On Midway Atoll National Wildlife Refuge, change is ever-present. However, to those of us who are fortunate to live (and have lived for some time now) among the species that define this place, these changes are somewhat subtle. Transitions between seasons seem seamless and it is sometimes only in retrospect that we become aware of just how much has transformed over the course of several days, weeks, or even months. What was once overwhelming (Laysan Albatross around each and every corner, Bonin Petrels taking to the sky in the thousands at dusk), now seems normal. And yet, these familiar avian staples of Midway are changing. Spring is afoot, a time of growth and renewal, and we find ourselves at the edge of the breeding season for numerous seabird species. Life is about to explode on Midway.
Since hatching in late January and early February, the Laysan (Phoebastria immutabilis) and Black-footed Albatross (Phoebastria nigripes) chicks are now quite substantial, some approaching the size of their parents. However, they are far from the majestic look of their parents’ sleek, tapered bodies. The chicks are rotund, fluffy masses of dark brown downy feathers accented with white tips (producing an uncanny Billy Idol appearance), often found napping contently, all curled up like a cat, snapping at dead leaves, pulling at vegetation, or busily re-arranging the structure of their nest cup. Some shuffle along the roadsides, scooting forward one foot at a time but unwilling to fully stand up like their mature counterparts. Others stand up momentarily, on their tippy toes, and flap their tiny (yet rapidly growing) wings and then, perhaps due to their plump physique, fall back unsteadily on their haunches (or, technically speaking, their tarsometatarsus, the equivalent of our ankle). Sometimes, we have to move these sizeable chicks as we pass by in our carts or bikes. Like their parents, they are becoming increasingly aggressive, pecking at anything that approaches them. Gingerly, carefully, we pick them up; the chicks are the size of large chickens but they feel like massive bowls of jelly, ready to spill over at any given moment. They are little more than squishy receptacles for fish oil so we pick them up gently (but quickly) to ensure that we do not stress them out and cause regurgitation of the precious food that their parents flew hundreds of miles to give to them.
But to focus only on the albatross would be to forget the various other species—more than 20 of them—that call Midway Atoll NWR “home” and return to breed here, year after year. White Terns (Gygis alba), Red-tailed Tropicbirds (Phaethon rubricauda), Red-footed Boobies (Sula sula), Great Frigatebirds (Fregata minor), Bonin Petrels (Pterodroma hypoleuca), Black Noddies (Anous minutus) and Sooty Terns (Onychoprion fuscatus) are all gearing up for what promises to be a busy breeding season.
For some species, like White Terns (Gygis alba), the breeding season is less of a dramatic pulse but more of an ongoing process. In fact, adults, eggs, and chicks alike can be found during all months of year, although there are “peaks” in breeding. Perhaps one of the most intriguing habits of the White Tern is the fact that it constructs no nest. Nope, nada, nothing. Instead, these pure white, graceful birds lay (or rather, precariously balance) their single egg on a branch, building, or rock. Or, in the case of Midway Atoll NWR, someone might wake up in the morning to find a freshly-laid White Tern egg on their bicycle seat (it has been known to happen!). Some White Terns have even tried to “nest” on the emergent, rocky reef surrounding Sand, Spit, and Eastern Islands that comprise Midway Atoll—the same reef that is pounded incessantly by ocean swells day after day. But, as clumsy as these birds may seem, the White Tern is a rugged species well-equipped for ever-changing conditions in this subtropical environment. With a year-round breeding season and a short relay period after egg or chick loss, White Tern populations can respond and rebound quickly. In addition, White Terns go through concurrent waves of feather replacement. Normally, feather molt is an energetically-intensive process that takes place after or before the breeding season, or even replaces the breeding season entirely, forcing the bird to sacrifice breeding over molting. But, because of their molt regime, White Terns are able to prolong their breeding season, sometimes successfully raising 2-3 broods within one season.
Other species take more time and effort in constructing their nests, such as Black Noddies (Anous minutus). Spend a few minutes in a stand of Ironwood trees (Casaurina equisetifolia) and you’ll be sure to find of them flying to the ground and back to the canopy again with nest material. In fact, Black Noddies are quite unique in that actually build a nest; they are the only marine tern species to do so and, along with the White Tern (Gygis alba), they are the only tree- and shrub-nesting species in their respective avian family (Laridae). All other species within this family, by contrast, nest directly on the ground or in small divots called “scrapes.”
