Songbird Males Incubating Eggs: It's Not A Simple Situation

I was sent a photograph of a bluebird sitting on a nest of eggs in a bird box.  The sitter was definitely a male; it's fairly easy to distinguish the genders with this songbird species.  The sender had been monitoring the bird box and knew there were eggs under this bird.  So, what was this male doing there?  Incubating the eggs while the female foraged, or covering the eggs in a protective posture during the female’s absence?  Is that a distinction without a difference?  (The picture does not appear here because I don't have permission to use it since it's appearing elsewhere.)

Incubation is the process intended to keep the embryos within eggs in the temperature range that supports their development.  The opportunity for males to be involved in this effort arises for two reasons:  (1) birds are oviparous, that is, egg laying, and (2) the eggs, once outside the female’s body, require incubation.  These two factors are the driving force behind the development of male and female pair bonds, the arrangement for most birds during the breeding season.  (See, Bart Kempenaers, Mating Systems in Birds, Current Biology, Volume 32, October 24, 2022.)  Taking care of eggs and, subsequently, tending to any nestlings is hard, demanding work, something many birds have evolved to deal with by including both partners.  Tomasz Wesołowski put it this way:

It seems that conditions for the evolution of biparental care arose only when eggs had become so dependent on care that they could not develop normally without it.  (On the Origin of Parental Care and the Early Evolution of Male and Female Parental Roles in Birds, The American Naturalist, Volume 143, Number 1, 1994, p. 53.)

Identifying male involvement in incubation of eggs is a challenge.  Consider the case at hand:  the bluebird male in the picture appeared to be incubating by virtue of sitting on the eggs, but was he actually helping to regulate the temperature of the eggs?  A couple of texts I had immediately at hand drew me into an ornithological black hole.  One source stated forcefully that incubation was solely the province of the bluebird female (Hal Harrison, A Field Guide to the Birds’ Nests:  United States East of the Mississippi River, 1975, p. 164), while another countered that incubation was a shared responsibility among bluebird males and females, though it was performed “mainly by the female” (Paul J. Baicich and Colin J.O. Harrison in their A Guide to the Nests, Eggs, and Nestlings of North American Birds, 1997, second edition, p. 252). 

To arbitrate this issue, I turned to the Cornell Lab of Ornithology, specifically its Birds of the World, an online resource bringing together the research on over 11,000 bird species.  (The full resource resides behind a paywall.)  In the eastern bluebird entry last updated in 2015, Patricia A. Gowaty and Jonathan H. Plissner have this to say about bluebird incubation (this particular text is behind the paywall):

By female only; scattered claims of male incubation probably based on observations of male attentiveness to incubating females or to nesting cavities. 

Hard to square that assertion with the photograph that started this exploration.  Possibly a better, though perhaps less authoritative, place to come down is the somewhat more nuanced position taken by Lawrence Zeleny, founder of the North American Bluebird Society:

The male bluebird sometimes sits on the eggs while the female is out of the box for brief periods, but it is doubtful that he contributes much to the incubation process since he does not have a brood patch.  (The Remarkable Process of Incubation, Bluebird, Spring 2021, an edited version of an article that first appeared in 1983, p. 23.)

The brood patch is a featherless area on the abdomen of a bird that develops to efficiently and effectively use the bird’s body heat to incubate eggs.  This anatomical feature is almost exclusively sported by females during the nest season.  In some species both genders develop brood patches, and in a very few, only the males do.

I take Zeleny's statement about male incubation to be based on his assumption that, because the bluebird male lacks a brood patch, he cannot be much of a partner in maintaining eggs in the optimal temperature range.  A fairly logical conclusion, but is it true?

I have come to believe it is not.  In much of the rest of the literature on avian nesting, if a male is seen to be sitting on a nest or present in a nesting cavity or box when the female is absent, the male is often deemed to be incubating the eggs, whether or not his precise impact on egg temperature is even measured.  Further, the presence or absence of a brood patch apparently isn't dispositive.

