Food habits of two fork-tailed swifts in Venezuela.
Abstract: The aerial arthropod prey of Neotropical Palm Swifts (Tachornis squamata) and Lesser Swallow-tailed Swifts (Panyptila cayennensis) in Venezuela included seven Orders and 60 families of insects plus spiders and mites. Diptera were the most numerous prey (>50%) taken by both swifts. Prey size ranged from 0.5 to 7.9 mm and averaged 2.43 and 2.77 mm, respectively. Both prey type and foraging habitat differences of these swifts could be interpreted as mechanisms for resource partitioning.
Article Type: Report
Subject: Foraging (Research)
Authors: Collins, Charles T.
Thomas, Betsy Trent
Pub Date: 03/01/2012
Publication: Name: The Wilson Journal of Ornithology Publisher: Wilson Ornithological Society Audience: Academic Format: Magazine/Journal Subject: Biological sciences Copyright: COPYRIGHT 2012 Wilson Ornithological Society ISSN: 1559-4491
Issue: Date: March, 2012 Source Volume: 124 Source Issue: 1
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: Venezuela Geographic Code: 3VENE Venezuela
Accession Number: 285207255
Full Text: The tails of birds are diverse in both morphology and function. The size and shape of most tails reflect their functional aerodynamic properties, but many others are highly modified for use in sexual displays (Thomas 1997). Deeply forked tails are found in many species including some swifts (Apodidae), swallows (Hirundinidae), and terns (Sterninae) and may reflect selection for both aerodynamic and display functions. Forked tails are thought to provide additional lift, when fanned, and increased turning agility (Thomas 1997). Deeply forked tails in aerial foraging insectivores may be adaptations for pursuit and capture of more agile prey or foraging low to the ground and close to vegetation (Waugh and Hails 1983). Fanning of the wing and tail feathers is a frequently observed behavior associated with rapid flight changes during prey captures by swiftlets (Manchi and Sankaran 2010). Differences in prey selection resulting from variable flight behaviors may be an important isolating mechanism for closely related species, particularly when in mixed species foraging flocks (Waugh and Hails 1983).

It is usually difficult to get detailed information on the prey taken by aerial foraging insectivores (Jahn et al. 2010). Direct observation can only detect or identify larger prey items and remains of smaller and more delicate prey may be undetected in gut contents or fecal samples. The boluses of arthropod material collected by swifts allow detailed analysis of both the prey taxon and prey size of even small insects and spiders. However, this information is only available during the chick-rearing period of the breeding season and may not be representative of swift diets at all times of the year. Prey type and availability may change substantially during the drier non-breeding season in tropical regions (Hails and Amirrudin 1981, Jahn et al. 2010).

The goals of our study were to: (1) describe the type and size of the prey taken by two neotropical swifts having deeply forked tails, the Neotropical Palm Swift (Tachornis squamata) and the Lesser Swallow-tailed Swift (Panyptila cayennensis); and (2) look for qualitative differences in prey taken by other swifts lacking deeply forked tails.

The Neotropical Palm Swift is a widespread and common resident of lowland sandy soil or wet savannas throughout its range extending from eastern Colombia and Venezuela south to eastern Peru, and Amazonian Brazil (Hilty and Brown 1986, Hilty 2003). It is usually observed circling at "low to moderate heights" over open areas where there are palms (Hilty 2003:389). It less commonly occurs in urban areas where palms have been planted (Chantler 2000, this study).

The Lesser Swallow-tailed Swift occurs from southern Mexico south to southeastern Peru, northern Bolivia, and Amazonian and southeastern Brazil (Hilty and Brown 1986, Sick 1993, Howell and Webb 1995, Hilty 2003). It is "uncommon and local in occurrence ... over humid lowland and foothill forest or partially forested terain" (Hilty 2003:389) at elevations of 1,000 to 1,400 m (Hilty and Brown 1986, Stiles and Skutch 1989, Hilty 2003). Its characteristic tubular nests occur at elevations from <100 to >800 m in Trinidad and Venezuela (pers. obs.), and in urban areas in Surinam (Haverschmidt 1958).

