Birds Caught in Spider Webs: a Synthesis of Patterns

—Results of queries through public avian list-servers and a thorough literature search formed a data base to synthesize patterns of birds trapped in spider webs. Sixty-nine cases of birds, representing 54 species in 23 families, were reported trapped in webs. Hummingbirds were the most diverse family (nine species) and had the most cases of entrapment (n 5 20). Archilochus colubris represented the species with the most cases of entrapment (n 5 6). Mean mass and wing chord length of all species trapped were 11 g and 61 mm, respectively. Eighty-seven percent of all individuals had mass #15 g and 88% had a wing chord ,90 mm. Phaethornis longuemareus and Mellisuga minima represented the smallest species (mass 5 2 g, wing chord 5 37 mm), and Streptopelia senegalensis was the largest (mass 5 80 g, wing chord 5 138 mm). Thirty cases of birds were entrapped without human intervention: 22 died and eight not wrapped in silk freed themselves. Those wrapped in silk invariably died unless freed by a human observer. One-half of all reported spider webs were of the genus Nephila, and all were orb weavers except for a single Latrodectus. Nephila clavipes entrapped nine species representing 14 cases, ranging from Mellisuga minima (mass 5 2 g, wing chord 5 37 mm) to Catharus ustulatus (mass 5 23 g, wing chord 5 93 mm). Patterns, causes, and consequences of birds entrapped in spider webs are discussed, including orb weavers as opportunistic predators of birds trapped in webs, and spider webs as a natural environmental hazard to birds. Received 31 August 2011. Accepted 7 December 2011. Birds have a wide variety of predators. Top predators to birds include humans, who have a long history of harvesting birds for sport, protein, and ritual (Brooks 1999). Other predators of birds include fish (Lockwood 1922), reptiles (Dove et al. 2011), amphibians (Norris-Elye 1944), mammals (Bisbal 1986), and a variety of avian species ranging from raptors (Mayr 1966) to certain passerines (Graves 1978). Hymenopterans (Grant 1959), odonates (Hofslund 1977), and mantids (Carignan 1988) have also been reported to attack birds, especially smaller species such as hummingbirds (Trochilidae). Other cases involving invertebrates include birds trapped in webs of orb weaver spiders, of which there are numerous accounts in regional journals (e.g., Coale 1912, Lockwood 1922, Kirkham 1925, Abbot 1931, Bent 1953, Grant 1959, Morris 1963, Doberski 1973, Pratt 1974, Vernon 1976, Hofslund 1977, Donnelly 1980, Dean 1984, Gosling 1984, Levy 1987, Carignan 1988, McKenzie 1991, Shaw 1994, Cheke and Mann 2001, Heck and Heck 2001, Riddell 2001, Engel 2006, Cox and Nesmith 2007, Peloso and de Sousa 2007, Brooks et al. 2008). The pattern of birds trapped in spider webs has not been properly synthesized to date despite extensive documentation because these observations are largely considered incidental and often dismissed by scientists as unremarkable (Graham 1997). For example, Lima (1993) broadly synthesized patterns of predator escape by North American birds but did not include fates of birds trapped in spider webs. Miller and Gass (1985) examined patterns of predation of hummingbirds and concluded the majority of the incidents were unusual and inconsistent. All cases they examined were of vertebrate predators except one, and that single case was not a spider. Two colleagues and I found a web-entrapped Swainson’s Thrush (Catharus ustulatus) in September 2007 while inspecting damage to wildlife sanctuaries on High Island (Galveston County, Texas) following Hurricane Humberto (Brooks et al. 2008). Subsequent literature review revealed a plethora of short published and unpublished reports of birds entrapped in spider webs with extensive variation in species and their respective fates. The objective of this synthesis is to examine patterns and fates of birds trapped in spider webs. This topic warrants investigation because birds trapped in spider webs have been reported in the literature on multiple occasions and may represent more than a series of trivial incidents. Even the most detailed reports to date (e.g., Graham 1997, Cox and Nesmith 2007, Peloso and de Sousa 2007, Sakai 2007) only examined a single or limited number of cases and lacked a thorough synthesis of multiple cases.