Flight Above, Science Behind, and Teaching Beyond:
Developing a Sense of Place through Intergenerational and Multidisciplinary Science Learning on Bird Migration in the Understanding by Design Framework
THE SCIENCE BEHIND THE CURRICULUM
MIGRATION AND NAVIGATION
When looking at the navigation strategies that birds employ for thousands of miles of seasonal travel, over mountains, desert, oceans, or cities, we must define the patterns seen in various migrations and identify the hierarchical arrangement of tools available. Birds do not all “fly south for the winter” as it can be oversimplified. Yes, many birds have developed patterns of north-south migrations and this is the main route used by birds of North America. Geographically, the Central Valley of California exemplifies topography for this migration route because of the funneling system of the Coast Range and Sierra Nevada Mountains. Most migrants, whether traveling north to south, east to west, or in a circular pattern assess advantages and risks associated with topography and oceanography, weather patterns, food availability, and their own health condition.
With all the diverse modes of migration, scientists have tried numerous approaches to studying the phenomena. Modern study of migration patterns began with John James Audubon, in 1803, tying string on the leg of a migrant bird and hoped to see it return in the spring. Later, scientists and naturalists counted bird silhouettes by moonlight and visualized the scale of bird movements with Doppler radar, and planted transmitters on individual birds to track their specific journeys (Gill, 2007). Throughout the advances in knowledge and technology, citizens have recorded sightings and banding birds all over the world which contributes significantly to evolving understanding bird migration and our relationship with this natural process.
To follow routes specific to their species, birds have innate and learned processes that are utilized hierarchically during different stages of development. These orientation mechanisms include landmark recognition, positioning related to the sun and stars, topology of earth’s magnetic fields, sensorial cues such as odor, and polarized light at sunset and rise (Gill 2007). While some studies increase the numbers of known mechanisms that assist in bird migration, others are investigating the hierarchical arrangement of known mechanisms and their interactions. As simple as these mechanisms may seem, there are many unanswered questions and potential conflict in understandings of these mechanisms, as well as the short comings of the popular research (such as the lack of rigorous study on East-West migrants for processes already observed in North-South migrants). Birds certainly employ these hierarchically arranged tools for navigation and orienteering, but the interactions between different mechanisms and appropriate temporal and/or spatial scales of utilization are still being examined (Alerstam, 2006, Table 1).
Before birds need to use their map and compasses instruments, they are cued into the migration season by their response photoperiod changes. The local photoperiod acts as an external time keeper synced with the internal, biological clock in the pineal gland (Gill 2007, Weidensaul 1999). Additionally, information gathered from the photoperiod is sorted in the retina. As photon absorption varies with light intensity it reorders the paring of electrons. The reordering of electrons in different spatial directions gives a bird another source of relativism for orientation using the earth’s magnetic field. This relativism between light, field, and electron ordering in the retina is explained by Wiltschko and Wiltschko whom also found that an oscillation of the magnetic field will disequilibrate the retina’s interpretation of photon absorption (2009). A question remains for me of how light-induced photon absorption is related to, or affected by, the magnetic field.
When the timing is right for optimal internal and external conditions, birds will launch into their migrations and may fly nonstop until reaching their destination. While on their journey, birds employ a combination of their map and compass tools. The orientation is the directional bearings, or compass system, while navigation within a specific landscape is left to the maps (Weidensaul 1999). Novice migrants will rely on compass mechanics such as polarized light, sun, and/or stars to guide them before they can use navigation by landmarks. By adulthood, many birds master the use of global magnetic field topography and often do not revert to using other tools. During the 1970s, biologists located cells in the nasal cavity containing magnetite crystals. These crystals allow birds to sense the earth’s magnetic field which was studied with homing pigeons by ornithologist Charles Walcott during this time (Weidensaul 1999).
As mentioned, the study of migration is still evolving as is the particular mechanisms that allow for the phenomenon. The pineal gland and magnetite in birds certainly regulate cycles associated with migration but a study published in Nature (2009) speaks more to the use of the visually-mediated orientation for migration. European Robins were used to distinguish which factors are the most crucial indicators for migration orientation when relying on the earth’s magnetic compass. Manuela Zapka, et. al. determined that the use of clusterN is required to activate orientation for migration using the earth magnetic compass (2009). ClusterN is a ‘specialized, night-time active, light-processing forebrain region” in which the light sensitive reorganization of electrons occurs, a process referred to as radical-pairing (Zapka, et al. 2009). Additionally, the study tests for the importance of other cues for migration compared with the visually-mediated mechanism. They find that separation of the clusterN from neural pathways does not affect the restlessness that the birds feel for migration, and does not affect their use of celestial or sun compass orientation. From this study, we can say that there is truly not one way for birds to process migration information, but different mechanisms are controlled by different centers in the brain. The hierarchy of migration mechanisms not only orders the appropriate use of these mechanisms based on needs, but also depends on diverse neural functions to determine the success of each mechanism.
