Effect of Wind on the Flight of Brown Booby Fledglings

Ornithol Sci 16: 17 – 22 (2017) SPECIAL FEATURE The effects of weather conditions on avian movements Effect of wind on the ight of Brown Booby edglings Takashi YAMAMOTO1,#, Hiroyoshi KOHNO2, Akira MIZUTANI2, Hanako SATO3, Hiroki YAMAGISHI3, Yutaka FUJII3, Miku MURAKOSHI4 and Ken YODA1 1 Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464–8601, Japan 2 Okinawa Regional Research Center, Tokai University, Uehara, Taketomi, Okinawa 907–1541, Japan 3 School of Marine Science and Technology, Tokai University, Orido, Shimizu-ku, Shizuoka 424–8610, Japan 4 Nagasaki Penguin Aquarium, Shuku-machi, Nagasaki 851–0121, Japan ORNITHOLOGICAL Abstract There is increasing evidence showing that wind velocity affects the flight and foraging behavior of seabirds; however, few studies have examined these effects SCIENCE on seabirds inhabiting tropical oceans where lighter wind conditions usually prevail. © The Ornithological Society The Brown Booby Sula leucogaster is an example of a tropical seabird with relatively of Japan 2017 low wing loading; strong wind conditions may be expected to impede the stability of their flight. We examined how different wind conditions affected the duration and flying behavior of Brown Booby fledglings during foraging trips by means of direct observation of nest attendance and by attaching video loggers to birds. The duration of foraging trips by fledglings decreased with increasing wind speed, and during flight, the body rotation of fledglings became greater with increasing wind speed. As expected, fledglings were buffeted by strong winds due to their relative inexperience in flight combined with their low wing loading. Fledglings were probably forced to flap against strong winds in order to adjust the stability of their bodies, offsetting the efficient use of wind for gliding. Furthermore, the height at which fledglings flew fluctuated more at higher wind velocities, which may have constrained their detection and capture of prey. In conclusion, our results indicate that the aerodynamic perfor- mance of Brown Booby fledglings is impaired by strong wind conditions, leading to poor flight stability and potentially reduced prey detection. Key words Fledgling, Flight, Tropical seabird, Video logger, Wind Seabirds are able to fly freely over barrier-free Previous studies into the effects of wind on the oceans; however, there is increasing evidence indi- flight of seabirds have mostly focused on species cating that wind conditions (e.g., speed and direc- inhabiting relatively windy areas at high latitudes tion) influence their flight and foraging behavior at (e.g., albatrosses). Most of these species have high sea (Finney et al. 1999; González-Solís et al. 2009; wing loading (i.e., a small wing area relative to mass), Dehnhard et al. 2013; Elliott et al. 2014; Tarroux et and often perform a special flight maneuver called al. 2016). Prevalent wind patterns have been found “dynamic soaring” (i.e., using wind shears in the to determine the timing and direction of foraging boundary layer above the ocean surface; Sachs 2005) excursions by a range of different seabirds including to utilize strong winds. Thus, such species actually gannets, petrels, shearwaters and albatrosses (Adams need higher wind velocities to remain aloft without & Navarro 2005; Garthe et al. 2007; Navarro & flapping (Weimerskirch et al. 2000; Amélineau et al. González-Solís 2009; Weimerskirch et al. 2012), with 2014). However, the flight style of birds is thought individuals taking advantage of favorable winds to to be adaptive, matching the effectiveness of their minimize their energetic expenditure during flight morphological characteristics to the wind regimes (Furness & Bryant 1996; Weimerskirch et al. 2000; of the regions they inhabit (Spear & Ainley 1997; Amélineau et al. 2014). Weimerskirch et al. 2005; Cairns et al. 2008). Winds over tropical oceans are typically much lighter (i.e., (Received 7 July 2016; Accepted 27 September 2016) less wind speed) than winds at higher latitudes # Corresponding author, E-mail: [email protected] (Suryan et al. 2008); furthermore, species inhabiting 17 T. YAMAMOTO et al. tropical regions appear to have relatively lower wing gate siblicide: Nelson 1978; Drummond et al. 2003). loadings (Spear & Ainley 1997), which is considered As part of a restoration program (Mizutani & Kohno to be an adaptation to exploit lighter winds (Hertel 2011), we transferred a total of 17 chicks of unknown & Ballance 1999; Brewer & Hertel 2007). Therefore, sexes that were abandoned by their parents to the tropical seabirds should be better adapted to weak ORRC during the study period, and raised them until wind conditions, which might determine their distri- they reached independence (see Yoda et al. 2004 butions (Cairns et al. 2008; Suryan et al. 2008). for more detailed information on our hand-raising The Brown Booby Sula leucogaster breeds in