When Jared Verner and Mary F. Willson reported the results of a massive review of the literature available to them (their paper was published in 1969) on how male passerines (members of the Passeriformes order that includes the oscines, i.e., the songbirds) participated in the nesting period, they noted it could not typically be determined from published papers whether a male was “simply covering the eggs or was actually applying heat to them.”  (Mating Systems, Sexual Dimorphism, and the Role of Male North American Passerine Birds in the Nesting Cycle, Ornithological Monographs, Number 9, 1969, p. 35)  Nevertheless, they considered the two actions to be roughly comparable, positing that “simply covering the eggs, however, can significantly reduce their heat loss and thus help in their development.”  (p. 35)  (No source cited in support of this conclusion.)

Frankly, there is no clear picture currently available of the impact on egg temperatures of a songbird male, lacking a brood patch, sitting on a nest.  The results from the few studies I've read that do measure egg temperature during male attendance suggest that there can be some impact, though it's not necessarily straightforward or consistent.  For example, in 1983, Gregory F. Ball published the results of research on barn swallow males sitting on nests.  (Functional Incubation in Male Barn Swallows, The Auk, Volume 100, Issue 4, 1983.)  Barn swallow males lack a brood patch.  Ball placed eggs with internal temperature devices in a series of barn swallow nests and determined that “when male Barn Swallows sit tightly on the nest, they can incubate as effectively as the female despite the lack of a brood patch.”  (p. 999)  He did acknowledge, though, that the brood patch might be necessary to maintain egg temperatures at night, when ambient temperatures were likely to fall (a period of “thermal stress” as he described it), thus this was a time when only females sat on the nest.

Some 25 years later, Margaret A. Voss, et al., reached a somewhat different set of conclusions, though they still found this male activity had positive consequences for egg temperatures.  (Adaptive Value of Thermally Inefficient Male Incubation in Barn Swallow (Hirundo Rustica), The Auk, Volume 125, Number 3, July 2008.)  As did Ball, they placed eggs with temperature gauges in barn swallow nests.  They found that, in those nests from which they could gather data, the temperature in the eggs often fell when males were sitting on them, and only rose when females did the sitting.  Though hardly the same results that Ball obtained in his study, Voss et al. actually found a subtle postive impact.  This male action slowed the rate of temperature loss compared to what transpired when the eggs were left unattended.  As a result, when males sat on the nest, the eggs were kept from dropping below the temperature level needed for embryo development.  Does this constitute male incubation?  I think it does.  Further, the authors concluded that this male involvement enabled females able to spend more time away from the nest foraging and rebuilding their physical reserves, without endangering the eggs.  They posited that this could make the females less likely to abandon their nests.

Of particular interest, one study fairly recent study found that incubation by the one species' males lacking a brood patch did actually raise the temperature of the eggs appreciably.  (Sonya K. Auer, et al., Biparental Incubation in the Chestnut-Vented Tit-Babbler Parisoma subcaeruleum:  Mates Devote Equal Time, But Males Keep Eggs Warmer, Journal of Avian Biology, Volume 389, 2007.)  The authors asserted:  "To the best of our knowledge, our results are the first to demonstrate significantly higher egg incubation temperatures in males relative to females."  (p. 281)

The proposition that sitting is incubating does make things neater and probably reflects reality.  Regardless of what the actual temperature effect is of a nest-sitting male that lacks a brood patch, a significant portion of all passerine species exhibit some degree of such male participation in incubation (as broadly defined in the literature).  Daiping Wang, et al., analyzed the variation in parental engagement in three aspects of the breeding cycle, including incubation, as those variations were reported in Birds of the World for over 1,500 bird species.  (Sex Differences in Avian Parental Care Patterns Vary Across the Breeding Cycle, Nature Communications, Volume 14, 2023.)  With regard to incubation, they reported that, in slightly fewer than half (48%) of the species they covered, incubation was reported to be a shared activity between males and females, while in about 50% of species it was solely a female responsibility, and in fewer than 2% was it only the male providing the care.  (The presence or absence of a brood patch in males was not considered, though, as already noted, most males lack that anatomical feature in the nesting season.)

Given how widespread male involvement is in incubation, it arguably is of some benefit for the nesting success of these species that practice it.  Males appear to be a valuable part of the process, however complicated or variable their contribution might be.

