Brooks • BIRDS CAPTURED IN SPIDER WEBS 
351 
in a spider web (20 cases). The small size ol 
members of this family makes them more 
vulnerable to web entrapment, especially the 
smaller species: six (67%) of the nine species 
and 14 (70%) of the 20 cases have a mass 2=4 g 
and wing chord ^50 mm. The next smallest birds 
trapped in comparison include only three cases in 
two families: Aegithalidae (Bushtit. Psaltriparus 
minimus: 5 g mass. 40 mm wing chord) and 
Regulidae (Goldcrest. Regains regains: 5 g mass. 
55 mm wing chord and Golden-crowned Kinglet, 
R. satrapa: 5 g mass. 57 mm wing chord). 
More than one-half {n = 36) of the cases ol 
birds entrapped in spider webs were released 
unharmed by the humans reporting the incident. 
When these cases were excluded, 73% (» = 22) 
died due to web entrapment, and the only cases ol 
birds naturally freeing themselves in = 8) were 
not wrapped in silk. Consequently, the chance for 
a bird to survive web entrapment is affected by its 
ability to free itself prior to being immobilized by 
the spider. The sheer size of a bird alone could 
deter a spider from immobilizing it, as spiders 
are often reluctant to wrap prey too large to 
successfully consume (Sakai 2007). 
\re Spiders and their Webs a Threat to Birds ?— 
Orb weavers will cut their web to rid it of 
undesirable debris (Robinson and Mirick 1971). 
However, it is not desirable for a web to be 
destroyed by a bird Hying through it, and one of the 
many functions of a web is to visually deter birds 
from flying into them (Bruce et al. 2005). This was 
illustrated by Robinson and Robinson (1976) who 
described how a tame, experimental Hooded 
Butcherbird (Cracticns cassieus) accidentally flew 
through a Nephila macula!a web with devastating 
results to both the spider with a destroyed web. and 
the bird which took several minutes to preen the 
web threading from its feathers and was cautious 
lor the subsequent 3 weeks. This case details the 
consequences to both bird and spider of a web 
collision but. more importantly, indicates that webs 
do not always serve as visible deterrents to flying 
birds. This synthesis presents 69 cases of birds 
entangled in spider webs, suggesting that spider 
webs can fail as visual deterrents for many species 
of birds, concordant with Robinson and Robinson 
(1976). 
The primary purpose ol venom in most species 
of spiders is to subdue insect prey rather than 
harm larger species ol vertebrates (Shear 1986). 
Certain tarantulas (e.g., Theraphosa, Avirularid) 
are sufficiently large to depredate eggs and 
nestlings of birds but do not specialize on them 
(Shear 1986). Orb weaver size, web radius, and 
web height are the most important factors 
affecting abundance and size of prey captured; 
these same parameters arc important for interspe¬ 
cific niche separation (Brown 1981). These 
spiders catch winged prey in higher webs whereas 
larger, jumping insect prey are caught more 
frequently at lower strata (Brown 1981). Orb 
weavers use a variety of tactics to immobilize 
prey. More primitive forms such as Nephila often 
bile to subdue their prey whereas Argiope and 
Eriophora wrap their prey in silk (Weems and 
Edwards 1978). Most research on orb weaver 
foraging has shown specialization on insects 
(Rypstra 1985. Higgins 1987). Orb weaver 
mouthparts are too small to specialize on birds 
(Sakai 2007) or lo suggest coevolution for bird 
specialization, but orb weavers will opportunisti¬ 
cally depredate a small bird that gets caught in the 
web. This review documents 18 cases of birds 
wrapped in silk for consumption, and each case 
resulted in death unless freed by a human 
observer. A more limited number of cases showed 
actual consumption by a spider without the bird 
being wrapped in silk (Levy 1987. Peloso and de 
Sousa 2007). These cases are contrary' to Gra¬ 
ham’s (1997) speculation that orb weavers do not 
prey on birds. 
Spider Webs and Natural Environmental Threats 
to Birds.—A variety of avian species feed on 
spiders and use spider web for nesting material 
(Waidc and Hailman 1977); birds in these 
situations are likely to be aware ol the web and 
do not become entangled (McKenzie 1991). Birds 
traveling along direct flight paths, the same open 
understory areas favored by orb weavers to build 
webs (Graham 1997). are more likely to become 
caught, just as a bird can collide with a mist-net. 
Trapped birds may have been moving within a lek 
site (Sakai 2007). chasing prey, fleeing danger, or 
traveling to a new site. Orb weavers are perhaps the 
largest arboreal spiders with a web that can attain 
>1 in in radius with strong and sticky fresh silk 
strands (Lubin 1978. Griffiths and Salanitri 1980). 
Over 50 different species of birds (Table 1) have 
been trapped in these large webs spanning open 
flight paths. Being trapped in a web also makes a 
bird vulnerable to predation by a larger vertebrate 
predator (Graham 1997), if the bird is not 
immobilized by the spiders themselves. 
Natural environmental hazards are rare in 
nature. Another example besides spider webs is 