Right now, the Black Noddies are building nests en masse and flutter back and forth between canopy and ground constantly. The breeding ecology of this species is somewhat variable; in some colonies, the breeding season is short and regular, while in other colonies it is prolonged and/or irregular. And in some cases, breeding can take place year-round. While the source of this variability remains unknown, some analyses have pointed to availability of prey species (such as lizardfish and dartfish).
Black Noddies are also interesting because of their gregarious nature; they will often nest, roost, and forage together in densely-packed groups. Looking up in the Ironwood canopy, you can spot several Black Noddy nests in a row, with only black tails jutting out from the messy nest cups composed of Ironwood needles and dried-out leaves. Males and females build nests together, but it is the male who brings up pieces of nest material (several times each minute) to the female as she busily puts everything in place.
At the base of many of the Ironwood trees that house Black Noddy nests, you’ll find another species gearing up for the breeding season: the Red-tailed Tropicbird (Phaethon rubricauda). Like their name implies, these birds are found throughout the tropics and also wander further into subtropic environments, such as the most northwestern tips of the Hawaiian Islands (including Midway Atoll NWR). While this particular species is considered the rarest of the three tropicbird species in North America, it is the most common tropicbird on Midway.
In flight, tropicbirds are incredibly graceful; they glide and soar with ease. And what is probably their most beautiful characteristic is their elongated rectrices--that is to say, the long, bright red “streamers” that extend from their tail and trail behind them as they slice through the air like slender kites. Their courtship displays are especially a treat to witness. In the sky, two or more tropicbirds will move towards one another, fly up and backwards in a vertical circle, and then forward and down again, like a massive, aerial Ferris wheel. Around and around and around they go, and as they do so, they flick their red tail streamers from side to side, with their tiny, black, webbed feet dangling limply. Flying is already an amazing feat in physics among avifauna, but flying backwards (typically only observed with hummingbirds) for such a large bird is especially astonishing.
But, for all their grace in the air, they are incredibly awkward on land. Believe it or not, they are more awkward than the albatross, who are the gods of lift and flight. Red-tailed tropicbirds are incapable of normal bipedal locomotion since their legs are situated rather far back on their body; because of this, they are unable to balance their body upright and must use both feet to lurch themselves forward and fall on their breast. Using their bill and wings as stabilizing aids, they scooch around the ground quite rapidly in this peculiar manner, even if it does look rather silly. And, at times, it seems that they know how ridiculous it looks; approach them and they will let out a loud, coarse squawk not unlike a rough cat’s meow. They are spunky birds that have some attitude!
The breeding season has just begun for this species. While most populations are seasonal, some populations can breed year-round (since they are a tropical species) but more leeward islands and environments (such as Midway Atoll NWR) experience more pronounced peaks in breeding, especially mid-December through May. Like many other seabird species, tropicbirds keep nests to a minimum. Most nest under a bush or shrub providing shade (such as Naupaka [Scaevola taccada]) or under the canopy of Ironwood trees. However, they are also opportunistic nesters and can be found under debris like cement slabs and around buildings as long as they can find a substrate to dig a small “scrape” into (like sand, soil, or fine coral). At this point on Midway, mates have started to pair up (tropicbirds are long-lived species and assumed to be monogamous) and select their nest sites. You can tell when a tropicbird is approaching their nest, especially nests that are under dense shrubs like Naupaka. For a few moments, as if unsure of how to land, the tropicbird will flutter hesitantly a few meters above the Naupaka; then, suddenly, it will drop straight into the Naupaka and bounce down the branches like a Plinko ball falling on the “Price is Right.” Some way or another, graceful or not, they will make their way to their nests.