The Challenge of Being Wrong: Are Scientists Different From the Rest of Us?

 In the opening of my most recent post where I wrote about the essential role in science of challenging conclusions,  I quoted Carl Sagan from The Demon-Haunted World (1996) about certitude being applicable only to pure mathematics.  I had only read the excerpt from Sagan's book that Richard Dawkins included in his always rewarding The Oxford Book of Modern Science Writing (2008).  I decided to read the book as a bit of after-the-fact due diligence.  What if I'd been wrong about what Sagan was saying?  (I wasn't.)

This explains why one day, recently, I had Sagan's book with me when I placed a carryout order at a nearby cafe and sat down at a table with someone else also waiting for his order to be filled.  He noticed the book and asked what it was.  I showed him the cover and, as I did so, someone else came and sat down, also waiting for her order.

The man said, with a relatively heavy accent, "Ah, Carl Sagan, I know of him.  He was a cosmo . . . ."  He struggled to complete the word.

"A cosmologist.  Yes." I said.

At that moment the newcomer to our table interjected forcefully, "No, he wasn't!!"

As I stared at this aggressive interloper to our conversation, I could feel that I was starting to blush and was at a loss for words.  My first thought was that I'd butchered the word and said something like "cosmetologist" (which Sagan certainly wasn't).  Then I recognized that, apparently, I had been wrong, seemingly stupid in calling him a cosmologist.

"He was an astronomer, very different from a cosmologist," she told us, with a tone that brooked no argument.

"Cosmology," I began , "... about the origins of the universe."

"Yes, a completely different field," she schooled me.

There are several explanations (excuses?) for why I said what I said, but, at that moment, I was, to use a word that will crop up later in this post, mortified by being wrong.  I was carrying the man's book, for God's sake, and had not immediately placed him in the correct scientific field.  What kind of poser was I?

This episode has stayed with me partly because the memory is painful, but also because I was already thinking about the scientific method where the search for wrongness apparently is unrelenting.  We humans hate to be wrong and are wrong much more often than we know.  We are also very bad at acknowledging when we have been wrong.  Clearly, this is true for me (see episode above), but is it true for scientists whose careers are forged in the crucible of the scientific method?

The ubiquity and challenges of being wrong are described nicely by journalist and writer Kathryn Schulz in her exhaustive and perceptive Being Wrong:  Adventures in the Margins of Error (2010).  She explores the factors that contribute to our being wrong, the ways in which we respond to being wrong, and, surprisingly, why being wrong is a good thing.  She also considers the scientific method.

Though her book is helpful in considering my minor instance of being wrong, her primary focus is on being wrong to the point of realizing a core belief of yours (scientific or otherwise) no longer holds, or finding that your world view or sense of self has come undone.

The experience of having been wrong can be excruciating.  Schulz writes, "Certain mistakes can actually kill us, but, many, many more of them just make us want to die.  That's why the word 'mortify' comes up so often when people talk about their errors."  (p. 26)

But she goes to great lengths to convince the reader that being wrong is actually "far from being a sign of intellectual inferiority, the capacity to err is crucial to human cognition."  (p. 9)  It's how we learn (yes, I did learn a little something from the episode above).  Further, she states, "Ultimately, then, we are transformed by error through accepting it."  (p. 252)  This leads to the salutary act of embracing doubt, that is, being aware of the possibility we could be wrong.

Schulz on the scientific method is quite interesting.

As an ideal of intellectual inquiry and a strategy for the advancement of knowledge, the scientific method is essentially a monument to the utility of error.  Most of us gravitate toward trying to verify our beliefs, to the extent that we bother investigating their validity at all.  But scientists gravitate toward falsification:  as a community, if not as individuals, they seek to disprove their beliefs.  Thus, the defining feature of a hypotheses is that it has the potential to be proven wrong (which is why it must be both testable and tested), and the defining feature of a theory is that it hasn't been proven wrong yet.   (p. 35)

For me, the key portion of this passage regarding scientists is:  "as a community, if not as individuals, they seek to disprove their beliefs."  (emphasis added)  That distinction is important and speaks directly to the question I posed above.  Do scientists, as practitioners of the scientific method, respond differently to being found to be wrong because that is always recognized as a possibility?  Schulz is positing that it's the scientific community that moves forward on this constant testing, not necessarily the individual scientist who might have been wrong.

