Written by Benjamin Van Doren

In North America, the vast majority of avian migrants engage in migration twice per year: northbound during the spring months (largely April and May) and southbound during the fall (late August through early November), and many of these species perform this migration during the night. These birds are not classified as nocturnal; the rest of their activities (e.g., foraging, breeding, defending territory and raising young) are performed during the day, yet nocturnal migration has developed as an important and beneficial behavior because its evolutionary benefits outweigh its costs. On occasion, however, nocturnal migrants also engage in sizable migratory flights during the daytime [1, 2, 3]. These "morning flights", also known as redetermined migration, are notable because, although they occur over a range of areas, specific criteria must be met in order for this behavior to take place: morning flight is only visible during major migration periods, and typically on days following large nocturnal migrations. The ubiquity and magnitude of this phenomenon under certain conditions suggests that it is an important part of migration [1]. A migrant in morning flight displays behavior analogous to typical migration, flying above tree height in one general direction [2]. In addition, these migrants often utter vocalizations known to be used primarily in extended nocturnal flights [2]. Interestingly, the headings of these morning flights often deviate from the expected migratory direction (e.g., northward flights in the fall), suggesting that the phenomenon is comprised of more than birds simply continuing their migration [1, 3].

Review of Literature

There are many benefits to migrating at night, such as: cooler temperatures, a more stable atmosphere and a decreased risk of predation [1]. Why these nocturnal migrants forgo the benefits of nighttime flying is not fully understood, although there has been much speculation as to what drives the behavior. One of the leading explanations is the "wind drift" hypothesis. Small passerines (songbirds) are easily blown off course by strong winds [4], and although they possibly attempt to correct headings to a certain extent while in nocturnal flight, morning flight may function as an opportunity for these birds to reorient and compensate for the drift incurred during the previous night's migration [1, 3]. For example, a migrant that had been pushed southeast of its destination by strong northwest winds could regain its former heading by flying northwest. Indeed, cage-orientation tests have found that birds attempt to compensate for wind drift after migrating [4]. Similarly, morning flight may be composed, at least partly, by migrants caught out over open ocean at dawn that have since reoriented toward the mainland [1, 3, 5]. For a migrant in the northeastern United States that finds itself several miles offshore, the most direct route back to land would be to fly northwest [3]. This dawn orientation has been observed off the Atlantic coast through the use of radar [5].

Morning flight may also be influenced by local topography [1, 2]. Migrating birds tend to congregate along a topographical edge, often a coast or river. Unusually-shaped land masses, such as peninsulas, may funnel birds unwilling to fly over water (usually smaller, weaker-flying birds) in a certain direction [1]. The habitat on the ground may also impact morning flight; the "habitat hypothesis" theorizes that morning flights may be comprised of birds attempting to find suitable stopover habitat to rest and refuel after migration [1]. The simple disorientation of birds may also play a part in morning flight, but this alone does not explain many of its observed characteristics. 

Finally, in certain instances birds may take part in morning flight merely as a continuation of nighttime migration. It has been shown that birds do this when a geographic barrier forces them to continue on (e.g. migration over the Gulf of Mexico). The majority of studies have examined morning flight on or near the coast, but an examination of inland morning flight in New York found that migrants maintained their seasonally appropriate heading [2]. 

Another important aspect of this behavior is its inherent relation to synoptic weather patterns. In autumn in the northeast, nocturnal migrants move in the largest congregations immediately following the passage of a cold front. This pattern appears to hold true for morning flight as well [1], but whether birds are actually reacting to a change in air pressure or simply taking advantage of the favorable wind conditions associated with cold fronts has yet to be determined.

Doppler radar has emerged as an extremely useful tool in tracking bird migration over large spatial scales [6]. In the United States, the National Weather Service allows open access to the products of its WSR-88D radar network. By examining the base reflectivity and radial velocity products from these radars, it is possible to determine the magnitude of bird migration taking place [6]. However, these radars are long-range, which means that, while they are excellent for judging the mean density and direction of all flying migrants, it is impossible to differentiate individual tracks.

