Swainson's Warbler

Limnothlypis swainsonii

Order:
Passeriformes
Family:
Parulidae
Sections

Diet and Foraging

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Swainson's Warbler foraging in leaf litter.
© Peter Hawrylyshyn , Florida , United States , 12 April 2018

Feeding

Main Foods Taken

Primarily ground-dwelling arthropods, including ants, beetles, centipedes, hemipterans, lepidopterans, and spiders. Not known to consume fruit or nectar.

Microhabitat for Foraging

Typically forages on the ground in leaf litter beneath vine tents and in small (1–40 m2), well-shaded glades with a low density of cane or shrub stems (7). During periods of high water, frequently forages at drifts of flotsam deposited along exposed floodplain forest ridges (7); one individual was observed foraging on suspended flotsam during a flood (16). In southern Illinois, although occupied habitat had an average of 34% herbaceous ground cover, foraging sites had almost no herbaceous ground cover (164). Barrow (176) found that birds in northeastern Louisiana foraged at sites with greater leaf litter cover and palmetto density than sites available at random. Anich (153) reported that foraging sites had more vine and subcanopy cover, more horizontal variation in understory density, and fewer cane stems than song perch sites.

Foraging height in Louisiana averaged 0.4 m, lower than for any other avian species surveyed (177). Birds do rarely feed at shrub level or even midstory (178, 176, 142), and during a flood several birds were seen foraging along large branches in the canopy (16), but under normal conditions, arboreal foraging is highly unusual. In winter, 96% of insects were taken on the ground or in leaf litter and 4% were taken from broad-leafed plants in the understory (101). Barrow (176) found the following substrates used for foraging substrates: 63% ground litter, 12% fallen debris, 15% live leaves, 7% dead leaves, and 3% herbaceous plants.

Food Capture and Consumption

Swainson's Warbler is a ground forager that specializes on leaf-litter invertebrates, obtained primarily by lifting and flipping fallen leaves, on both the breeding and overwintering grounds (7, 12). Forages by probing bill under dead leaves, quickly pushing leaves up, and examining the ground or leaves underneath for prey. Sometimes flips leaves by thrusting their bill upward and back, or sideways; sometimes open curled leaves by inserting and opening their bill (1), and also opportunistically glean insects from the exposed surfaces of leaf litter (7). Rarely hawk insects from low perches in trees and forage on top of fallen logs, but occasionally glean insects from lower parts of tree trunks and in low undergrowth foliage (178, 176, 7). Barrow (176) recorded a mean of 5.6 (range 3.5–9.6) prey attacks/min for foraging birds, using 63% flaking, 32% gleaning, and 5% sally-gleaning as foraging methods (foraging terms defined by 179). Given the nature of these maneuvers it appears that leaf litter is a critical component of Swainson's Warbler habitat (7; see Distribution, Migration, and Habitat: Habitat).

Diet

Stomach content analyses in various parts of the range have revealed consumption of butterfly and moth larvae (Lepidoptera), beetles (Coleoptera: Carabidae, Cryptophagidae, Staphylinidae, Tenebrionidae, and Nitidulidae), ants and wasps (Hymenoptera: Formicidae, Ichneumonidae, Platygasteridae), spiders (Araneae), crickets (Orthoptera: Gryllidae), grasshoppers (Orthoptera: Acrididae), katydids (Orthoptera: Tettigoniidae), stinkbugs (Hemiptera: Pentatomidae), other true bugs (Hemiptera), flies (Diptera), centipedes (Chilopoda), and millipedes (Diplopoda) (180, 150, 12, 181, 182).

Detailed analysis of stomach contents of 4 birds collected in canebrake habitat in Georgia by Meanley (150) revealed prey in the following percentages (volume): Coleoptera: Carabidae 12.6%; Cryptophagidae 1.0%; Staphylinidae 0.7%; Tenebrionidae 0.7%; Coleoptera larvae 2.4%; undetermined Coleoptera 2.4%; Hymenoptera: Formicidae 7.2%; Ichneumonidae 4.8%; Platygasteridae 0.2%; miscellaneous Hymenoptera 0.7%; Orthoptera: Gryllidae 20.8%; Acrydiinae 7.2%; Tettigoniidae 0.5%; Lepidoptera larvae 7.7%; Homoptera 2.1%; Diptera pupae 0.5%; undetermined insect adults, larvae, or eggs 19.2%; Arachnida: Araneida 2.0%; Arachnida eggs 7.2%; Diplopoda 0.2%.

On the overwintering grounds, based on an analysis of stomach contents collected using emetics, beetles (39%), spiders (22%), and ants (19%) were the most commonly consumed prey items (12).

Unexpected finds included the discovery of bones of small lizards (Iguanidae) in gizzards of Swainson's Warblers in Cuba (180), and bones of geckos (Sphaerodactylus goniorhynchus) occurring in > 60% of samples in Jamaica (12).

Food Selection and Storage

On breeding grounds in South Carolina, Savage et al. (182) reported that larval and adult spiders, larval butterflies and moths, and larval wasps and bees were strongly selected based on an analysis of items flushed from crops. Additionally, the proportion of each arthropod order in crop samples did not differ between adult males and females. On the overwintering grounds, spiders, beetles, lepidopterans, and true bugs were selected (180, 12).

Swainson's Warbler is not known to store food.

Nutrition and Energetics

Little information on nutrition. In coastal Louisiana, Swainson's Warblers (n = 6) had an average mass gain of 0.58 g ± 0.49 SD (0.41 g/day ± 0.46 SD) after a northward trans-Gulf flight (137). This gain equaled 4.3% (± 3.9 SD) of a bird's body mass, or a 3.0% (± 0.8 SD) increase/day. Five of 6 Swainson's Warblers in this study gained weight during their brief stopover; one that lost weight was in good condition initially.

Very little information on energetics.

Metabolism and Temperature Regulation

No information.

Drinking, Pellet-Casting, and Defecation

In many hours of field observations by the authors, this species was observed drinking and bathing from a muddy puddle of water only once (JDB). Swainson's Warblers may obtain most of their water through food items, but more study is needed.

Not observed to cast pellets. Defecation occurs while individuals are foraging, perched singing, or have just taken flight. Based on many hours of video observation, adults do not appear to defecate near nests, but remove or consume fecal sacs produced by nestlings.

Recommended Citation

Anich, N. M., T. J. Benson, J. D. Brown, C. Roa, J. C. Bednarz, R. E. Brown, and J. G. Dickson (2019). Swainson's Warbler (Limnothlypis swainsonii), version 3.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.swawar.03