The Contradiction of Robins

In woods in mid-winter, living trees, shorn of leaves, join their dead companions in extending naked branches to the sky.  What was hidden in summer behind a green curtain is revealed.  The track along the edge of the pond near my home takes me to a point where, across the frozen surface, I now can see through the trees to where I started.  As a result, though the horizon has expanded in this season, this pond today seems much smaller.

In the mid-Atlantic region of the United States, this February has been bitterly cold and battleship gray with little hint that we are heading to spring.  Yet, the days are inexorably growing longer.  A full hour of sunlight will be gained between now and mid-March, though it won't feel that way amid the rain, the snow, the gray.

One of the contradictory phenomenon of winter throughout much of the central and eastern U.S., particularly at lower latitudes, is the bird we salute as the herald of spring, the American robin (Turdus migratotius).  Although it is actually with us through much, if not all, of this bleak season.  On my winter walks, robins have been my frequent companions, sometimes signaling their presence just by calling, and, sometimes, by swooping in large numbers onto the trees and bushes that still bear fruit (or, at least did, until the marauding robins descend).

I recently read Margaret Renkl's inspiring and graceful The Comfort of Crows:  A Backyard Year (2023).  Its essays track the passage of a year in the events and changes in the natural world, largely in Renkl's Nashville backyard.  The life and death, losses and gains, the comings and goings in that world move and educate her as she deals with changes in her own life, and responds to the chaos that has overtaken the country.  Ultimately, the persistence and resilience of the natural world offers her hope.  In nature, she remarks, "so much life springs from all this death that to spend time in the woods is also to contemplate immortality."  (p. 259)

What prompted me to write this post is a passage early in the book.  In a chapter set in winter, she cites a tradition among birders that the first bird seen in a new year will characterize the kind of year you will have.  One year, she spots a passing woodpecker but cannot determine if it is a downy or a hairy, so she goes with the second bird sighted, a robin.

I love robins. . . .  I love the way they flock up in winter, with the locals and their new offspring welcoming the migrators to a season-long family reunion.  (p. 5-6)

Renkl's portrayal of the local robins sharing their winter home with those migrating through pleased me, prompting research on the bird's migration patterns, including the relative balance between robin homebodies and travelers, and how far the latter actually migrate.

I admit that I've always found the robin's scientific genus name a bit off putting, in part because it evokes South Park.  But the Latin root for Turdus means "thrush" which the robin is, being a member of the thrush family Turdidae.  It's the species name that I think revealing because its dual meanings capture the bird's contradiction I just cited.  The Latin root of migratorius means both "migratory" and  "wandering."  Robins are restless creatures, they all wander, even those who seldom travel far from home.  Their search is constant for a supply of soft-bodied invertebrates in some months and fruit in others.

Populations of robins may be comprised of both migrants and year-round residents.  Indeed, this is possibly true of the majority of bird species.  For robins, this phenomenon has been explored by biologists David Brown and Gail Miller, using data on bird banding and recovery from 80 years of Federally supported bird banding.  They looked at robins in these data for which there were observations in both the breeding season (May to August) and the wintering season (November to February).  They defined individual, local/non-migrant robins as those for which the distance between locations of observations in breeding and wintering months was less than 100 km; all others were considered migrants.  (Band Recoveries Reveal Alternative Migration Strategies in American Robins, Animal Migration, Volume 3, 2016.)

Their findings document the overall complexity of the migratory behavior of T. migratorius.  Overall, they find 80 percent of recoveries to be migrants.  The distance traveled by migrants is stunning.  Of migrants, some 96 percent (nearly 77 percent of all robins) traveled 500 to 2,100 km from their breeding sites.  Curiously, the further south a migrant robin wintered, the further it had migrated.

A fifth or 20 percent of robins covered by their data stayed locally across breeding and wintering seasons, a phenomenon that has been increasing since 1980.  Overall, non-migrants moved on average slightly less than 21 km between these seasons.

A more detailed study of the movements of a small number of robins over a year suggests just how complicated migration can be for these birds, and how generalizations may fail to capture the reality.  Alex E. Jahn, et al. tagged 31 robins in the 2017 breeding season with devices that recorded their movements.  (First Tracking of Individual American Robins (Turdus migratorius) Across Seasons, The Wilson Journal of Ornithology, Volume 131, Number 2, 2019.)  Although only 7 of the birds were recaptured in the 2018 breeding season, the journeys of these birds over the course of the year were fascinating, almost idiosyncratic.