Many species of nocturnal migrants utter short (<1/5 sec) flight call notes with regularity during extended migratory flights [7]. Using sound analysis software, it is possible to identify these call notes to family and sometimes to species, and therefore gain insight into what species of birds are migrating overhead [7]. This method does have its pitfalls; since call rates are variable, it is very difficult to determine the exact number of individuals migrating overhead. Additionally, some species that migrate at night do not give flight calls. However, this approach is excellent at documenting the presence, and to a lesser extent, quantity, of passing migrants on a local scale. In addition, these vocalizations are given more frequently by birds when conditions for orientation are subpar but winds are still favorable. For example, a low, thick cloud ceiling, thought to impede birds’ celestial navigation capabilities, often induces migrants to call more frequently [7]. A similar effect has also been noted with artificial lighting during the night (e.g., buildings and communication towers) [8]. It is evident that there may be a link between navigational obstacles and disorientation, and flight call frequency.               

Morning flight is largely understudied for such an integral part of bird migration, and the experiments that have been conducted took place in vastly different areas, both topographically and geographically. The protocols developed, data collection methods used, and environmental variables correlated were also dissimilar, and these disparities, compounded by the wide range of different, sometimes contradictory, proposed hypotheses mentioned above, prevent a unified picture of morning flight from being created.

Hypotheses

·      The magnitude of morning flight will increase after nights in which wind speed and direction force birds off track.

·      When migration during the night deviates from a normal southwest track, morning flight the following morning will be oriented to correct for this difference.

·      Morning flight will be more prevalent on the days immediately following the passage of a cold front.           

·      The amount of flight calls uttered during nocturnal migration will be higher on nights preceding morning flight as birds deviate from the course they are trying to follow.

Methods and Materials

 This study will attempt to bridge the gaps created by the discrepancies in current ornithological knowledge pertaining to morning flight by using a suite of modern technologies partnered with volunteer observation. There are certain groups of birds that are more likely to engage in morning flight migration than others (e.g., Parulidae warblers), but this study will collect data from all species observed in continuous flight at observation locations in order to gather the most complete data set possible.

An accurate representation of bird migration on specific nights will be created using Doppler radar to discern overall bird density and acoustic monitoring set-ups to identify species migrating overhead. A team of volunteers will observe morning flight on certain mornings by recording all bird species observed in sustained flight and their headings. Data from winds aloft and synoptic weather systems will be collected to determine if they have an effect on nocturnal migration or morning flight. All of these variables will be correlated to evaluate the validity of the aforementioned proposed explanations and formulate others.

. . . . (material omitted)

References

[1] Wiedner, David S., Paul Kerlinger, David A. Sibley, Paul Holt, Julian Hough, and Richard Crossley. "Visible Morning     Flight of Neotropical Landbird Migrants at Cape May, New Jersey." The Auk. 109.3 (1992): 500-10. Print.

 [2] Bingman, Verner P. "Inland Morning Flight Behavior of Nocturnal Passerine Migrants in Eastern New York." The Auk. 97.3 (1980): 465-72. Print.

 [3] Able, Kenneth P. "The Orientation of Passerine Nocturnal Migrants Following Offshore Drift." The Auk. 94.2 (1977): 320-30. Print.

 [4] Moore, Frank R. "Evidence for Redetermination of Migratory Direction Following Wind Displacement." The Auk. 107.2 (1990): 425-28. Print.

 [5] Richardson, W. John. "Reorientation of Nocturnal Landbird Migrants over the Atlantic Ocean near Nova Scotia in Autumn." The Auk. 95.4 (1978): 717-32. Print.

[6] Gauthreaux Jr, Sidney A., John W. Livingston, and Carroll G. Belser. "Detection and discrimination of fauna in the aerosphere using Doppler weather surveillance radar." Integrative and Comparative Biology. 48.1 (2008): 12-23. Print.

 [7] Farnsworth, Andrew. "Flight Calls and Their Value for Future Ornithological Studies and Conservation Research." The Auk. 122.3 (2005): 733-746. Print.

 [8] Evans, William R., Yukio Akashi, Naomi S. Altman, and Albert M. Manville, II. "Response of night-migrating songbirds in cloud to colored and flashing light." North American Birds. 60.4 (2007): 476-88. Print.