Of this constant challenge in science, Sagan in The Demon-Haunted World writes:

If you examine science in its everyday aspect, of course you find that scientists run the gamut of human emotion, personality, and character.  But there's one facet that is really striking to the outside, and that is the gauntlet of criticism considered acceptable or even desirable.  (p. 31)

He then asks:

Why do we put up with it?  Do we like to be criticized?  No, no scientist enjoys it.  Every scientist feels a proprietary affection for his or her ideas and findings. . . .  [T]he hard but just rule is that if the ideas don't work, you must throw them away.  (p. 32)

I take Sagan as suggesting that, given their experience in the constant testing of the scientific method, scientists, after being knocked down by finding they erred, will pick themselves up, dust themselves off, and move forward.  Is that true?  Is it mostly true?  As I've discovered, possibly not.

My interest here isn't how scientists respond when a major scientific belief system falls apart and is replaced with another, i.e., a paradigm shift as described by scientific philosopher Thomas S. Kuhn in The Structure of Scientific Revolutions (1970).  Rather, it's when a scientist finds they've been wrong about a finding or an idea, something important to them, but not upsetting of a scientific belief system.  Are they just like the rest of us in their responses to this kind of error, or are they different, having been living within the embrace of the scientific method which schools them in understanding that they could be wrong?

Turns out that, upon learning they've been wrong, many scientists don't behave gracefully.  Consider my favorite story about a scientist getting something very wrong.  It involves paleontologists Edward Drinker Cope (1840-1897) and Othniel Charles Marsh (1831-1899), who, in the beginning, worked together amicably.  Then they had a falling out and things turned nasty.  (I have written about the Cope-Marsh feud several times, including the post here.)

Contributing particularly to the animosity that Cope felt toward Marsh was a specific event that, in some ways, was simply part of what happens in science.  In 1868, Cope, at the time a young scientist making his way, received a jumbled mix of fossil bones of a plesiosaur (an ancient marine reptile) from a contact out west.  He spent a year or more cleaning and piecing together the reptile's skeleton.  He followed an established method for assembling the creature and, when finished, was convinced this was a new plesiosaur taxon, one distinguished from all others by its distinctive, long neck topped with a small skull.  He named his fossil and wrote it up for publication by the American Philosophical Society.

When Marsh took a look at what Cope had put together, he let him know that he had it wrong (perhaps not gently, though that's how Marsh characterized it).  He told Cope that he had placed the animal's skull on the animal's tail.  In essence, Cope had things ass-backwards, literally.

Cope's first response was to be defensive and he sought to prove Marsh wrong.  Neither man gave in, until the renowned and respected paleontologist Joseph Leidy (1823-1891) was brought in to settle matters.  Leidy concurred with Marsh.  As Mark Jaffe writes in The Gilded Dinosaur:  The Fossil War Between E.D. Cope and O.C. Marsh and the Rise of American Science: 

Cope was mortified.  He quickly set about buying up all the copies he could find of the transactions of the American Philosophical Society, although some remained rattling around town.  (p. 14)

So, Cope's first response was denial and argument, then a digging in, until it was incontrovertibly shown that he had erred.  This was followed by a desperate effort to cover things up, make it seem like the error had never happened.  Though this particular incident was of very low stakes for paleontology, it was of high stakes for Cope.  This paper was to have been a major breakthrough for him, an establishing event.  Instead, he'd been wrong and, after being called out on it, knew it.

That scientists don't always readily admit to being wrong, even if the evidence seems overwhelming that they are, is the subject of Michael D. Lemonick's article titled When Scientists Screw Up (Slate, October 15, 2012)  He recounts several incidents in which scientists made significant claims only for their evidence to be shown to be wrong.  Admit defeat?  Hardly,  some stick to their disproven guns for many years.