METHODS

Food samples of the two species of swifts were collected as boluses of food being brought to nestlings at two locations near Maracay, Estado Aragua, Venezuela. Seven partial or complete boluses from Neotropical Palm Swifts were obtained from adults captured in a mist net on 27 July 1976 as they were entering a nesting colony in dense dead palm fronds near the base of a tree on the grounds of the Hotel Maracay (10[degrees] 17' N, 67[degrees] 35' W). Boluses were collected from the mouth of the netted adult or from a cloth on the ground below the net if the bolus was ejected by the adult when captured. The exact number of nests could not be ascertained due to the density of the fronds but there were 28 captures of adults at this site. It is unlikely that more than one bolus was collected from any single individual. A single complete bolus and one partial bolus were collected on 16 July 1976 from the mouths of Lesser Swallow-tailed Swift nestlings in a nest on a roadside overhanging rock cleft in the lower portion of Henri Pittier National Park, just north of El Limon (10[degrees] 19' N, 67[degrees] 39' W). These two locations are [+ or -]9 km apart. No measurements of insect abundance in the study area were made as part of this study. All of the food boluses were stored in alcohol and later examined under a dissecting microscope. Identifications were made to the family level; the two psocopterans were identified as Caecilius antillanus (Turner 1984). Body size of prey items was measured with an ocular micrometer to 0.1 mm from the tip of the head to the tip of the abdomen excluding antennae or any caudal appendages.

RESULTS

The 381 prey items obtained from Neotropical Palm Swifts were distributed among seven Orders and 62 families of insects in addition to spiders and mites (Table 1). Diptera (52.2%), Homoptera (18.1%), and Hymenoptera (10.5%) were the most numerous insects recorded. The 108 prey items obtained from Lesser Swallow-tailed Swifts were similarly diverse with spiders and five Orders of insects distributed among 22 families. Diptera (62.2%), Homoptera (17.6%), and Hymenoptera (13.9%) were the most numerous insects recorded (Table 1).

The size of the prey items taken by Neotropical Palm Swifts ranged from 0.5 to 7.8 mm with a mean [+ or -] SD of 2.43 [+ or -] 0.9 mm. The size of the prey items taken by Lesser Swallow-tailed Swifts ranged from 1.1 to 7.9 mm with a mean [+ or -] SD of 2.77 [+ or -] 1.49 mm. There was an abundance of smaller prey (<4 mm) and few larger prey items in the boluses from both species (Fig. 1).

DISCUSSION

Both Neotropical Palm Swifts and Lesser Swallow-tailed Swifts took a preponderance (>50%) of Diptera in the samples collected. There are no detailed data on flight characteristics of most insects, but it has been suggested (Hespenheide 1975) that flies are more agile fliers than representatives of other Orders with the possible exception of some hymenopterans. There is some support for the suggestion that deeply forked tails in these two swifts are adaptations for pursuit of agile prey. Further support can be found by examining the prey types taken by three other Venezuelan swifts which have square or only slightly forked tails. Diptera comprised only 27-35% of their diets while Hymenoptera, particularly slower flying winged ants, made up 28-51% of their diets (C. T. Collins, unpubl, data).

The food of the Lesser Swallow-tailed Swift in Costa Rica included flying ants, termites, small beetles, bugs, flies, and wasps (Stiles and Skutch 1989); in Brazil "small ants and termites comprised about 90% of the diet" of both species (Sick 1993:313). Winged ants and termites were rare or absent from the prey identified in our study (Table 1) but expectedly would occur in their diets at times when these swarming insects are temporarily abundant.

Diptera made up 43% of the prey items of the White-rumped Swiftlet (Aerodramus spodiopygius) in Fiji (Tarburton 1986b), 24% of the prey of this species in Queensland, Australia (Tarburton 1993), and 25.8% of the prey of the Glossy Swiftlet (Collocalia esculenta) in Malaysia (Hails and Amirrudin 1981). Diptera were more commonly taken in open areas than in forested areas in Fiji, and taken more frequently than their representation in aerial insect samplings (Tarburton 1986b). Diptera averaged 22.1% (range = 5.0-57.2%) of the prey of seven larger swifts in Europe and Africa (Cucco et al. 1993; Collins et al. 2009, 2010; Garcia-del-Rey et al. 2010). Only the Common Swift (Apus apus) had an occurrence above 50% of the prey items and then in only one of three separate prey collections (Lack and Owen 1955; C. T. Collins, unpubl, data).

Previous studies have also documented substantial differences in prey taken by individual species on different days or under differing weather conditions (Lack and Owen 1955, Hails and Amirrudin 1981, Waugh and Hails 1983, Tarburton 1986b, Garcia-del-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 both 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 = 15.823.0 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 = 9.012.4 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" (Hilty 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 (Chaetura 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. obs.). These swifts, when actively foraging, tend to fly "in very jerky erratic manner with many zigzags and sudden shifts of direction" (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.

