June 19, 2011
For birders visiting the Yucatán usually the most cherished sightings are those of endemic species -- species occurring naturally nowhere else on Earth. In the bioregion comprising the Yucatan Peninsula, parts of Belize, northern Guatemala and western Tabasco, about 14 bird species are recognized as endemic. One of those is the Yucatan Woodpecker, CENTURUS PYGMAEUS.
Before coming here I'd only seen Yucatan Woodpeckers once before -- in arid coastal scrub near Río Lagartos in the northern Yucatán. Here I've seen several. A picture of one with a beakful of grubs is at http://www.backyardnature.net/n/11/110619yu.jpg.
That bird displays much more yellow around the base of its beak than do illustrations of the species in field guides. Typically Yucatan Woodpeckers are shown with small, yellow "nasal tufts" at the base of the top of the bill. Is the pictured individual atypical, or are the faces of some Yucatan Woodpeckers simply more yellow than the books show? Two more pictures appear at http://www.backyardnature.net/n/11/110619yv.jpg.
In that picture I'm not sure whether we have two different birds or one bird shot twice. Both images were taken at the same hole but on different days. Whatever the case, both pictures show birds with much more facial yellow than field guides do. On the Internet I find a few more such yellow-faced Yucatan Woodpeckers so ours aren't just birds who have stained their faces by eating something yellow.
Up at Hacienda Chichen so often I've seen Golden-fronted Woodpeckers identified as Yucatan Woodpeckers that it's worth dwelling on the differences between the two species. First, a Golden-fronted is shown at http://www.backyardnature.net/yucatan/wp-g-f.htm.
The Golden-fronted Woodpecker is the very common red-headed woodpecker with narrow zebra barring across the back occurring throughout the Yucatán and far beyond, while Yucatan Woodpeckers are much less in evidence, or completely absent. I'm guessing that here we have ten Golden-fronteds for each Yucatan. My impression is that Yucatan Woodpeckers require less disturbed habitat than Golden-fronteds.
Yucatan Woodpeckers at 6.7 inches long (17cm) are significantly smaller than Golden-fronteds, at 9.8 inches (25cm). The Yucatan's beak is proportionately shorter than the Golden-fronted's, plus the Yucatan in flight displays a more spherical or compact, darker body. Even with these distinctions, though, under field conditions often what you think are Yucatans turn out to be Golden-fronteds -- unless you see yellow at the beak's base.
The Yucatan and Golden-fronted Woodpeckers are members of a cluster of similar species. In Mexico there's also the Gila, Golden-cheeked, Gray-breasted and then in the eastern US there's the very common Red-bellied Woodpecker. More species in the cluster occur farther south. Clearly, at some point in evolutionary history Nature came up with an especially effective "woodpecker theme," part of which was the red-headed, zebra-barred-back combination, and now that theme is being refined with local variations.
What a pleasure to be in the field experiencing these "variations on a theme."
HERON WITH A BULGING THROAT
Out in the mangroves a loud, guttural, croaking WOK WOH-WOH erupted from not far away. I climbed onto a hummock and amidst a tangle of branches saw the bird at http://www.backyardnature.net/n/11/110619ti.jpg.
It was a big heron. Any Northerner at first glance would have called it a Great Blue Heron, but that species is mostly just a winter visitor here, plus Great Blue Herons have very slender, snaky necks, while this bird looked like he had a huge fish stuck halfway down. Also Great Blues have neither this bird's strikingly yellow gullet nor its narrow, oscillating barring. This was definitely something different.
It was a Bare-throated Tiger-Heron, TIGRISOMA MEXICANUM, one of only a handful of herons found here but not also in North America. Bare-throated Tiger-Herons are distributed in various freshwater and mangrove habitats, especially those with wooded edges, from Mexico to northwestern Columbia. Very rarely one makes a foray across the Rio Grande into southern Texas, where it's received by birders with jubilation.