The admission in science of one's wrongness before being called out on it is apparently rare enough that it's a unicorn.  Lemonick cites astronomer Andrew Lyne who, in a presentation about a major discovery he'd announced several months earlier, proceeded to tell his colleagues that he'd made a mistake in his calculations.  The reaction?  A standing ovation!  Perhaps these scientists realized that what Lyne had done in admitting his error was rarer than what he originally claimed to have discovered.

Lemonick's article doesn't describe Lyne's reaction when he first realized his mistake.  For a first hand account of that by someone who went on to own up to their mistake, I recommending reading the article that psychologist Julia Strand penned about being wrong.  (Julia Strand, Scientists Makes Mistakes.  I Made a Big One, Elemental, March 24, 2020.)

In 2019, she was a young academic up for tenure.  A year earlier, she had made an exciting discovery of an intervention that significantly reduced the time needed (listening effort) by test subjects to detect speech in a noisy environment, compared to that required by control subjects without the intervention.  (I have only a tenuous grasp on that intervention and what it did.)  She'd published her results and her findings prompted an NIH grant and became the basis for an app in development.

But when difficulties replicating her findings arose, she went in search for an explanation.  Ultimately, she figured out that she had incorrectly programmed the timing of the testing for the control subjects:  the clock had started before their testing actually began.  As a result, it only appeared that the test subjects were able to detect speech quicker than the control subjects; in effect, they'd gotten a head start.

Here's how Strand describes the moment of finding she was wrong and her reactions, including one worthy of Edward Drinker Cope:

When I found the mistake, I was home alone on my laptop - working late in the evening.  While I sat in the dark (crying), I briefly consider what would happen if I never told anyone.  The bug was hard for me to identify; maybe no one else would ever find it.  I could just go on with other research and nobody would ever know.

But clearly she doesn't do that, though the consequences she foresaw were dire.

I felt deeply embarrassed to have made such a stupid mistake, disappointed that my finding was junk, guilty for wasting everyone's time and polluting the literature, and worried that admitting the error and retracting the paper would jeopardize my job, my grant funding, and my professional reputation.

The next day, she owned up to the mistake, contacted everyone involved, including the journal that published her study and the chair of her tenure committee.  Turns out that Strand received the equivalent of Lyne's standing ovation:  her reputation wasn't trashed, people she contacted were supportive and comforting, and, yes, she received tenure.

Just how rare this self-correction of research results might be is suggested by the data from the Loss-of-Confidence Project which, roughly a decade ago, queried the psychology research community on the extent to which its scientists had lost confidence in the validity of one or more of the findings of their prior research and what they'd done about that, if anything.  (Julia M. Rohrer, et al., Putting the Self in Self-Correction:  Findings From the Loss-of-Confidence Project, Perspectives on Psychological Science, Volume 16, Number 6, 2021.)

Psychology researchers were invited to submit descriptions of their own research in which they had lost confidence.  The project ultimately received just 13 statements describing research from scientists, among them one from Julia Strand.  In essence, the scientists admitted publicly, perhaps for the first time, there were problems with these research findings.  In addition, an online, anonymous survey garnered 316 responses about loss of confidence (not just from research scientists in psychology fields).

According to Rohrer et al., the researchers submitting statements admitted to committing methodological errors; making invalid inferences; and resorting to so-called p-hacking (in effect, cherry-picking results from multiple statistical analyses).

Of the researchers responding to the online, anonymous questionnaire, almost half had experienced a loss of confidence in one or more of their findings.  The predominant reason for the loss of confidence was the recognition that they had engaged in "questionable research practices."  Tellingly, only a distinct minority (just 17%) had made their loss of confidence public (and when made public, it was not often done very aggressively).  Rohrer et al. acknowledge that their respondents are not statistically representative of scientific research in general, but suggest that their finding may mean that "thousands of articles published each year . . . will eventually lose the confidence of their authors - often because of known errors yet typically without any public disclosure."  (p.1263)

In the article's abstract, the authors are quite blunt:

Far from evaluating new evidence dispassionately and infallibly, individual scientists often cling stubbornly to prior findings.  (p. 1255)

That's only human of them.