Of the 11 tagged in Alaska, 4 were recaptured.  By the middle of September, these robins had left Alaska, flying into western Canada.  A month later, some had reached Montana and North Dakota.  One ultimately reached Texas, 4,480 km from where it originated; another migrated 4,508 km to Oklahoma.  Only one of the 14 birds tagged in Massachusetts was recaptured a year later.  That bird first lingered during the fall near its breeding grounds, only leaving Massachusetts in November.  Then, over the course of 20 days in early November, it winged its way to South Carolina, a distance of 1,210 km, where it overwintered.  Finally, of the five robins tagged in the District of Columbia, only two were recaptured and both stuck around their home territory for the entire year, migrating not at all.

The routes migrating robins follow during any particular journey south may not be fixed, possibly varying from year to year (bird to bird?).  The Cornell Lab's entry for the American Robin in Birds of the World, posits:

The term 'routes' does not really apply to robin migrations and there does not appear to be strong connectivity between overwintering and breeding grounds.  Indeed, evidence from banding records shows that robins in a particular area originate from widely scattered areas to the north.  (E. Natasha Vanderhoff, et al., American Robin, Birds of the World, Cornell Lab of Ornithology, 2020.)

Indeed, migrant status from year to year may not be immutable for individual robins.  Though Brown and Miller concede that their data do not allow them to determine whether any specific bird migrated one year but not the next, they add:  "It is possible, especially given the typically vagrant nature of robins during the non-breeding season."  (p. 43)  I am really taken with the idea that the impulse to migrate at all might be up for grabs for individual birds from year to year.

Ultimately, I believe that the robin's migratory behavior, bridging the seasons, offers some solace in this world where life inevitably leads to death.  Robins are heralds of restorative spring, the season when life is new and renewed, and, in the dark days of winter, they show that, even then, life persists.

Sea Urchin as Canvas

The fossil pictured below is an echinoid (sea urchin) some 84 to 90 million years old from the middle of the Upper Cretaceous period.  This post explores how this particular specimen, as well as most of the other members of its specific taxon, served as a "canvas" that was intricately "decorated" postmortem.  It concludes with a very plausible explanation of why.

The first photo shows the apical (apex) side of the fossil (the side where, in life, waste products were expelled); the second is of the oral side.

Echinoids originate in the fossil record in the Middle Ordovician period (471.3 to 458.2 million years ago) and are with us still.  They come in a variety of spherical, usually globular, shapes.  A calcium carbonate skeleton or test made up of interlocking plates encloses the internal organs of the animal.  In life, the test is covered with spines giving the animal the name "urchin" which, in Middle English, means "hedgehog."  (Sea Urchins:  Strange and Spiny Wonders of the Ocean, Holly Chetan-Welsh, Natural History Museum, London.)  The spines are typically lost when the animal dies.

In my well worn (and surely dated) copy of Invertebrate Fossils (1952), geologist Alfred G. Fischer notes that the echinoids are divided into two groups:  Regularia and Irregularia.  The former have a clear and distinctive pentagonal symmetry; the latter show a bilateral symmetry.  The two groups differ as to their life style.  Regular urchins live on the surface of the sea bottom.  Fischer quaintly says of the regular urchins:  "some wander about on their spines, as on stilts, while others clamber over submarine cliffs by means of their prehensile tube feet, or nestle in rock cavities."  (p. 705)  In contrast, the irregular urchins are adapted to soft, muddy sea floor sediments, often burrowing into the substrate.

The echinoid pictured above has the distinctive overall heart shape that marks it as an irregular urchin of the spatangoida order.  The distinguishing heart shape evolved, according to Fischer, to facilitate the animal's deep burrowing into the sea floor.

The apical side of this specimen displays a set of radiating plates, all but one in a quite distinctive petal shape.  That outlier, the one pointing to the top of the picture and leading to the indentation of the test, appears slightly less well defined.  This specimen is of the genus Micraster, and I accept the original collector's identification of it as a M. coranguinum.

This specimen was collected in Spain from a portion of the Olazagutía Formation that spans the Coniacian (89.9 to 85.7 mya) and Santonian (85.7 to 83.6 mya) ages of the Cretaceous period.  These fossils are very abundant in this formation which is composed of sedimentary layers of marl and marly limestones.  In general, marls are a blend of carbonate (usually calcite) and siliciclastic (silt and clay) muds:  the marl tends to marly mudstone as the siliciclastic proportion increases and to marly limestone as the carbonate proportion increases.  (Samuele Papeschi's "Marl" entry on his Geology is the Way website is an excellent introduction to marls.)  The ocean floor in the Olazagutía area during the Upper Cretaceous was soft and muddy, ideal for a burrowing irregular echinoid like Micraster.