Often when you see this bird it appears to have a much shorter neck, like a bittern or night heron. The one in the picture was stretching his neck, looking at me. Also, juvenile birds are strikingly barred with many narrow, dark bands across their bodies, inspiring the name "tiger-heron."
Some herons dramatically stalk and chase their quarry but watching this bird hunt was fairly boring. He just waited, or sometimes very slowly waded about, his neck held diagonally with his beak pointed downward.
BLACK HAWK NEST UPDATE
In this May 8th's Newsletter we saw a Common Black Hawk perched in a snag in the mangroves, looking at her new nest. About 18 days ago the mother began spending more time perched on the nest's rim than sitting inside it. I assumed then that the egg or eggs had hatched, though I couldn't see what was inside.
This week I got my first glimpse of a fuzzy, white nestling head. You can see the nestling peering over the nest's edge and the watchful mother perched nearby at http://www.backyardnature.net/n/11/110619bh.jpg.
So far I've just seen one nestling. Howell says that nests usually contain one to three eggs.
Wednesday as the sun was setting the sky was clearer than usual and a full moon was rising in the east, out over the water. Sea turtles are laying eggs about now, and I couldn't imagine a better egg-laying night than that one. In the day's final, dim light I mounted the bike and headed toward a known nesting site.
About halfway there I barely could make out a blocky silhouette inching onto the white sand road. I figured it was a crab, but I didn't really see it until I'd photographed the form with a flash, then viewed the image on the screen. You can see what I came up with at http://www.backyardnature.net/n/11/110619cb.jpg.
Because of its large size -- the boxy body was about four inches across (10cm) -- I'd figured that it would be one of the big, blue crabs so often seen along the road in early mornings, so I was surprised by this one's body color, with only its big claws being a little bluish. Well, I knew that volunteer identifier Bea in Ontario would clarify the matter, and she did.
What's in the picture is indeed the Blue Land Crab, CARDISOMA GUANHUMI, despite its color. This species' color can range from dark blue to brown or pale gray.
Blue Land Crabs are distributed from southern Florida along Caribbean coasts to South America. Omnivorous, they eat leaves and fruits close to their burrows, as well as insects, carrion and much more, including other Blue Land Crabs.
Mainly the species is nocturnal. However, finally I'm realizing that many of the rustling, scampering sounds issuing from roadside bushes as I bike down the road on early mornings, instead of being raccoons, coatis and agoutis, which I'd supposed, are Blue Land Crabs. The crabs dig sizable burrows in the sand, often below tree roots, and hang around their den openings. When anything disturbs them, like me on the bike, they stampede across dry leaves into their dens.
By the way, Bea says that you can distinguish land crabs from underwater crabs by the way land crabs have tall, boxy bodies like our Blue Land Crab. Aquatic crabs usually are flatter and more streamlined to prevent them from being knocked about by waves and currents. Also, on underwater crabs the back two legs are usually flattened into paddle-like "swimmerets" used for swimming. On land crabs the back legs are slender like its other walking legs.
By the way, I didn't see turtles laying eggs that night. I did find one nest that had just been robbed. A couple of mornings later Marcia found some new nests, some already robbed, and a couple marked with upside-down Coke bottles on wooden sticks, for later robbery. Marcia moved the Coke bottles, so maybe there's hope for those nests.
One morning Marcia found on her doorstep what's shown at http://www.backyardnature.net/n/11/110619br.jpg.
The creature was dead and dried stiff. She'd seen such a thing before, but in the ocean, as she snorkeled. These little animals are aquatic, so how'd this one get to her door? She's guessing this scenario:
The previous day as she was snorkeling she'd found the colorful shell of a Queen Conch illegally killed within the last day. Its edible body had been cut from the shell. Marcia brought the shell home, and she's betting that the animal in the picture had been hiding in the shell, and fell out upon arrival at the door.
