Download 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
Flight Pattern Utilized by Seabirds
Proportion of Individuals Using Strategy
0.9
0.8
0.7
0.6
0.5
Frigates
0.4
Pelicans
0.3
Raptors
0.2
0.1
0
Circling
Solo
Diving Sinusoidal
Ess Line
Line
Group,
circular
Group,
scattered
Solo
Straight
V
Line, formation
group
Figure 1. The flight strategies used by seabirds differs substantially between species (X 2 <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).
Flight Patterns Frequency: Frigates
1%
5%
12%
5%
Ess
Line
Sinusoidal
Line
Group, circular
Group, scattered
Solo
Straight Line, group
77%
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.
Flight Pattern Frequency: Pelicans
2%
11%
1%
Circling Solo
Diving
42%
16%
Group, circular
Solo
Straight Line, group
V formation
28%
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 nonmigratory 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.
Flight Pattern Frequency: Raptors
39%
46%
Circling Solo
Group, circular
Solo
15%
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.
Average Size of Flight Aggregations
V formation
Straight Line, group
Group, scattered
Group, circular
Sinusoidal
Ess Line
Line
Diving
0
2
4
6
8
10
12
Number of Individuals
14
16
18
20
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
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
Sinusoidal Line: single-file line of birds
traveling in the same direction but varying
in individual altitudes
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.