Rather than birds relying on external clues to orientation, vector navigation is a genetic instruction to fly for a certain length of time (not distance) then follow the next heading in the instructions. Tests on caged birds proved the passage of time rather than distance flown that triggers course change (Weidensaul, 1999). This the perhaps the very first guide that birds instinctively rely on in their first migration, correlating this information on the journey with the orientation to the sun, stars, landmarks and polarized light.
Looking at early Walcott studies and later concepts of vector navigation, Wiltschko and Wiltschko contribute to our understanding of bird navigation systems even further. In 2009 they published a study that details the specific use earth’s magnetic field as a navigation tool. It became clear in their study that birds do not respond to earth’s magnetism as a simple compass. Their directional response is to the inclined compass, or the directionality of the field of magnetism that differs at either pole. The field line leaves the earth at the South Pole, arcs over the equator at a nearly perfect horizontal to the earth’s surface and then curved downward at the North Pole. Birds at attuned to these up and downward inclination of the magnetic field, which is the key factor in helping them determine their initial whereabouts within the scope of the global magnetic field. It remains unclear how birds react at the equator, in the absence of this strong directional cue, but solutions may be to utilize other compass tools such as celestial orientation. (Wiltschko and Wiltschko 2009)
As mentioned earlier, the simplification of ‘birds flying north for winter’ is further dismantled. Birds using the magnetic fields are not detecting changes from the North to South poles, but rather the gradient of magnetic field from pole to equator, and now we know, they can sense the particular direction of travel from the inclination of the field as they head towards a pole. Often this system fails birds traveling close to the equator for the fields are at their least potency here. Additionally, Wiltschko and Wiltschko found that for European Robins, employ a sense of relativism between their current location in the magnetic field and the intensity of the field that they are able to process (2009). It takes but a short time for them to ‘recalibrate’ for the new intensities of the field. Perhaps this chance to recalibrate is another benefit to stopovers during long migrations. In the end, there always is the possibility of vagrant species showing up in unexpected places. With all the ingrained mechanics for migration, birds can still be led off track by crosswinds, storms, and misread environmental cues.
EVOLUTION OF MIGRATION
How and why did birds develop the behavior of migration in the first place? Genetically birds are predisposed to respond to changes in photoperiod, but not all responses result in long-distance migrations. Even within a species, not every individual will migrate with the voracity as another. Reasons supporting the behavior begin with the longevity of daylight hours. Other factors, particular for those birds of North America that travel to temperate latitudes for breeding, include low-density breeding habitat, reduced nest predation threats as a result of low-density, and the abundance of insects during the spring hatch. Birds that winter in the tropics may depend on fruits for a majority of their diet but insects provide much needed protein boosts during egg and chick development in the north. (Gill 2007)
The evolution and character of migration is still being studied but the original theory for this annual movement is driven by the availability of food. This does not explain the whole story, for instance, it does not account for the evidence of hyperphagia before migration. There is obviously food available, so did hyperphagia develop before or, concurrent, to migratory behavior? This one of the questions that will shape the path to resolving the mystery of migration patterns and behaviors.
Overall ornithologists agree that bird migration evolved from sedentary species populations that experienced environmental pressures, moved, and then shifted back to their ancestral range. This theory is based on examining partial migrant species that exhibit sedentary and migratory behaviors.
The “why” of migration is a stickier question that again is not clearly addressed in our ornithology textbook. The possible theories for migration evolution we discussed in Professor John Marzluff’s Bird Biology and Conservation course included the effect of continental drift newly isolated populations. This theory remained popular for many years but advances in phylo-genetic studies proved that continental drift had ended previous to the advent of our modern bird species (Marzluff 2009). We also discussed the influence of Pleistocene glaciers retreat in preserving northern ancestral homes which became possible feeding grounds above the northern extent of the glaciers. The southern lands became a ‘refugia’, or escape, from advancing glaciations. An opposing theory states that migratory birds originated in the southern lands and as glaciers retreated they took advantage of the food resources further and further north.
Volker Saleski and Bruno Bruderer, of the Swiss Ornithological Institute, simplify these theories by stating the question differently: Did birds develop migration patterns to increase reproductive success or non-breeding season survival? Their synthesis in Naturwissenschaften determines that birds did indeed develop migration patterns for the sake of non-breeding survival strategies (Salewski et al. 2007). Their spin on this highly debated issue is this- birds arriving to a breeding area with enhanced fitness will be expected to achieve high reproductive success. Therefore, migration evolved in birds that were searching out the best places for survival and enhanced fitness in the non-breeding season, for the benefits of higher reproductive success at the original breeding grounds. This modern synthesis of old theories begins to explain phenomena such as the temporal and spatial scales of diverse migration patterns and the success of micro-evolution of migration due to climate change (which could influence adaptive management projects in the near future). This synthesis can also advocate for better ecological understanding and stronger conservation action on non-breeding habitat along migratory routes, which will be discussed here in a case study of the Central Valley of California.