You might guess that the five-armed creature in the picture is closely related to starfish, and that's right, both being echinoderms. What's in the picture is a "brittle star," a common species known by several English names, including Banded-arm and Harlequin Brittle Star, Banded Brittle Starfish, and Serpent Sea Star. It's OPHIODERMA APPRESSUM, one of the most common brittle stars in the Caribbean region.
On the disc's underside was the five-slit opening seen at http://www.backyardnature.net/n/11/110619bs.jpg.
I'd guessed that the above opening was the mouth, but I was only half right. The star-shaped hole serves as both mouth and anus.
This species is nocturnal and does what it can to avoid light, so hiding in a conch shell is about right for it. Marcia says that sometimes brittle stars get inside her bodysuit as she snorkles, apparently seeking dark hiding places. Despite their interesting look, brittle stars disappoint aquarists because as soon as the light is turned on they go hide beneath something. If they're mishandled, their arms shatter.
By the way, how did I know that this brittle star wasn't an octopus? Octopuses are mollusks, not echinoderms, and mollusks -- like their snail and slug cousins -- have rubbery flesh with no interior or exterior skeletons. But this brittle star felt brittle, as it should have, being an echinoderm, for echinoderms have internal skeletons (endoskeletons) right below the skin. Their endoskeletons consist of interconnecting, subepidermal calcareous plates.
LIKE A SAND-COVERED BANANA
Walking the beach during the lowest tide I've seen yet, on the sand a couple of feet from the water's edge I saw what looked like a sand-encrusted banana. A toe nudge indicated that it was firm, so I picked it up. It felt leathery and its surface was studded with thousands of low, conical, dullish spines. You can see what the thing looked like with the sand washed off at http://www.backyardnature.net/n/11/110619cc.jpg.
Not "banana," but "cucumber," I realized. It was a sea cucumber. A lot of Googling later I couldn't be sure which species it was, but it may have been HOLOTHURIA IMPATIENS which, since it's found in tropical waters throughout most of the world, goes by several English names, including Brown Spotted, Mottled, Bottleneck and Impatient Sea-Cucumbers.
This was my first sea cucumber, but only a couple of feet away lay my second one. Or at least the remains of one. One end of it was stuck into the sand, it was flat and mushy looking, and flies were buzzing it. These were the only two sea cucumbers I could find all up and down the beach, so what were two doing here together, with one of them dead and with one end in the sand?
I imagined a scenario in which the male guarded the egg-laying female, but as soon as I began reading about sea cucumber life history, that notion had to be abandoned. Most sea cucumber species simply release sperm and ova into ocean water, though a few species keep the eggs inside the female's body and "give birth" when the eggs hatch through the cucumber's ruptured body wall. Therefore, there's no nest building or laying of eggs belowground for sea cucumbers. It just remains a small mystery as to why two sea cucumbers in their respective conditions should have been found as they were.
Like the above brittle star and starfish, sea cucumbers are echinoderms with endoskeletons just below their skins. Sea cucumber endoskeletons consist of tiny, calcified structures called ossicles joined by connective tissue. You can see a close-up of the cucumber's skin bumpy with subepidermal ossicles at http://www.backyardnature.net/n/11/110619cd.jpg.
Some sea-cucumber species move about on tube feet equipped with suckers, but on ours I couldn't find anything like feet. Sea cucumbers also display five- fold (pentaradial) symmetry, but that's visible only internally.
I wasn't sure whether our live sea cucumber was dangerously stranded by the unusually low tide, or whether it was doing something there important in its life cycle. I left it in a shallow pool of water. After a few minutes it began expelling jets of clear water from one end, though from the mouth or anus I don't know.
I read that Holothuria impatiens feeds on microorganisms and organic matter in sand, and often is found on submerged sand flats and among sea grass. Ours were near large beds of Turtle Grass.
Here's another local bush that's so pretty and robust that it deserves to be widely planted horticulturally: http://www.backyardnature.net/n/11/110619su.jpg.
