and light intensity (measured in LUX). The evening arrival time of maximum num- bers of birds is significantly different between 7 8 9 10 11 12 13 14 15 16 17 18 19 the two species:Great Frigatebirds return ear- TIME OF DAY lier than the Red-footed Boobies. This was FIGURE 1. The number of Red-footed Boobies and quantified by ranking the percent of birds ar- Great Frigatebirdsroosting on SandIsland, JohnstonAtoll, riving for each time interval based on the total central Pacific Ocean, in relation to time of day. The mean number that were present at midnight, and &one SD is graphed. using the Wilcoxon’s test for unpaired data, P < 0.05, for a one-tailed test (Fig. 2). For Great Frigatebirds, approximately half the dai- for trace, six times during each of two corre- ly arrivals occurred by sunset,65% by 10 min lation analyses. after sunset, and 75% at last light (25 to 30 min past sunset).For Red-footed Boobies, only RESULTS one quarter of the daily arrivals occurred by During this study, roughly a dozen pairs of sunset, 30% by 10 min after sunset, and 45% Red-footed Boobies and fewer than 100 pairs by last light. Only 12 to 18% of the total roost- of Great Frigatebirds nested on Sand Island. ing population of both specieswas present dur- However, the roosting populations were con- ing mid-morning to early afternoon. Early siderably larger and were composed of indi- morning departures occurred at such low light viduals coming from outside the immediate levels and so rapidly that estimatesfrom counts atoll on a daily basis (Harrington 1977). taken at that time are unreliable. Verner (1965) found a diurnal movement Our data indicate that numbers of roosting pattern for boobies at Half Moon Cay, Belize. boobies and frigates increaseover several days We also documented a predictable diurnal pat- when winds remain calm (i.e., Fig. 3: 25 to 31 tern of movement to and from a roost site by December, 15 to 23 January), and the birds do Great Frigatebirds and Red-footed Boobies not leave in the morning when winds are light. (Fig. 1). The majority of both speciesof birds For both species, the numbers of birds and left the wires near dawn and returned after mean daily windspeeds are significantly neg- sunset,reaching maximum numbers after last atively correlated (Figs. 4, 5) (Great Frigate- light, based on counts at midnight (Figs. 2, 3). bird: II = 45 days, r = -0.799, P -c 0.01; Red- We found that the number of birds counted at footed Booby: n = 33 days, r = -0.8157, and 10 min after sunsetwas correlated to the max- t = 7.846, P < 0.01). imum numberscounted at midnight (Y= 0.747, Numbers of frigatebirds and percent possi- t = 2.75, P < 0.05), and can be used to predict ble sunshine,amount of sky cover and air tem- the maximum bird population for that eve- perature were not significantly correlated. ning. A significant negative correlation exists Numbers of frigatebirds and precipitation between the percent arrival of both speciesand levels were negatively correlated (r = -0.345, light intensity (Fig. 2) (Great Frigatebirds: Y = t = 378, P < 0.05). Numbers of boobies were -0.980, t = 11.02, P < 0.01; Red-footed Boo- not significantly correlated with temperature by: r = -0.986, t = 13.18, P < 0.01.) or amount of rainfall. Numbers of boobies and BOOBY AND FRIGATEBIRD ROOSTING 489 700 600 GREAT FRIGATEBIRD 500 az 400 m 0” 8 a 300 GREAT FRIGATEBIRD ii I = 200 100 0 8 16 24 32 40 WINDSPEED (Km/HR) FIGURE 4. The relationship between wind speed and number of roosting Great Frigatebirds based on countsat 10 min after sunset. index to the numbers of birds roosting on a given night, at all seasonsof the year. Estimating the total population of boobies t 1 10 20 JO and fiigatebirds using Johnston Atoll is prob- ably impossible because of the movement of FIGURE 3. The numbers of Red-footed Boobies and individual birds through the region (Harring- Great Frigatebirds roosting at noon, dusk (10 min after ton 1977). Data collected at sea in this region sunset),and midnight from 22 December 1966 to 3 1 Jan- uary 1967, in relation to wind speed (km/hr). of the central Pacific indicate that within about 120 to 160 km of an atoll, distinct movements by Red-footed Boobies occur in the mornings percent possible sunshine (an index to cloud away from land and in the evenings toward cover) are negatively correlated (r = -0.429, land. Inward movement continued to occur P < 0.05).We found no significantcorrelations after dark. At greater distances such move- between precipitation, cloud cover, and wind ments were not noted.
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