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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 1. March 2012 
and Amirrudin 1981. Waugh and Hails 1983, 
Tarburton 198(>b, Garcia-dcl-Rey et al. 2010), in 
different parts of their geographic range (Collins 
et al. 2009) and in samples taken from the same 
population in different years (Collins 2010). Thus, 
variation in type of prey captured is difficult to 
analyze, particularly when only limited food 
samples are available. The size of prey items 
tends to be more uniform despite appreciable 
differences in prey types taken (Collins 2010). 
The mean size and range in size of prey items 
taken by the Neotropical Palm Swift and Lesser 
Swallow-tailed Swift in this study were highly 
similar. There is an abundance of smaller prey 
items for bolh species and a lesser number of the 
larger items (Fig. 1) as true for other swifts 
(Collins et al. 2009, 2010; Garcia-del-Rey et al. 
2010). Body weight for a variety of insectivorous 
birds, is "a better predictor of the size of prey 
taken" than morphological characteristics such as 
bill size or shape (Hespenheide 1971:63). Recent 
studies of other swifts have shown there is a 
significant linear relationship between swift body 
weight and mean prey size (Collins et al, 2009). 
Thus, the larger Lesser .Swallow-tailed Swift 
(mean body weight = 20.9 g, range = 15.8- 
23.0 g, n = 5; C. T. Collins, unpubl. data) might 
have been expected to take, on average, larger 
prey items than the smaller Neotropical Palm 
Swift (mean body weight = 10.4 g, range = 9.0- 
12.4 g. n = 47; C. T. Collins, unpubl. data). This 
is not indicated by the data presented. However, 
prey taken by these two swifts may also have been 
influenced by their foraging habitat. Neotropical 
Palm Swifts, as their name implies, forage almost 
exclusively in lowland wet palm savannas where 
they “circle at low to moderate heights’* (Hilly 
2003:390) and at times low over grassy areas 
(C. T. Collins, pers. obs.) where agility would be 
important. Other species of swifts are generally 
uncommon in this foraging area. Lesser Swallow¬ 
tailed Swifts are frequently observed flying and 
foraging high to very high above the ground 
(Stiles and Skutch 1989, Hilty 2003). They are at 
times found loosely associated with feeding flocks 
of other species of swifts (Chaeiura spp.), at 
which time Lesser Swallow-tailed Swifts appear 
to fly and forage well above the others (Stiles and 
Skutch 1989; Hilty 2003; C. T. Collins, pers. 
o .v). hese swifts, when actively foraging, tend 
to fly very jerky erratic manner with many 
KqT Shm ‘ S of (Hilty 
2003.389). The tail which is usually closed and 
spike-like is widely fanned during abrupt changes 
in direction (Stiles and Skutch 1989). Foraging at 
higher altitudes may reduce the encounter rate 
with larger and slower flying prey such as winged 
ants and termites, and require more active pursuit 
of smaller and faster flying prey, such as Diptera. 
Glick (1939) reported Diptera were nearly three 
times more abundant than insects belonging to 
any other Order at elevations from 30 to 60 m 
while there is a greater proportion of larger and. 
presumably, slower flying insects nearer the 
ground (Johnson 1969). The low wing loading 
and high aspect ratio typical of most swifts and 
swallows favors the gliding flight often seen in 
these birds as well as their low flight metabolism 
(Hails 1979). However, no attention has yet been 
given to measuring the energetic cost/benefit ratio 
of the different foraging behaviors and their 
relationship to aerial prey availability. 
The maneuverability or agility of birds is 
difficult to quantify for purposes of interspecific 
comparisons. Two maneuverability indices have 
been proposed: a wing index (the ratio of the wing 
length to body weight; Tarburton 1986a) and a tail 
index (the ratio of the length of the outer and 
longest rectrix to body weight: Hails and Atnimi- 
din 1981). The tail index of 6.71 for the 
Neotropical Palm Swift is among the highest ot 
44 species reviewed by Tarburton (1986a). It is 
exceeded only by the 7.79 for the African Palm 
Swift ( Cypsiurus pan'us) and the 6.97 for the 
Black Saw-wing (Psidoprocne pristoplera). Both 
of these species are light bodied and have deeply 
forked tails. The tail index for the Lesser 
Swallow-tailed Swift is 2.67 (C. T. Collins, 
unpubl. data). The Black Saw-wing forages low 
across forest and woodland clearings, along forest 
tracks, and over tallgrass savanna at an average 
height of 7 m hut is often observed skimming the 
ground (Turner and Rose 1989, Keith et al. 1992. 
Sinclair et al. 1997). Its deeply forked tail, which 
presumably increases its agility, would be of great 
advantage, and selected for, in habitat' requiring 
careful maneuvering. 
It is not clear as to whether the forked tail of the 
Neotropical Palm Swift is selected for lower level 
flight in its more restricted palm savanna foraging 
habitat or more for pursuit of agile prey, 
particularly Diptera. The forked tail of the Lesser 
Swallow-tailed Swift seems more likely to be an 
adaptation for capturing agile prey in the an 
column at higher altitudes above ground level. 
The forked tails of these two swifts seem to be 