Birds Above All Else

There is an exchange between two characters in Tom Stoppard’s mind-bending play Arcadia (1993) that captures in a clever way the essence of science. In Act Two, Scene VII, two characters in the present day on the Coverly estate are discussing events that may or may not have occurred there in 1809. Hannah Jarvis, an author, is skeptical of the conclusions being advanced by another character, an academic, regarding what happened over a century ago. Valentine Coverly, a graduate student in mathematics, is the scion of the Coverly family that owned, and still owns, the estate.

Valentine It may all prove to be true.

Hannah It can’t prove to be true, it can only not prove to be false yet.

Valentine (pleased) Just like science.

Hannah’s statement, with its double negatives, does address a core tension in science: no conclusion, no matter how thoroughly tested, is above challenge. As Carl Sagan wrote, in The Demon-Haunted World: Science as a Candle in the Dark (1996), “Except in pure mathematics nothing is known for certain (although much is certainly false).”

But there are some propositions in science that have moved beyond Hannah’s cautionary statement into the realm of nearly universally accepted scientific facts. Evolution through natural selection is one of them. The subject of paleontologist Steve Brusatte’s masterful new book, The Story of Birds: A New History from Their Dinosaur Origins to the Present (2026) should be another, if it isn't already. Brusatte writes:

Birds are dinosaurs.

This realization is one of the greatest achievements in the history of paleontology.

(I read an e-book version of this book so I won’t be providing page numbers for quotations.)

The book reaches two conclusions: birds are dinosaurs, and birds are marvelous creatures. It is a heady and graceful work, serving not just the fossil lover, but the bird lover as well. It's also rich fare for someone in search of how science has been done in the past and is done now. Basically, it's a good read.

That he is completely smitten by birds, ancient and present, is, if possible, an understatement. To offer just one example of the language he uses when extolling birds, early on he writes that birds are

some of the most unusual, the best adapted, the most fine-tuned animals that live today, and that have ever lived, in the 4.5-billion-year history of the Earth.

Though Brusatte describes his personal connections to this work, he is restrained and does not fall victim to the temptation, succumbed to in much popular science writing, of becoming the hero of the story.

Undoubtedly and deservedly so, Archaeopteryx is the hero of the paleontology side of the story that Brusatte tells. The fossils of this species, first discovered in Germany in the early 1860s, show that the animal sported feathers. It is considered the oldest bird, dating from the late Jurassic (about 150 million years ago). The picture below shows the so-called “Berlin” fossil of the species. (This image is in the public domain one of the several fossils of the species. It is published under the Creative Commons Attribution-Share Alike 3.0 Unported license.)

There are human heroes in this story, as well. Though Brusatte profiles many of the scientists who have done significant, groundbreaking work exploring the evolution, development, and family ties of birds, I am most taken by two who were involved in either first enunciating that birds are dinosaurs, or reviving that position after years of willful neglect: biologist and Darwin champion Thomas Henry Huxley (1825-1895) and paleontologist John Ostrom (1928-2005).

Huxley is a fascinating personality, faithful to his working class origins and dogged in his commitment to what he perceived as true. Brusatte highlights the bold steps Huxley took in lectures in the 1860s to lay out why he believed: "Aves (birds) has its root in the dinosaurian reptiles." The key elements to his argument were the combination of bird and dinosaur attributes that he detected in two recent fossils discoveries; these approached the issue from different directions. First, there was the earliest bird, Archaeopteryx, which featured some characteristics of reptiles, and, then, there was Compsognathus, a reptile also from the late Jurassic that sported some avian features. Huxley was brilliant in building his argument to a striking and irresistible conclusion.

Brusatte describes how, in time, this conclusion about the relationship of birds to dinosaurs fell out of favor and faded into the background. Some hundred years after Huxley had staked this claim, paleontologist John Ostrom revived it in a profound way. Ostrom had found and named Deinonychus, a coelurosaur theropod dinosaur that lived during the early to late Cretaceous (115 to 98 million years ago). Ostrom argued that the coelurosaurs, unlike their lumbering, huge brethren (think T. rex), were small, agile, and quick predators. They were also light weight, given that their bones were hollow. (Coelo, the Greek root, for their genus name means “hollow,” and, by the way, birds have hollow bones.)