methods). This project has provided us with a unique subtropical and tropical oceans between 25°S and opportunity to understand the fledgling behavior and 25°N latitude (often termed the “pantropical” region) ecology of this species (Yoda et al. 2004; Kohno & (Harrison 1985; Nelson 2005). The species also Yoda 2011; Yoda et al. 2011). In the years follow- breeds in Japan. The colony breeding on Nakanoka- ing fledgling independence, one juvenile has been mishima Island, Japan, is close to the northern limit observed to return to the area where it was reared. of the species’ breeding range, located to the north After the chicks had fledged (i.e., the post-fledging of the Tropic of Cancer. This region is characterized dependence period), their cages were opened at sun- by seasonal changes in wind speed. Calm weather rise and closed at sunset, allowing them to make trips conditions prevail from spring to summer, but strong to and from the sea throughout the daytime. During northeasterly winds caused by the East Asian winter the period between fledging and independence, we Monsoon prevail from November to February (Zhang observed fledglings and recorded the attendance of et al. 1997; Ikema et al. 2013). Boobies migrate to their nest in 2005, 2006, 2008, 2009, and 2010 (N=13 the south of the Philippine Islands, located 1,000– birds) to quantify the time spent at sea (i.e., forag- 2,000 km away from their natal colony, usually by ing trip duration). In 2009, 2010, and 2011, we also November. This migratory behavior is assumed to attached video loggers (LY30, 1.9 cm×6.8 cm×1.9 be triggered by an increase in wind speed (Kohno cm in diameter, Benco, Taiwan) to 10 birds. In brief, 2000). However, the effects of wind conditions on the we first attached small plastic bases to the back feath- behavior of this species at sea remain poorly under- ers of fledglings with adhesive tape (Tesa, Hamburg, stood. Germany) and glue (Loctite 401). We then attached In this study, we examined how different wind the video loggers to the bases using cable ties. This conditions affected the duration and flying behav- technique allowed us to collect video footage repeat- ior of Brown Booby fledglings during foraging trips edly throughout the study period (Fig. 1a). The video through the direct observation of nest attendance and lens faced forward to provide a bird’s eye view of the attachment of video loggers to birds. the environment. Each video logger had a 280 mAh Li-polymer battery and 4 GB of memory, and could record for up to three hours. We deployed the loggers MATERIALS AND METHODS once or twice each day, and retrieved them on the 1) Fieldwork same day. We then downloaded the video footage to This study was carried out on Nakanokamishima a computer. The total weight of each logger including Island, Japan (24°11′N, 123°34′E), and at the Okinawa the cable ties (27 g) corresponded to less than 2% Regional Research Center (ORRC), Tokai Univer- of the mean mass of the equipped birds (mean±SD: sity, Iriomote Island (24°19′N, 123°41′E) from May 1,361±163 g). Previous studies have recommended to December in 2005, 2006, 2008, 2009, 2010, and that loads exceeding >5% of body mass should be 2011. Birds on Nakanokamishima Island have been avoided for flying seabirds (e.g., Phillips et al. 2003). disturbed since 1906 by human activities, including commercial hunting for feathers, specimens, and oil 2) Data analysis (Mizutani & Kohno 2011). These actions led to a The video footage was used to extract scenes dur- major decline in population size to ca. <200 breeding ing which the fledglings flew linearly and the sea pairs (Hiroyoshi Kohno unpublished). Each Brown horizon was visible (Fig. 1b, also see Appendix for Booby lays two eggs, both of which hatch. However, an example). We then measured the roll angles of the one of the chicks (usually the first one to hatch) rou- body axis (i.e., transversely crossing the body from tinely kills its sibling by pushing it out of the nest right to left) for each second in relation to the horizon within a few days of the second chick hatching (obli- using protractor software (available at http://www. 18 Eff ect of wind on the fl ight of Brown Booby fl edglings was examined in relation to 10-min means of wind (a) Photo by H Yamagishi direction and speed data corresponding to the time when each fl ight scene was recorded. Wind direction, relative to the bird’s heading, was separated into two categories: head wind and tail wind. These criteria were selected because, although the broad direction in which the birds were heading was detectable using the geomorphic characteristics of Iriomote Island, the exact direction that birds headed was not clear. Data were analyzed using R version 2.15.1 (R Development Core Team 2012). As fledglings increased the time spent at sea and developed their fl ight skills gradually towards independence (Yoda et al.

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