610 
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 3. September 2012 
TABLE 2. Why the reported incident of infanticide does not appear to support established evolutionary hypotheses. 
Hypoihesis 
Predict urn 
Citation 
Sexual selection 
Female from nest # 37 did not mate with 
male from nest # 106 
(Trivers 1972. Hrdy 1979) 
Resource competition 
Nest # 37 and # 106 were not synchronous: 
resource availability was not dramatically 
different at time of event 
(Hrdy 1979. Sherman 1981. 
Ebensperger 1998) 
Exploitation 
Female from nest ft 37 did not eat or further 
manipulate nestlings from nest # 106 
(Lancaster 1971. Fox 1975. Hrdy 
1979. Quiatt 1979) 
Parental manipulation 
Nestlings did not belong to female from 
nest # 37: there were no size or mass 
differences among the nestlings and no 
nestlings were left unharmed 
(Low 1978) 
2011 season was a particularly productive year 
(JKH. pers. obs.) suggesting food resources were 
plentiful and environmental stressors were rela¬ 
tively mild. Moreover, there was no significant 
change in local climate conditions (temperature 
and rainfall) that may have affected food 
resources leading to this event. 
This particular case of infanticide is difficult 
to explain, and is perhaps an example of a 
pathological behavior, but it does raise the 
question of how often infanticide is mistaken for 
predation. Missing eggs and nestlings, as well as 
nestlings that are found dead in nests, are typically 
attributed to predators such as domestic cats (Pel is 
cat us) or Black-billed Magpies (Pica hudsouia) or 
competitors for space such as House Sparrows 
(Passer domes ticus). However, in light of this 
observation, as well as relatively high rates of 
infanticide in European populations of Barn 
Swallows (Moller 2004), it seems probable a 
portion of these instances could actually be 
cases ot infanticide. Furthermore, if infanticide 
is performed to gain access to mates, we are 
overestimating the strength of natural selection 
imposed by predation and underestimating the 
strength of sexual selection. Dark coloration is 
sexually-selected in the North American subspe¬ 
cies (H. r. erythrogaster) (Safran et al. 2005) and 
in our population, darker males are more likely to 
have their nests depredated (A. J. Flynn, unp'ubl. 
data). This event poses an alternative hypothesis: 
darker males arc more likely to experience 
sexually-selected infanticide by potential female 
mates. We have only witnessed one case of 
infanticide in our population, and future work- 
should attempt to differentiate between infanticide 
and predation to better understand the selective 
pressures imposed on traits related to fitness. 
ACKNOWLEDGMENTS 
Wc thank J. A. Barringer-Richers, Greenwood Wildlife 
Rehabilitation Center. Autumn Hill Equestrian Center, and 
the Safran Laboratory. Wc thank W. M. Shields and an 
anonymous reviewer for helpful comments on a previous 
draft of this manuscript. This work was funded b\ a 
National Science Foundation Grant (10S-0707421) to R J. 
Sal ran. Bioscience Undergraduate Research Skills and 
Training award to A. L. Tobin, and a Graduate Student 
Research Grant from the University of Colorado Ecology 
and Evolutionary Biology Department to J. K. Hubbard 
LITERATURE CITED 
Cut.K. A. A. and R. J. Robertson. 1991. Infanticide in 
female Tree Swallows: a role for sexual selection. 
Condor 93:454-457. 
Crook. J. R. AND W. M. Shields. 1985. Sexually selected 
infanticide by adult male Bam Swallows. Animal 
Behavior 33:754-761. 
Em NSPLRGLR. L. A. 1998, Strategies and counter-strategic 
io infanticide in mammals. Biological Reviews 
73:321-346. 
Fox, L. R. 1975. Cannibalism in natural populations. 
Annual Rev iew of Ecological Systems 6:87-106. 
Hansson, B.. S. Bensch. and D. Hasselqiist. I99h 
Infanticide in Great Reed Warblers: secondary female* 
destroy eggs of primary females. Animal Behavior 
54:297-304. 
Hrdy. S. B. 1979. Infanticide among animals: a review, 
classification, and examination of the implications tor 
the reproductive strategies of females. Ethology and 
Sociobiology 1:13-40. 
Lancaster, J. 1971. Play-mothering: the relations between 
juvenile females and young infants among tree- 
ranging vervet monkeys (Cercipithecws aethiop ' 
Folia Primatulogica 15:161-182. 
Low, B. S. 1978. Environmental uncertainty and t- 
parental strategies of marsupials and placental' 
American Naturalist 112:197-213. 
Moller. a. P. 1988. Infanticidal and anti-infanticukd 
strategies in the swallow Hirundo nistica. Behavioral 
Ecology and Sociobiology 22:365-371. 