I noted at the opening of this post that this echinoid and others of its ilk were thought to have been decorated after death (more on that timing below).  The "artists" in question were so-called sclerozoans.  These are animals that, in this instance, attached themselves to the surface of the test after the urchin died, some to drill into or otherwise penetrate the urchin plates, presumably in search of whatever remained of the dead urchin, and some to secure a solid perch.

The term sclerozoan was proposed in 2002 by paleontologists Paul D. Taylor and Mark A. Wilson to describe an animal "fouling any kind of hard substrate."  (A New Terminology for Marine Organisms Inhabiting Hard Substrates, PALAIOS, Volume 17, Number 5, October 2002.)  I like the term "fouling" applied to what these animals and their plant counterparts (sclerophytes) do to these surfaces.

In the images below, I have focused on several of the "ornamentations" left by the sclerozoans on this particular Micraster specimen.  In general, I can only suggest the broad taxa from to which these animals belong.

The first shows a close up of the worm tube on the oral side of the test.  This could be the work of worms in the Serpulidae or Spirorbidae families.

This next picture is the signature of a bivalve that encrusted the test on the edge of apical side of the test.  The species responsible could be Atreta sp.  I do not know what the little oblong object is that appears above and to the right of the shell, but it would appear that it dug into the urchin test.

This last picture is of ichnofossils, that is, the traces left by the action of some living organism.  What entity is responsible for an ichnofossil is not often known with any certainty.  In this case, the little cuts into the urchin test were left by some boring animal; these markings are identified by paleontologists as the ichnogenus Rogerella.  If evidence from extant animals is any guide, the actor in question was probably a barnacle of some sort.

These are only a very few of the traces of sclerozoan activity on this test.  A fascinating study of Micraster echinoids from the Olazagutía Formation by researcher Samuel Zamora and his colleagues lays out the wide range of signs of sclerozoan activity upon and in Micraster tests from this formation.  In their paper titled The Infaunal Echinoid Micraster:  Taphonomic Pathways Indicated by Sclerozoan Trace and Body Fossils From the Upper Cretaceous of Northern Spain (Geobios, first available online January 11, 2008), Zamora et al. present the results of analysis of 100 Micraster specimens collected from the Cementos Portland quarry near the village of Olazagutía, Spain.  These fossils were found in a layer dating from the Lower Santonian age.

I would note that it is a section of the Olazagutía Formation exposed in this particular quarry which has been accepted as defining the base of the Santonian age.  The bivalve Platyceramus undulatoplicatus first occurs there.  (M.A. Lamolda, et al., The Global Boundary Stratogype and Section Point (GSSP) for the Base of the Santonian Stage, "Cantera de Margas", Olazagutia, Northern Spain, Episodes, Volume 37, Number 1, March, 2014.)

What I found remarkable about the study by Zamora and colleagues was that 95 (95 percent) of the 100 Micraster echinoids they analyzed showed evidence of activity by sclerozoans, either living on the test surface (94) or digging into the test (71).  Though my specimen shown above may not be from precisely the same location as those considered in the study, it certainly reflects the same evidence of sclerozoan activity as those analyzed by Zamora et al.

This raises the question:

Why did these urchin tests prove so irresistible to sclerozoans - those encrusting or drilling animals - in this general time and place?

 Zamora and colleagues proffer an answer I find elegant and convincing.  First, as I noted earlier, this activity on and in the urchin tests likely occurred after the death of the echinoids.  Zamora et al. posit this timing is true because, while alive, these urchins lived burrowed into the muddy ocean floor and were protected by spines.  Both of these attributes ward off sclerozoans.  Second, after death, the echinoids' tests lost their spines and the tests were likely to have been unburied at some point.  Third, in a key assertion, the authors write:

In the argillaceous-carbonate, muddy bottoms of the Upper Cretaceous marine platform of the Olazagutía area, exhumed endobenthic echinoids tests constituted small, yet stable island environments on which several biological groups found a place for settling (94% of the tests found are colonised).  (p. 23)

"Small, yet stable island environments . . . ."  Lovely image.  Given the abundance of Micraster echinoids, their tests constituted many, many such small islands.