Anyone who has spent much time watching birds in natural and human-altered habitats probably wonders why some species seem able to adapt and others seem to suffer in the face of change. Species have this intangible quality that scientists have labeled ecological plasticity, or the ability to adapt to changing conditions. Or put in a more timely way, this quality distinguishes the ivory-billed woodpecker from its commonplace cousin, the pileated woodpecker. But naming something does not necessarily mean we understand it.
So what sets apart the adaptable generalist from the rigid specialist? A quick answer would be that the success of species is driven by their evolutionarily shaped adaptations. So we need to assess their morphology, physiology, dietary requirements, and the like to understand and predict how they may respond to any given change in their environment. But more and more, scientists are turning their attention away from the physical bird to study its psychology in order to further understand the nature of adaptability. Like us, bird make thousands of decisions everyday, and how birds makes these decisions appears to be a central feature of what we call plasticity.
A long time ago, I studied the ecology of two species of warblers wintering in the tropical forests of Central America. One species, the bay-breasted warbler, can easily be classified as highly adaptable and ecologically plastic. It moves through the different layers of this complex ecosystem with ease, feeding on a wide variety of seasonally available insects and fruits. The other species, the chestnut-sided warbler, is more of a specialist. It forages at the same height, in the same way, and follows the same resident species all winter long.
Morphologically and genetically similar as they were, I wondered about the different approaches the species might use to explore and learn about their winter quarters. Through some relatively simple experiments, I uncovered a single large behavioral difference between the two species. Whenever a new object was placed next to the favorite feeding dish of a chestnut-sided warbler, its normally enthusiastic feeding response was suppressed. Although their beloved mealworms were in plain sight, the warblers refused to feed. In fact, they were afraid to feed and showed many of the behavioral signs associated with fear. The plastic and adaptable bay-breasted warblers paid no heed to the novel objects and feed with alacrity.
This fear of novelty is known as “neophobia.” From my observations I developed the Neophobia Threshold Hypothesis, which holds that more specialized species have a lower threshold for being fearful in the face of novelty. A shift in this threshold, which is easily selected for in domestic animals, may portend large ecological differences between species. This hypothesis is attractive because it is simple and because it resonates with our own human experience. How many of us have friends or children who shy away from any flavor but vanilla, unwilling even to try something new?
This hypothesis has been further tested by myself and others working on sparrows, West Indian songbirds, primates, rodents, and other animals and has been confirmed in many of these studies. However, like many scientific theories attempting to explain something complex and biological, results have been admittedly mixed.
One of the surprising discoveries is that some of the species that have the closest association with humans and human-created habitats are actually among the most neophobic species around. Last winter I conducted neophobia experiments on the wild ducks that come to the ponds at the National Zoo. I had good numbers of mallards and wood ducks and I predicted that the mallard, which is very widespread and closely associated with humans, would be less neophobic than its forest-loving cousin.
Instead, I found that mallards were amazingly neophobic. They were habituated to people and tame in their presence, but would often wait hours before they fed ambivalently at the feeder with novel objects, and even then they appeared ambivalent about doing so (view video clips of the experiment). Other species closely associated with humans, such as Norway and black rats, ravens and crows, and house sparrows, also show very high levels of neophobia.
All of these species have the common challenge of being both dependent on humans and highly persecuted by theml. Humans are unlike other predators in their ability to rapidly change their behavior and attempt to outwit their adversaries. This selects for a behavioral arms-race where attraction to novelty (a potential new food source) must be tempered with initial caution (as it may be a trap or contain poison). So I believe that these persecuted commensals are uniquely adapted by being both very curious (neophilic) and neophobic at the same time, with the balance shifting over time and with experience.
However these theories eventually fall out it is clear that animals pay an inordinate amount of attention to novelty in their world and that novelty responses are central to how individuals and species cope. And it is fun stuff to think about, because our modern culture depends upon that subtle dance between our own fear of and attraction to novelty as well. Just watch the commercials on TV, exciting us about a “new and improved” product and at the same time assuring us that it has the same “old-fashioned” quality, and you may feel what it is like to be a bird.