[FIGURE 1 OMITTED]

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 Amirrudin 1981). The tail index of 6.71 for the Neotropical Palm Swift is among the highest of 44 species reviewed by Tarburton (1986a). It is exceeded only by the 7.79 for the African Palm Swift (Cypsiurus parvus) and the 6.97 for the Black Saw-wing (Psidoprocne pristoptera). 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 but 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 habitats 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 air column at higher altitudes above ground level. The forked tails of these two swifts seem to be related to their exploitation of habitats and non-uniformly distributed types of aerial insect prey not as intensely used by other species. This may be viewed as forms of resource partitioning allowing coexistence of otherwise similar species of aerial insectivores (Waugh and Hails 1983, Collins 2000). Additional attention needs to be given to the food and foraging of these and other swifts to confirm the interpretations presented here.

ACKNOWLEDGMENTS

I am grateful to Gonzalo Medina P. for permission to conduct studies of swifts in Henri Pittier National Park and to Robert Smice and Robert Waggenstein for identification of the prey items. Michael K. Tarburton, two reviewers, and the editor made helpful suggestions which improved an early draft of this manuscript. The field portions of this study could not have been conducted without the participation and cheerful companionship of the late Betsy Trent Thomas.

Received 6 March 2011. Accepted 29 September 2011.

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Charles T. Collins (1,2) and Betsy Trent Thomas (3)

(1) Department of Biological Sciences, California State University, Long Beach, CA 90840, USA; e-mail: ccollins@csulb.edu

(2) Current address: 6001 Fairbrook Street, Long Beach, CA 90815, USA.

(3) Deceased.
TABLE 1. Prey Items taken by Neotropical Palm Swifts and Lesser
Swallow-tailed Swifts in Venezuela.

                      Neotropical   Lesser Swallow-tailed
                      Palm Swift            Swift

Prey taxon               %     n           %     n

Araneae                8.6               1.9
  Thomiscidae                  8                 2
  Clubionidae                  5
  Lingphiidae                  4
  Zoridae                      1
  Salticidae                   7
  Ctenidae                     3
  Sparassidae                  1
  Lycosidae                    1
  Theridiidae                  3

Coleoptera             7.6               3.7
  Curculionidae               10                 2
  Dryopidae                                      2
  Histeridae                   1
  Nitidulidae                  1
  Scaphidiidae                 1
  Anobiidae                    1
  Bostrichidae
  Chrysomelidae               13
  Lyctidae                     1

Homoptera              1.1                 0
  Lygaeidae                    1
  Piesmatidae                  3

Homoptera             18.1              17.6
  Aphididae                    3                 2
  Delphacidae                 33                13
  Cicadellidae                20                 4
  Psyllidae                    8
  Membracidae                  2
  Cixiide                      3

Hymenoptera           10.5              13.9
  Formicidae                   7                 2
  Braconidae                   1                10
  Chalcididae                  1                 1
  Eulophidae                   2                 1
  Pteromalidae                 3                 1
  Ichneumonidae                1
  Chrysididae                  2
  Encyrtidae                   5
  Torymidae                    1
  Cephidae                     1
  Sphecidae                    1
  Agaonidae                    7
  Cynipidae                    8

Psocoptera             0.2               0.9
  Pseudocaeciliidae            1                 1

Diptera               52.2              62.1
  Syrphidae                    4                 3
  Tephritidae                  2                 4
  Calliphoridae                                  3
  Muscidae                     2                 8
  Sciaridae                    8
  Dolichopodidae              21                20
  Ephydridae                  26                 2
  Lauxaniidae                  2                 1
  Chloropidae                 68                21
  Chamaemyiidae                                  1
  Agromyzidae                 16                 3
  Drosophilidae                6                 1
  Stratiomyidae                4
  Empididae                    1
  Phoridae                     5
  Chironomidae                 2
  Milichiidae                  7
  Mycetophilidae               2
  Cecidomyiidae                1
  Pipunculidae                 1
  Otitidae                     1
  Sepsidae                     4
  Anthomyzidae                13
  Trixoscelididae              2

Lepidoptera            0.5                 0
  Coleophoridae                1
  Pyralidae                    1

Acarina                1.1     4           0
  Totals                     381               108
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