The Wilson Journal of Ornithology 124(3):518-524. 2012 
CAPSAICIN AS A DETERRENT AGAINST INTRODUCED 
MAMMALIAN NEST PREDATORS 
SHANE M. BAYLIS, 1 4 PHILLIP CASSEY,- AND MARK E. HAUBER'' 
ABSTRACT.—We investigated use of capsaicin, a chemical that evolved as a mammal-directed fruit-consumption 
deterrent for chili (Capsicum annum) fruits as a nest-predation deterrent. Capsaicin is unpalatable to mammals, but 
apparently undetectable by birds, and has been used as a selective mammal-repellent in several commercial applications, 
e placed nmtanon thrush (Turdus spp.i nests containing model and real eggs in a suburban site near Auckland. New 
ealand. The majority of observed predation attempts in our experiments were attributed to introduced rats (Ramis spot 
used on comparisons of tooth-marks on damaged plaster eggs and tooth-marks made with rat skulls. Predation rau-von 
imitation eggs seated with adhesive chili powder were lower than predation rates on eggs in other treatments (W = 201. 
17 H e l g " on - adhesive adhesive paprika (capsaicin-free chili powder), non-adhesivc paprika, and 
n reated eggs. Successive replicates of the same experimental paradigm in the same site revealed the predation rate on all 
* deCI ' eaSe , ci W,th Placement of imitation ncs,s and eggs. These results Support the potential Vtdu 
use ol capsaicin-heated nests to deter mammalian predators of natural bird nests. Received 15 July 2011. Accept S 
The time spent as an egg. hatchling, or juvenile 
by birds is a period of high vulnerability to 
predation (Weatherhead and Blouin-Demers 
2004). Reproductive losses of focal nesting birds 
or populations due to mammalian nest predation 
are typically controlled by the use of poison baits, 
often used just prior to the breeding season, in a 
lorm to target a particular predator species, but 
often affecting others as collateral damage (Eraser 
and Hauber 2008). For instance, widespread aerial 
releases of 1080 bait in New Zealand, mainly 
targeting brush-tailed possum (Trichoswus vulpe- 
cula) as a tuberculosis reservoir, also have a 
significant negative impact on introduced rat 
populations and secondarily poison ermine (Mus- 
lela emiinea), all ol which are major nest predators 
of native birds (Innes et al. 1995, Murphy ct al. 
1999). The primary conservation benefit of poison 
baits is that predator numbers are diminished when 
native birds are at the greatest risk of nest 
predation, leading to markedly increased Hedging 
and recruitment rates (Powlcsland ct al. 1999). The 
wide-scale application of poison baits is frequently 
unpopular with opponents observing the poisons 
are non-specific and may affect non-target animals, 
including pets and even protected avian species 
(Fraser and Hauber 2008). Thus, narrowly targeted 
'School ol Biological Sciences. University of Auckland 
Auckland. PB 92019. New Zealand. 
School of Earth and Environmental Sciences. Univer¬ 
sity of Adelaide, SA 5005, Auslralia 
'Department of Psychology, Hunter College of ,he City 
University of New York. NY 10065. USA. 
‘Corresponding author; e-mail: 
shane.m.baylis@gmail.com 
518 
and/or non-lethally toxic alternatives to traditional 
baits should be useful, particularly near areas of 
human habitation. 
Capsaicin is the active ingredient in ‘hot’chili 
peppers, responsible for the immediate ‘spicy’ 
sensation upon ingestion and the further ‘burning’ 
stimulus immediately following gut passage, as well 
as the similar ‘burning* feeling associated with 
prolonged skin contact (Szallasi and Blumberg 
1999). Capsaicin is thought to have evolved as a 
selective seed-predation deterrent by the chili 
(Capsicum annum) plant, although there are some 
cultivars that produce fruits lacking capsaicin, 
including sweet paprika (Tewksbury and Nablian 
2001). Hot* C. annum fruits are extremely offensive 
to potential mammalian seed predators in the plant's 
native range; the obvious evolutionary, ecological, 
and cognitive consequence of the noxious ’hot' taste 
is that mammals do not willingly consume fruits or 
seeds of C. annum (Tewksbury and Nabhan 2001). 
Avian seed dispersers, and birds in general, are 
unaffected by capsaicin with Curve-billed Thrashers 
(7 oxnstatna curvirostre) recorded as important 
dispersers of viable C annum seeds, as well a 11 
being major consumers of its fruits (Tewksbury and 
Nabhan 200 1 ). 
Several practical uses have been made of the 
deterrent capacity of capsaicin against mammals. It 
has been recently patented as a selective deterrent, 
when mixed with commercial seed mixes, against 
squirrels (Sciurus spp.) in bird feeders in North 
America (Blumberg 1998), as well as being 
investigated as an agent to protect longleaf pine 
(Pinus palustris) seeds from predation by mice 
(Nolte and Barnett 2000). It is also used to protect 