Along with the Red-tailed Tropicbirds, we’ve spotted another tropicbird species on Midway Atoll NWR: the White-tailed Tropicbird (Phaethon lepturus). Although it is the smallest and most common of the three tropicbirds of North America, it is somewhat rare on Midway. Only a few weeks ago we noticed a pair of white tail feathers poking out from an Ironwood tree. Pulling up on our bikes to investigate the mystery, we discovered the first White-tailed Tropicbird of the season nesting in the crotch of an Ironwood. Like its red-tailed cousin, the breeding season for this species varies, ranging from year-round in tropical areas to seasonal peaks at higher latitudes. For the White-tailed Tropicbirds, nesting phenology is most likely governed by combination of food availability, photoperiod, weather (including cyclone cycles), and perhaps competition for nest sites. In addition, this species shares many of the same attributes as other tropicbirds, such as nest structure (or rather, a lack there of). But, due to its slightly smaller size (than other tropicbirds in any case), the White-tailed can exploit other nesting sites, ranging from bare, rocky terrain to caves to craters to tree hollows and crotches.
The avian diversity across Midway Atoll NWR is quite high (over 20 different species) but what is perhaps even more intriguing is the difference in species composition between the islands (Sand, Spit, and Eastern). Eastern Island is slightly smaller than Sand Island (where the population of scientists, field technicians, Fish and Wildlife staff, and contractors reside… along with several hundred thousand albatross) but it seems completely different. Walk around on Eastern Island, and you’ll see almost only native Nohu (Tribulus cistoides) and Bunchgrass (Eragrostis variabilis) sprawling uninterrupted in every direction, along with Alena (Boerhavia repens), Sweet Alyssum (Lobularia maritima), and Lesser Swinecress (Coronopus didymus); beyond this plant community, the beaches are marked with dense stands of Naupaka (Scaevola taccada) and the occasional Tree-Heliotrope (Tournefortia argentea). From afar, you might notice white splotches in the Naupaka. While it might seem like large patches of bird poop, these spots are actually Red-footed Boobies (Sula sula) sitting patiently on their nests. Midway Atoll NWR is home to three different booby species: the Red-footed (Sula sula), Brown (Sula leucogaster), and Masked Booby (Sula dactylatra). The Red-footed booby is the smallest of the six booby species found worldwide. Of course, it does indeed have bright red feet, all the more accented by this species’ pure white plumage (with streaks of black on its wings). The Red-footed are colonial tree-nesters and in some cases, they may nest in colonies of up to several thousand pairs. Perched on the crowns of Naupaka and Tree-Heliotrope lie the large, messily built nests of the Red-footed boobies. Each pair builds their respective nest together, often from twigs, grasses, and other green vegetation; many times, pairs will try to steal nest materials from neighboring unattended nests. As such, Red-footed Boobies guard their nests quite vigorously, especially from their most stealthy and thieving neighbors: Great Frigatebirds.
Described as looking haunting and ominous as they glide through the air, these aerial acrobats have a reputation as “full-time pirates” because they often chase and harass other seabird species, particularly boobies, to steal a meal or nesting materials. For example, Great Frigatebirds (Fregata minor) can be found waiting in dense shrubbery for an unsuspecting seabird returning to feed its chick. Once the frigatebird spots its target, it pursues the seabird with astonishing dexterity, even pulling at its tail feathers. The seabird may drop its meal or regurgitate it out of stress or surprise upon which the frigatebird drops down and catches the food before it hits the water. However, for all the negative connotations that the frigatebird carries, this species retrieves the majority of its food on its own without nagging other birds. In a way, frigatebirds “surface feed” at sea; they will glide along the surface of the ocean, waiting to find a flying fish to come up, or catch fish and squid swimming just a few inches below the surface.
Like other avian species specialized towards the sea life, the Great Frigatebird was made for gliding and flying. In fact, it has a greater ratio of wing area to body mass than any other bird; that is to say, they have very long, tapered wings and a relatively small and light-weight body, thereby maximizing lift and soaring abilities. Along with a deeply forked tail for precise maneuvering and tiny legs and feet (like tropicbirds, they are unable to use bipedal locomotion), the Great Frigatebird can glide effortlessly and quite gracefully through the air.
