Day 10 — February 11
Grande Pass, Aldabra  Today’s Zodiac excursion brought birders face-to-face with nesting lesser frigatebirds ( Fregata ariel ), red-footed boobies (Sula sula), and graceful red-tailed tropicbirds ( Phaethon rubricauda ). From our Zodiac-borne perspective we found it easy to glide past hundreds of these primitive-looking pantropical seabirds, each members of the order Pelecaniformes, which exhibit specialized gular pouches, totipalmated feet, and unique beaks in which to exploit their respective feeding niches. For example, whereas male frigates inflate their bright-red gular pouches during nuptial display in an effort to attract females, all of these seabirds use their gular pouches to radiate body heat through capillary action. This process is called gular flutter, and since birds do not sweat, they literally have to mechanically flutter this thin, membranous skin to avoid overheating. Blood, within a network of capillaries, releases excess body heat to the external environment while bringing cooler blood back into the birds’ bodies.
Totipalmation — or totally webbed feet — is another common trait. But the birds separate out into their respective feeding niches using their unique beaks — the frigates snagging fish from the surface or stealing it from other seabirds (even boobies) with their hooked beaks; boobies plunge-diving with their sharp, bone-reinforced bills; and tropicbirds employing a tern-like feeding strategy but with a twist — they may pursue their quarry under water using feet set far toward the posterior of their bodies.
Though the frigatebirds and boobies construct stick nests amid the branches of Aldabra’s abundant mangroves, and the tropicbirds choose to place their nests beneath the protection of xeric brush on uplifted fossil coral islets, all must go out to sea to find food. Amazingly, though the kleptoparasitical frigates unmercifully hound the boobies and sometimes even the tropicbirds at sea, all nest and coexist in close proximity without problems. During today’s visit, we observed nearly fledged young of all species. Additionally, we spied an immature Aldabra drongo ( Dicrurus aldabranus ), perhaps the atoll’s rarest endemic, with an estimated population of less than 1,000.
The hard-core snorkelers went with Jack Grove this morning, and although there was some discussion about avian life, the focus of our adventure was drift snorkeling in the main channel. Things got off to a lively start when we first jumped into the water near a five-foot-long barracuda in the process of devouring a large red snapper. A few members of the group had retreated to the Zodiac when things had gotten real exciting, but everybody had a good look at what a large picivore looks like in action. Numerous other events took place this morning that will afford each of us with lifelong memories of our visit to one of the world’s great natural history destinations.
The ship repositioned back to Picard for the afternoon program. After lunch snorkeling was offered on the outside of the reef slope, west of the research station. Visibility was limited because of the low tide, but there were some great fish observations, including our first clown triggerfish. Once the tide was high enough to get the Zodiacs across the reef, we landed on the beach in front of the research station. Here we had our last chance to do some shopping at the research station. Although the selection of shirts and books was limited, everyone understood that purchases would help fund important, ongoing research at Aldabra. From the station, most of the group followed Greg Homel for a late-afternoon nature walk along the trail skirting the beach.
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