Ostrom knew the coelurosaurs from his find and others which dated back to the Jurassic.  After closely analyzing the four Archaeopteryx fossils known at the time, he came to a profound conclusion. In a brief, one-page article, The Ancestry of Birds, which appeared in the March 9, 1973, issue of Nature, Ostrom listed the 21 specific morphological attributes of Archaeopteryx that he posited were shared by coelurosaurs. He introduced the list by writing:

The following coelurosaurian features of Archaeopteryx collectively are here considered as prima facie evidence of a coelurosaurian (Theropoda) ancestry of birds.

He concluded that Archaeopteryx had gained these features through "direct inheritance from a small coelurosaurian ancestor."

Of Ostrom's article, Brusatte writes:

It is probably the single most important page ever written about dinosaurs.

He summarizes what Ostrom was demonstrating as follows:

Birds are a type of coelurosaur, which is a type of theropod, which is one of many types of dinosaurs.

In support of the book’s central conclusion, Brusatte delineates in detail how the pedigree of many typical bird traits and features is actually dinosaurian in nature. These traits range from hollow bones, to feathers, to nesting, to the laying of colorful, hard-shelled eggs, to an efficient respiratory system, and to relatively small size (true of many dinosaurs), as well.

Of course, what possibly most marks an animal as a bird is its ability to fly. Archaeopteryx, the most primitive bird, was a flyer, but only a crude one. Brusatte outlines how flight probably arose and, most importantly, uses this as one of several powerful examples in the book to illustrate a central aspect of evolution. A feature that arises for one purpose may, over time, come to serve another, different one. Flight is a clear example of this. Feathers, so critical to birds' flight, arose among dinosaurs, not for flight, but possibly initially as "sensory devices," and then for insulation. Indeed, Brusatte calls feathers a "default condition" for dinosaurs (think, hair for mammals). In this case, over time, evolution repurposed the early feathers into an essential, powerful component of a constellation of features, including the forearms evolving into wings, that would be necessary for birds to take flight. Light weight from hollow bones clearly contributed.

If one were foolish enough to skip the paleontological heart of this book with its treatment of how and why birds evolved and, instead, just read the last chapter, it would still be worth the price of admission. That chapter, titled Marvelous Modern Birds: Intelligence, Cognition, Birdsong, and How Birds Are Still Evolving Today, is a précis of much that makes extant birds so universally fascinating. In this chapter, Brusatte makes good on his assertion that "birds are among the smartest, cleverest, and most cognitively advanced animals on the planet." Some of today's birds, he shows, think about the future, engage in abstract thought, fashion tools, among other impressive mental and physical feats. I was particularly taken with the oscines, the songbirds, whose songs are not innate or instinctive, but are, instead, learned by immature birds from their elders. They are "vocal learners," no mean feat, considering the complexity of many of their songs.

Finally, birds have faced dramatic threats to their survival from their origins, threats that have included such events as the mass destruction at the end of the Cretaceous, and never-ending challenges from climate change. Sadly, in recent centuries, the impact of Homo sapiens on these dinosaurs has been profound, with many species lost to hunting and wanton destruction of the environment, and billions of individuals killed by the domestic cats we seem to be unable to keep indoors (but, despite claims to the contrary, only a piddling number are lost to wind turbines). Brusatte demonstrates how birds have endured these challenges, showing that they are truly survivors. He writes:

The future is uncertain. And paleontologists like me are always more comfortable looking to the past than predicting what might lie ahead. But as the world changes rapidly around us — because of us — I am willing to make a bet. The Age of Birds will persist, even if the Age of Humans does not.

Climate Change Hits Home: A Review of The Lost Trees of Willow Avenue

Mike Tidwell, climate activist and founder of the Chesapeake Climate Action Network, posits that, if you were to throw a dart at a spinning globe, no matter where the dart punctured the surface of the globe, that actual spot on the Earth would have an ominous tale to tell of the effects of climate change.