Currently, frigatebirds have not yet started to nest--but it is likely to start soon. Instead, we have enjoyed the fantastic show that the male frigatebirds put on when attempting to attract a female over on Eastern Island. Perched at the tips of Naupaka and Tree-Heliotrope, males join together to form display groups, all advertising together to the females flying overhead. The show starts when the males begin to inflate their “gular sac,” a bright red pouch of skin below the male’s bill. Once fully inflated, the male tips his head back and points his bill skyward, tracking the females as they peruse their choices below. The show continues as the males outstretch their huge, butterfly-like wings, trembling them as they do so, and holding them forward and upward for inspection. Finally, the males add an eery, descending “warble,” followed by bill-rattling. If a female lands by a male she finds to be of interest, the show explodes. Gular sacs inflate to the maximum and a cacophony of warbles and bill-rattling breaks out among the group; males nearby the female push their gular sacs up to her face and let their heads loll from side to side with their eyes half shut. It is quite a spectacle!
In addition to the rollicking frigatebirds, there another great gig in the sky that is just starting to unfold: Sooty Terns (Onychoprion fuscatus). A few weeks ago, only a few of these slender birds were spotted drifting along high in the sky. Now, their numbers are growing and ever-increasing flocks hang over Eastern Island, filling the air with a quiet, but constant, chatter of calls (check out the video below--it may not be Midway, but the sound is the same). They spiral around in clouds of flitting black-and-white shapes, as if unsure where to go. For some of the Sooty Terns, this may be the first time that they see and actually contemplate landing on solid ground. Sooty Terns are renowned as a species that truly “lives on the wing.” Years may pass between when Sooty Terns fledge and first breed--and never during this time do they land or even rest on water. Rarely, at sea, they may momentarily perch on floating marine debris or the backs of surfaced sea turtles, but otherwise, they are in the sky all day, every day. But, come nesting time, they leave their realm of the sky and return to the terrestrial zone in the millions. If you visit Eastern Island once all the terns have returned, be sure to bring a pair of earplugs…Sooty Terns are notorious, noisy neighbors!
Sooty Terns are common throughout tropical and subtropical oceans all over the globe, nesting on remote islands, just like Midway Atoll NWR. While these terns used to breed on almost every island group in the Tropics, they have been extirpated from most, unfortunately due to human presence and activities, specifically alteration of habitat, persecution, and introduction of predators into breeding colonies. However, the Sooty Tern population is still quite robust on a global scale, totaling between 60-80 million.
From the tips of Ironwood trees to several feet underground, avian life abounds. But below ground, changes are happening. The Bonin Petrel (Pterodroma hypoleuca) is a major mover of sand and soil all over the atoll; thousands of these birds dig surprisingly deep and extensive burrows for the breeding season. Look across any patch of ground at Midway Atoll NWR and you’ll spot small heaps of sand or soil next to a dark opening. While these burrows may be a source of frustration to some (walk off of any road or trail and you’ll soon discover a minefield of small burrow openings), these burrows are quite remarkable in terms of nest architecture. For months, we have watched Bonin Petrels come in from the ocean at night, swirling around in the moonlit sky, crooning and perusing, eventually spiraling down to the ground and crawling to their burrows, slowly digging deeper each night, kicking out the sand behind them. As careful as we are in the field, we sometimes fall into burrows (they extend laterally and vertically… even seeing the burrow opening cannot help you to guess which way the burrow goes) and dig them out to check for buried Bonin Petrels or their eggs or chicks. Burrows twist and turn, sometimes going deep down into the soil, further than our hands can reach, and sometimes looping off into other chambers--quite amazing! In December, we would sometimes find adults in these burrows. Come late January/early February, we found the first few eggs in burrows; now, fluffy chicks have taken their place, comfortably residing in these cozy, underground dens.
The change in avian activity on Midway Atoll NWR is a thrill to experience. Laysan chicks to the left of you, Bonin Petrels to the right, and Red-tailed tropicbirds circling overhead…you are surrounded by wildlife wherever you go. It is during the spring that you truly become aware that you are indeed living in a seabird colony. To be surrounded by such a dense and diverse array of species is incredible and to catch a rare glimpse into the lives of these birds is all the more extraordinary. For all the avian life of the atoll, though, little is known about most of these seabirds’ basic ecology and biology due to their remote location and nomadic nature at sea. Where do Bonin Petrels forage in the vast Pacific Ocean? What environmental factors trigger breeding for White Terns? What sort of migration patterns do Great Frigatebirds follow? The breeding season brings life, but it also brings the chance for discovery—and we look forward to all we can learn from our feathered friends.
Wieteke Holthuijzen: budding environmental scientist, passionate birder.