The Five-Thirty Commute: Flight Preferences of Seabirds

The Five-Thirty Commute: Flight Preferences of Seabirds

By Ashley Jones

Introduction

Seabirds are extremely specialized animals, with the ability to occupy a niche that consists of air, land, and sea. Flight is one of the most interesting and complex of these adaptations, and is dependent on several biotic (skeletal structure, energy levels) and abiotic factors (wind patterns, time of day) (Simons 2010). Flight methodology varies from solo flight to scattered groupings to strictly maintained linear formations, differing not only between species but between groups of individuals as well. Several theories have been proposed to explain how birds choose which flight strategy to utilize, including the historic argument that certain formations provide an aerodynamic advantage by reducing drag (Heppner & Bajec 2009). However, more recent studies suggest that these flight strategies might not be as aerodynamically favorable as once suggested, and instead, the organization and history of the community dictates when, where, and how a group of birds flies together (Hainsworth 1988, Seiler et al. 2002).

Seabirds that “flock” demonstrate synchrony in at least one aspect of flight, such as takeoff, landing, or rapid change in direction (Heppner & Bajec 2009). Formations can be classified in one of two categories, linear or clustered formation. Linear formations may be straight, staggered, or V-shaped, and are usually observed in relatively large birds such as geese, cormorants and ducks (Heppner & Bajec 2009). Meanwhile, cluster formations are typically made up of large numbers of smaller birds, flying in irregular arrangements, perhaps to avoid aerial predators, or find a collective roosting site (Major and Dill 1978). Yet, small social groups of seabirds within a population have been observed to initiate either type of flocking without any effect on their neighbors, suggesting a complex interplay of attractive and behavioral forces between individuals in addition to behaviors characteristic of each species (Heppner & Bajec 2009).

Should flocking strategies be dependent on these factors, we might expect flight patterns to differ between species, as opposed to a single technique used by all seabirds for a particular function. For example, frigate birds (Fregatidae) and pelicans (Pelecanidae), both members of the Pelecaniform group, each possess unique foraging techniques. While frigate birds scavenge, pelicans forage in shallow water by swimming or diving, and this likely influences which flight pattern each species uses (Simons 2010). Moreover, flight strategies between intraspecific groups may be affected by complex social and organizational behaviors. Therefore, if individuals consciously choose their flight pattern, strategies should differ between and within seabird populations.

Methods and Materials

I observed coastal seabirds of Ecuador for two consecutive days during their daily migration, which tended to begin near 5:30 pm and lasted for 20 to 30 minutes. My sample consisted of frigate birds, pelicans, and vultures, which were the only species to participate in this migration. I recorded the number of birds as they passed overhead, noting the species, direction, altitude, and size of each group. I classified flight patterns in the following categories: ‘solo flight’ (further classified as circling, diving, or flying linearly), or ‘group flight’ (further categorized as a sinusoidal line, v-formation, scattered group, or circular group). I defined groups as two or more individuals of the same species flying at the same speed and direction in a recognizable formation. A description of each group flight pattern can be found in Appendix A.

I then calculated the proportion of birds within each species that utilized each flight strategy and compared the results using a Pearson’s chi-square goodness-of-fit test. I also compared the average number of birds per flight pattern to the average group size of the same flight pattern in the other two species using an ANOVA test. Finally, I performed three separate paired student’s t-tests, one for each species; comparing the proportion of observed individuals against the expected proportion of individuals per flight pattern should the species show no preference. Where applicable, I conducted further calculations based on origin, direction, and altitude of travel within the populations.

Results and Discussion

Seabirds appear to exhibit preferential flight strategies, each species utilizing unique formations for solo and group flight (X2 <0.001, 14df, Fig. 1). Group flight is confined to frigates and pelicans, which fly in groups 94.17% and 73.8% of the time, respectively. Species differ significantly in the size of their flocks as well, with frigates preferring large aggregations (mean flock size = 15.6 individuals), pelicans favoring smaller, more organized groups (x = 8.1) and raptors remaining solitary (x= 1.2). Yet there is also significant variability in flight pattern chosen between individuals of the same species; for example, although frigates are capable of five flight strategies, most individuals chose to fly in scattered aggregations (t-test, 7df, p<0.001, Fig. 2). Likewise, pelicans favored v-formations for migration and solo-diving for feeding purposes (t-test, 7df, p<0.001, Fig.3).

Because flight patterns appear to be somewhat species-specific, the average number of birds in a flock should mirror the same variability; for example, scattered groups and sinusoidal lines are performed almost entirely by frigates and v-formation only by pelicans. However, there is no significant difference in average number of birds based on pattern alone, indicating that other factors limit the size of a group (ANOVA, 14df, p=0.22, Fig. 5). Such results are consistent with Seiler’s theory that flock size is limited in order to maintain positioning within the group (Seiler et al 2002). The combined data of species and individual preferences reveal that flight patterns differ not only between species, but between distinct groups within an intraspecific population.