Tidwell has chosen a single year, 2023, and the small block on which he lives in Takoma Park, Maryland, in the Washington, D.C., suburbs, from which to sound the alarm over the damage being done by our sharply warming planet.  His book, The Lost Trees of Willow Avenue:  A Story of Climate and Hope on One American Street (2025), tells how climate change has come to his own block.  It’s an eminently readable volume, but, to be blunt, it scared the shit out of me.

This account came into being when Tidwell realized the giant oaks on his block and in his town were in crisis.  These trees, which had survived for decades or, sometimes, for a century or more, were dying in staggering numbers.  The loss of giant oaks in a single suburban block cannot be missed, because they leave such a massive empty space at eye level and in the overhead canopy.  That is not the only impact.  He writes, “Oaks are the miracle trees of both urban and rural forests.”  (p. 128) They are crucial to the entire local ecosystem, hosting and supporting a wide range of insect and bird life.  And it wasn’t just the oaks in peril.  During the period from 2019 to 2022, Takoma Park, a town of just 2.1 square miles with about 18,000 residents, lost 1,200 trees, the largest portion of them oaks.  A study from 2022 reported the loss of a startling 45 acres of the town’s tree canopy.

The causes for this still ongoing season of death are myriad, but ultimately mostly tied to the increasingly weird and changing weather patterns that have taken hold as the worldwide climate warms.  Rising temperatures change weather patterns, fostering prolonged droughts and storms whose flooding downpours are fed by the ever greater amounts of moisture held in the warming atmosphere.  Seasons are marked by atypical temperatures throwing trees’ internal cycles out of sync with damaging consequences.  The deluges of rainfall saturate the ground fostering the spread of insects and fungus, which attack the already stressed trees.  Takoma Park experienced all of this.  A recipe for a local, national, and global ecological disaster.

The stakes associated with the death of trees are high.  By sequestering a significant portion of the carbon dioxide we’ve poured into the atmosphere since the dawn of the industrial age, trees have helped mitigate some of the worst effects on the climate of that greenhouse gas.  Though the loss of trees reduces their important contribution to the planet’s health, what particularly worries Ning Zeng, one of Tidwell’s neighbors, is the prospect that the carbon dioxide contained in those myriad felled trees will be released back into the atmosphere.  Zeng, a professor of climate science at the University of Maryland, wants to bury as many of them under conditions that will preserve them for centuries.  Tidwell recounts in some detail the time-consuming challenges Zeng faces as he works to arrange for the burial of 5,000 tons of dead trees.

I would characterize it as a noble response to the crisis, but a drop in the bucket.  For instance, at the time Zeng worked to remove 5,000 tons from a site in Baltimore, there were many thousands of tons of dead trees still there poised to release their carbon dioxide.

Tidwell documents the broad scope of climate change-induced blows that his block and his town has endured, a tale told well with vignettes of neighbors, experts, and local politicians who are engaged in trying to respond to the damage being done.  The efforts range from trying to divert rainfall from low lying areas, from streets and basements, to planting trees that have evolved to thrive in a wet environment, to installing solar panels on roofs now exposed to the sun due to the loss of canopy cover, to promoting electric vehicles, and so on.

It’s quite clear to him that more, much more is needed on an unprecedented scale and that it must have a potential impact that dwarfs what’s been done so far.  Clean energy isn’t enough to reverse the damage.  Geoengineering on a vast scale may well be needed.  He describes many of the proposed methods being explored to draw carbon dioxide from the atmosphere.  But he is clearly attracted to seriously researching a different approach:  solar radiation modification, that is, seeding the atmosphere with aerosols (particulate emissions) to reflect sunlight away from the planet.  It’s a bold approach that may hold great potential to addressing the immediate crisis, but also may bring serious, negative outcomes.  At this stage, we just don’t know.

He is influenced by The Open Letter Regarding Research on Reflecting Sunlight to Reduce the Risks of Climate Change, issued in early 2023 and signed by more than 110 of the world’s leading climate scientists.  The letter posits that reducing greenhouse gases at this stage is too late to address the planet’s current and future warming.  Instead, it is urgent that we explore quickly whether and how to put aerosols into the atmosphere.

It’s a sobering thought that solar radiation modification is the “hope” in the book’s title.