608 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 3. September 2012 
The Wilson Journal of Ornithology 124(3 ):608—611. 2012 
Malicious Motherhood: Instance of Infanticide by a Female Barn Swallow 
Joanna K. Hubbard 1: and Audrey L. Tobin 1 
ABSTRACT.—Infanticide, the killing of dependent 
offspring, often provides direct or indirect fitness 
benefits to the perpetrator. Infanticide in socially 
monogamous systems, like that in many passerine 
birds, is typically performed by males in an attempt to 
gain access to potential mates. We observed infanticide 
by a female Barn Swallow (Hirmulo nistica ) in North 
America and. to the best of our knowledge, this is the 
first documentation of this behavior. A female pecked 
and threw out nestlings belonging to a neighboring pair 
of swallows. There were no obvious fitness benefits 
gained by this female, thus established evolutionary 
explanations arc not applicable. Further investigations 
into the frequency of female infanticide, easily mistaken 
for predation, should be pursued to better assess the 
selective pressures driving this behavior. Received 17 
January 2012. Accepted IS April 2012. 
Infanticide occurs in many animal laxa, and 
many hypotheses attempt to explain this behavior. 
These include: (1) sexual selection. (2) resource 
competition. (3) exploitation of the infant, and (4) 
parental manipulation (Hrdy 1979, Sherman 1981. 
Ebensperger 1998: Table 1). Unmated males, or 
males new to a social group, are typically (he 
culprits in sexually-selected infanticide; by killing 
a female's dependent offspring, a male increases 
their chance of mating w ith that female and siring 
offspring (Trivets 1972. Hrdy 1979). Infanticide is 
also commonly the result of resource competition 
when resources are limited. A parent will kill 
unrelated offspring in the area to increase the 
available resources and the chances of survival for 
its own offspring (Hrdy 1979. Sherman 1981. 
Ebensperger 1998). Infanticide can also be a form 
of exploitation or parental manipulation. Infanti¬ 
cide is considered direct exploitation when an 
individual directly gains from the act. for 
example, by cannibalizing the infant (Fox 1975) 
or “play mothering' or “aunting to death' (Lan¬ 
caster 1971, Hrdy 1979, Quiatl 1979). Alterna¬ 
tively, an individual may increase its own survival 
Department of Ecology and Evolutionary Biology, Uni¬ 
versity of Colorado at Boulder. Boulder, CO 80309, USA. 
' Corresponding author: 
e-mail: Joanna.Hubbard@colorado.edu 
or the survival of healthier offspring by killing 
unhealthy or debilitated infants (Low 1978). The 
parent manipulates the situation to increase its 
fitness directly or indirectly by increasing the 
resources available to viable offspring. 
Barn Swallows (Hirmulo nistica) are a socially 
monogamous passerine with a Holarctic distribu¬ 
tion (Moller 1994). They breed semi-colonial!} 
with North American groups typically ranging in 
size from nine to 35 pairs (Safran 2004). Sexually- 
selected infanticide by Barn Swallow males to 
gain access to a female has been observed (Crook 
and Shields 1985; Mpller 1988. 2004). Mollcr 
(2004) proposed that infanticide by males is more 
common in benign years that allow individuals of 
lower condition to survive spring migration, The 
number of unmated males increases in these years, 
and the average condition of mated pairs is lower, 
thus reducing their ability to defend nests and 
prevent infanticide, Moller (2004) directly ob¬ 
served infanticide by males in European popula¬ 
tions and estimated the rate to be —25$; this 
behavior has also been documented in North 
American swallows at low frequency (Crook and 
Shields 1985). Infanticide was not directly 
observed until 2011 in our study population in 
Colorado, USA, which has been under intensive 
observation since 2008. We report infanticide 
performed by a paired female Barn Swallow: to 
the best of our knowledge, this is the first 
observation of female infanticide in Bam Swal¬ 
lows and suggests a need for future studies to 
explore the prevalence of this behavior. 
OBSERVATIONS 
Our study population breeds in bams and 
culverts in Boulder. Weld, and Jefferson counties. 
Colorado, USA. Autumn Hill, the site where 
infanticide was observed, is a large equestrian 
center in Boulder County (40 08' N. 105 lb VV| - 
the barn houses —30 horses with a large indoor 
arena in (lie center. There is a relatively high 
amount of human activity; although, the swal¬ 
lows do not appear disturbed. The site has T 1 
breeding pairs, —40 of which we are able to 
monitor; nests in the arena are too high to access. 
