14 
Psyche 
[Vol. 87 
1975; Tolbert, 1975) based on observations on natural populations. 
However, Robinson and Robinson (1970) performed experiments 
using model stabilimenta which indicated that birds could use 
stabilimenta as an aid to prey location. Thus, the defensive role of 
stabilimenta is not clear. 
I performed experiments in aviaries which consisted of exposing 
blue jays to a simultaneous choice among A. aurantia or A. trifasciata 
which exhibited different degrees of web completion, in order to 
assess the effect of the stabilimentum on avian predation. 
Materials and Methods 
The experiments were performed in aviaries which consisted of 
1.6X6.2X2.3 m wire mesh enclosures located on the roof of Morrill 
Science Center, University of Massachusetts, Amherst. The basic 
experimental design was to offer blue jays ( Cyanocilla cristata) a 
choice among three spiders, each in a web at a different stage of 
completion; (1) suspended in the characteristic resting position, but 
entirely out of a web; (2) in a web without a stabilimentum; and (3) in 
a web with a stabilimentum. 
The blue jays used in the experiments had been taken as nestlings 
from their nests in the Amherst area and reared by hand. Thus they 
were naive with respect to spiders and their webs prior to experi¬ 
mentation. The blue jays were kept individually in the aviaries for 
several days prior to experimentation. Each bird was trained to 
recognize spiders as food by placing living spiders in its food tray (i.e., 
spiders free of webs). A given bird was used in the experiments when 
it took spiders readily (three within five minutes) without having 
previously been deprived of food. Prior to experimentation the birds 
were deprived of food overnight. Nine birds wre used, each bird being 
used one to four times. An attempt was also made to use starlings 
(Sturnus vulgaris ) and robins ( Turdus migratorius) in the experi¬ 
ments; they were found to be unsuitable. 
Penultimate and adult female A. aurantia and A. trifasciata were 
collected from field areas in and near Amherst, Massachusetts. The 
size and species of the spiders were the same within each choice set. In 
some cases, 6— 12 spiders were released into the aviary during the 
evening prior to experimentation, while the bird was isolated in a 
small cement-block enclosure (1.5X2.IX 1.4 m) at the rear of the 
aviary. Most of the spiders selected suitable web sites and built their 