Down at the southern rocky point this waist-tall bush has been sculpted by the constant wind. In less exposed areas the plant takes on a dense, pleasing shape like a garden Forsythia. And like Forsythia it produces thumbnail-sized yellow flowers. You can see some blossoms and the bush's narrow, soft leaves at http://www.backyardnature.net/n/11/110619sv.jpg.
You can see a flower in which its five petals and ten stamens have shriveled and fallen away after pollination, leaving five yellowing, leathery sepals looking like petals, and five slender, brownish styles (ovary necks) rising in the flower's center, at http://www.backyardnature.net/n/11/110619sw.jpg.
Here we're talking about what's often called Bay Cedar in English, though it's hard to see what's cedary about it. It's SURIANA MARITIMA, a bush found in thickets, on sand dunes and rocky shores, often just beyond the high tide line, as in the picture. It occurs on tropical coasts throughout the world. In Florida it's listed as an endangered species. Local gardeners there recommend it for planting in buffer strips around parking lots, in highway median strips and at borders. It's regarded as a butterfly bush.
Bay Cedar's floral anatomy is unusual enough for it to belong to a family most Northern botanists have never heard of -- the Bay Cedar Family, or Surianaceae. The family is a small one with most species found in Australia. One unusual feature of flowers in the family is that their styles arise from the bases of the ovaries' separate "carpels," a carpel being a subdivision of an ovary.
WHERE LIMESTONE MEETS WATER
Maybe more than 99% of the coast here is sandy beach, with limestone jutting into the sea only locally here and there. Where limestone has been subjected to wave action for thousands of years, the stone face often develops very irregular, sharp-edged features, as seen at http://www.backyardnature.net/n/11/110619ls.jpg.
In such spots you'd like to sit and watch fish and crabs in water swirling at your feet, but sitting there is just too painful. Imagine yourself having been shipwrecked, and after a long swim finally reaching shore, and then before you can crawl onto land the waves bounce you among these knife-edged rocks! Or trying to get a canoe ashore through them.
A geological term describing the process of water and/or wind eroding stone into such a pockmarked surface is "alveolization." An alveolus is a small cavity, pit or hollow. An alveolized surface might otherwise be described as honeycombed, though to my mind a honeycombed surface has more structure, like a honeycomb's neatly arranged lines of hexagonal cells.
Limestone is sedimentary rock. It's to be expected that marly mud from which limestone derives would not be perfectly homogenous in terms of composition and texture. Softer parts or those higher in carbonate content might erode at a different rate than harder parts or those with less carbonate content. After billions of wave splashes, you get jagged "alveolization."
The key: Early morning, showers offshore, on the beach just breezy enough to keep the mosquitoes down, high tide.
The tempo: Small, nervous, indecisive waves washing among rocks maybe every eight seconds.
Instrumentation: Wind-thump in ears, wave sloshes between rocks, constant breaker roar from offshore reef, woodpecker tapping on dead palm trunk.
Theme: The sun rising among slate-gray, tall- clustering cumuli, a thin layer of whitish, scaly alticumuli above them, pale blue sky above that, and then the sea leaden at a distance, green-blue nearer, transparent at my feet, thus double themes trending toward ever greater clarity, except, at me, non- understanding, non-analysis, all stimuli filtering through getting confused and lost in passage.
Melody: A single white gull, silent, sailing up the beach, ever so slowly, gazing onto the line below of ankle-deep golden Sargasso washed ashore with a single green and red watermelon rind from someone's boat, the passing gull not noticing me, soloing on up the beach, then gone, as if never there, the empty sky and me on a rock, me.
Accompaniment: The process of increasingly whiter clouds with better delimited borders and sunlight increasingly hotter from a sun a little higher up; mid-session a 90-second rainbow beside a purple shower crosses the beach down below.
Improvisation #1: But, no metaphors, please, and no memories either, please, just rocks and clouds and waves and wind, please.
Improvisation #2: The sun, the wind, the salt spray toughen the skin, they say. But, that's only the skin, I say.
Best wishes to all Newsletter readers,
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