Conclusions

Variability in flight strategies suggest that flight patterns differ not only in form, but also in function, and are thus selected by individuals for specific reasons. Group flight may be utilized when a large number of individuals share a common goal: feeding, nesting, breeding, or migration. Individual birds, perhaps the “leaders” within their social group, likely utilize social and/or behavioral cues in order to express their intentions to other individuals, and this communication creates organized flocking (Couzin et al. 2002). Once organized, birds in a flock demonstrate aerial group dynamics to maintain shape, direction, and altitude of the group, as well as to avoid collisions and maintain distance between neighbors (Couzin et al. 2002). It has been further postulated that these traditional behaviors form a ‘collective memory’ which is passed on to future generations, thus becoming a defining feature of the species or the population as a whole (Heppner & Bajec 2009). To understand the potential benefits of flocking, future research should focus on confirming the presence of these social cues: before, during, and after flight.

Figures

Figure 1. The flight strategies used by seabirds differs substantially between species (X2 <0.001, 14df) as observed by the different proportions of frigates, pelicans, and raptors that utilize a given pattern. Sinusoidal lines and scattered groupings are restricted solely to frigate birds, while v-formations, straight lines, and dives were observed only in pelicans. Species also differ in proportion of individual birds that choose to utilize each pattern, indicating behavioral differences not only between but among species (frigates: p<0.001, pelicans: p<0.001, raptors: p<0.003).

Figure 2. Frigates differ significantly in their preferences for flight strategies, with a strong bias for large scattered aggregations, especially for migratory flight (p<0.001). Frigate birds do not exhibit more structured V-formations nor do they dive for feeding purposes, indicating a significant difference between species of Pelecaniformes (p<0.001). Note solo circling, V-formations and dives are not included in the chart, as they were not observed.

Figure 3. Pelicans show a significant preference for group flight opposed to solo flight, especially for migratory purposes, choosing to fly in group arrangements 73.8% of the time. Pelicans opt for solo flight for feeding or non-migratory reasons, such as diving for fish or making reverse migrations. We may reject the null hypothesis that pelicans prefer every flight pattern equally by comparing expected and observed frequency of use (p<0.001). Note sinusoidal lines and scattered groups are not included in the chart, as they were not observed in the pelicans.

Figure 4. Raptors show very specialized flight patterns, with group flight almost absent and only three out of eight potential flight strategies utilized. Raptors engage in solo flight regardless of altitude or direction, indicating significant preference for circling or flying alone (p<-0.003). Note sinusoidal lines, scattered groups, linear groups, V-formations, and dives were observed 0% of the time and are not represented.

Figure 5 Differences in the average size of flight aggregations does not differ significantly between flight pattern used, with average size of any given strategy between 1 and 16 individuals (ANOVA, 14df, p = 0.22). Variability of flock size may be more accurately explained by the species of birds using the pattern, frigates averaging large groups (15.6 individuals), pelican groups smaller (8.1 individuals) and raptors solitary (1.2 individuals) (p<0.001)

Appendix A

Circular: multiple birds traveling in a clockwise or counterclockwise direction, orbiting a central point

Linear: single-file line of birds traveling in the same direction and altitude

Sinusoidal Line: single-file line of birds traveling in the same direction but varying in individual altitudes

Scattered: a large aggregation of birds traveling in the same direction within close distance, frequently passing each other

V-formation: a well-maintained formation of a leading bird and two parallel rows of followers in synchrony

Literature Cited

Couzin, I. D., Krause, J., James, R., Ruxton, G. D. & Franks, N. R. 2002. Collective memory and spatial sorting in animal groups. Journal of Theoretical Biology, 218, 1–11.

Hainsworth, F. R. 1987. Precision and dynamics of positioning by Canada geese flying in formation. Journal of Experimental Biology, 128, 445–462.

Heppner, F. & Bajec, I. 14 August 2009. Organized flight in birds. Animal Behaviour, 78, 777–789.

Major, P. F. & Dill, L. M. 1978. The three-dimensional structure of airborne bird flocks. Behavioral Ecology and Sociobiology, 4, 111–122.

Seiler, P., Pant, A. & Hedrick, J. K. 2003. A systems interpretation for observations of bird V-formations. Journal of Theoretical Biology, 221, 279–287.

Simons, E. 2010. Forelimb skeletal morphology and flight mode evolution in pelecaniform birds. Zoology, 113, 39–46.