albatross distress call and helicopter rotor 
noise. The authors stated that high-intensity, 
low-frequency noises appeared to hold promise 
for alleviating the gooney bird problem at 
Midway (52). Further research in the develop- 
ment of acoustical controls was recommended. 
There has been much interest in evaluating 
the use of alarm and distress calls of several 
gull species commonly found in greatnumbers 
at certain airports. Gulls are involved in 
many bird-plane strikes and have caused much 
damage to aircraft. Experiments with gull dis- 
tress cries began in Holland in 1956, and by 
1958, loudspeakers had been installed along a 
runway at a military airport plagued by gulls. 
The distress calls of three species of gulls are 
broadcast just prior to takeoff. Hardenberg 
(pers. commun., 1962) reported that the sys- 
tem has proven extremely effective in reducing 
the number of bird-plane strikes. In France, 
black-headed gulls (Larus ridibundus) and 
herring gulls were effectively repelled from 
the Nice Airport from March through October 
1963 using a sound truck (96). In England, 
bird distress-call experiments begun in 1959 
were less successful than in Holland and 
France. Mobile sound trucks were used at 
airports, with limited success, to repel lap- 
wings (Vanellus vanellus) and starlings. Suc- 
cess in repelling herring gulls was poor 
(Brown, Sugg, and Brough, pers. commun., 
1962). Preliminary work with distress calls 
of ring-billed gulls (Larus delawarensis) and 
herring gulls in Canada indicated that distress 
calls should be useful in the dispersal of 
birds at airports, but that the calls probably 
will have to be reinforced with other frighten- 
ing devices (89). The use of mobile sound units 
for certain airports is being considered in 
Canada. 
During the past 2 years, gull distress calls 
have been tested by the U.S. Department of the 
Interior on gulls feeding at garbage dumps, on 
loafing gulls at airports, and on gulls on a 
breeding island. Varying degrees of success 
were achieved in modifying herring gull be- 
havior and movements (81). Gulls at feeding 
sites (garbage dumps) were prevented from 
feeding for intervals ranging from a fewhours 
to more than 12 days depending on the availa- 
bility of alternate food supplies. Distress calls, 


188 
augmented with shell crackers, appeared tobe 
more effective than distress calls alone. 
Broadcast distress calls repelled gulls from 
breeding islands for only a few hours, but 
showed no loss of effectiveness in 8 days at 
an airfield. 
Results of these studies indicate that the 
success of sound stimuli depends on how they 
are used and on environmental and behavioral 
factors. The effectiveness of broadcast dis- 
tress calls and shell crackers for repelling 
herring gulls from feeding and loafing sites 
depends on the strength of the drive to remain 
in a feeding or loafing area (site tenacity) in 
relation to the strength of the reaction to the 
stimuli, Since the strength of these reactions 
changes with time and place, it is unlikely that 
the success of a given stimulus can be pre- 
dicted for all situations (81). Care must be 
exercised when inferring success or failure 
of a particular experiment. Apparent success 
should be viewed with just as much suspicion 
as apparent failure. Factors to consider in 
determining the success of any repellent 
stimulus include the strength of the native 
reaction to the stimuli, the availability and 
suitability of alternate sites to which birds 
may be dispersed, site tenacity, the presence 
of abnormal or nonreacting individuals that 
act as decoys, weather factors, and the stage 
of the birds' annual cycle, i.e., migration, 
breeding, or wintering (81). 
Results of work in various places indicate 
that nuisance birds can be repelled for at 
least short periods with a variety of sounds. 
Results have been particularly encouraging 
with broadcast alarm and distress calls, used 
alone or in combination with other stimuli. 
Most experiments in repelling birds have been 
conducted with no knowledge of the basic be- 
havior of the species involved, however. Such 
studies are therefore very muchoverdue. Em- 
phasis should be directed toward finding the 
strongest sound stimuli and the strongest 
combination of sound and other stimuli that 
will affect the movements of problem species. 
Practical use of acoustical bird-control meth- 
ods will require cost appraisals as well as 
basic studies of life history, ecology, and 
behavior. 

