The author clearly describes the subtle and often quite complex ways the details of the particular sensory systems of different bird species are used in their respective ecologies, that is, their natural contexts. He explains the diversity of these systems and experiments that try to confirm the mechanisms by which the systems are used to lead birds to do what is observed, and how these experiments confirm the 'why' as well as the 'how' of the behavior.
|Snowy owl; Wikimedia|
Adaptation as the default assumption
Bird Sense is a long, enjoyable, and informative string of stories about birds and their sensory systems. A characteristic of the presentation of some of the sensory systems whose mechanisms are better understood is that the resulting behavior is described not just by showing its mechanisms, but by describing the mechanism in terms of adaptation. Thus, an owl's eyes or hearing are 'adapted' for hunting in poor lighting conditions.
There is a subtle but important difference between describing how a system functions in a given species or its setting, and saying that it is an 'adaptation' for that function. Indeed, many chapters in Bird Sense stick to the means and functions. The word 'adaptation' can mean that, just as a hammer drives nails, the mechanism is suitable for that particular job. But 'adapted' can also imply a designer with a problem to be solved in mind, and when used in the biological context the latent assumption is that the function we see today was 'designed' to do that by natural selection. It is very common if not often routine for biologists to equate functional description to 'adaptation' in this selection sense, and, further, that the selection was for the particular function being described.
This is not just implicitly strong but is also specifically genetic determinism, by which one would say that genes leading to a particular shape of an eye or ear evolved because of the light or sound it can respond to. It implies that in the past, bird-ancestors that didn't have the genes 'for' that trait didn't survive or reproduce as well, to the point of extinction (or, alternatively, to replacement of earlier versions of the trait because of the present-day observed function). We often write here about how such explanations are used in interpreting human behavior and, especially hominid fossil finds.
Inferring specific natural selection in a specific context of present-day observations, but applied to the slow and gradual process that evolution generally seems to be (even when selection is involved) can be a far stretch from what happened in the actual history of the trait. Even if selection is the explanation, indeed even if the assumed type of selection is true, we have little way of affirming that inference. Just because a story is plausible doesn't make it true.
One standard response to such points is that, "well, the mechanism today is so well-organized and highly functional, that the specific inferred selection/adaptation scenario is most likely the true one." But that is only an assumption, not really a scientific statement, because it is not tested or compared in any rigorous (or, often, to any) other possible explanations that might also be plausible (indeed, without observing past behavior directly, the alternatives may be impossible even to identify or speculate about). What makes it 'most likely' is really the subjective judgment of the scientist making the assertion, not any formal kind of likeliness analysis.
Sometimes an explanation seems very persuasive. Protective coloration is a good example, and some instances seem really striking. Maybe the explanation is entirely correct. But maybe there are other things involved and we just don't realize it. It took Rick Majerus a lot of work to confirm the standard explanation for industrial melanism in moths in the UK, and non-visual hypotheses have been suggested even there. So we should keep an open mind.
However, it is typical to accept someone's proffered scenario rather readily. One might ask why we would even raise this as an issue. Aren't the experts right? Our reason is simple: we know that the kind of assumed genetic determinism that underlies such inferences can easily be wrong.
Indeed, Bird Sense includes numerous examples of context-specific behavior of birds. That means the behavior only occurs in some situations or settings. Also, for a given function, there are related birds, who don't have but might well benefit from the examined bird's mechanisms. (We'd like to be able to see better in the dark, for example!) And Birkhead describes many instances in which the same birds behave very differently in different ecological circumstances, or only behave in a given way with some probability. This is living proof of genetic non-determinism, and should be a coup de grace against highly specific selective explanations.
When the same structures are used differently in different circumstances, such as when birds learn to sing in a volume or tone that conveys sound most effectively given the nature of the plant species in their particular environment (dense trees or not, etc.), then what is 'adaptive' is not the specific behavior, but the ability to evaluate circumstances and modify behavior.
The ability to evaluate circumstances and behavior accordingly is much harder to understand than it is to show that, say, a given length of cochlea is most sensitive to sounds of a given frequency, or eye shape and size to focusing or resolving in the dark. But all too often in evolutionary biology the implication that a suggested mechanistic adaptive scenario might not be correct is not acknowledged. The variation is treated, implicitly, as an exception--as if exceptions don't challenge the the adaptive assumption about the rule.
What is adaptation 'for'?
The adaptation problem is basically the stereotyping of a trait or equating what is seen in some particular setting, by some particular observer, as s/he chooses to look at it, treated as universally true, to the selective reason in the past that is simply assumed to have produced the trait we see today. Indeed, Birkhead is explicit in acknowledging how these stories, in his case about birds, change over time and with new knowledge. He calls this 'truth for now', and that's quite appropriate. What's inappropriate is the way that biologists, including anthropologists and human geneticists, are so assertive of the 'truth' they happen to see or think of.
Again, the scenarios suggested may be true in whole, or at least in part, but there may be some very different scenarios we just can't think of today, which may just as plausibly have laid the groundwork for what we see today. Current practice in evolutionary biology simply does not square up to these important problems.
Nor are these new thoughts on our part; they have been around, largely in smaller circles in evolutionary biology for many decades. Bird Sense is a compelling explanation of many means by which birds manage to grace this world today. In that sense, in most instances adaptive phraseology isn't necessary.
Bird Sense concludes with a section discussing some of the unknowns and, in particular, of how senses can be adapted based on such circumstances as damage to one of an individual's senses. To us this shows the cogency of the question how organisms are programmed not to be too tightly programmed.
Nobody except a few ornithologists really cares how a given species of bird finds its food--how much it relies on sight, smell, or touch. But when it comes to humans, assertitions about specific behavior that the asserter implicitly or explicitly claims was favored by selection and hence is now genetically mandated, can as history shows lead to devastating treatment of people by other people.
The same adaptationist habits are predominant in the technical as well as popular science literature today, and we think one should resist the tendency to treat explanations of what is present today as if we know that they are what was going on in the slow, deep past, when birds took flight.