SHORT COMMUNICATIONS 
393 
The Wilson Journal of Ornithology 124(2):393-396, 2012 
Brood Sex Ratio of the Lilac-crowned Parrot {Amazona finschi) 
Shannon M. Pease . 1 Alejandro Salinas-Melgoza,' Katherine Renton , 2 
Patricia Escalante , 3 4 and Timothy F. Wright 14 
ABSTRACT.—Evolutionary theory predicts birds 
should adjust the sex ratio of their broods in response 
to external factors that differentially affect the repro¬ 
ductive value of each sex. We examined the brood sex 
ratio in the Lilac-crowned Parrot (Amazona finschi) in 
relation to climate, hatching date, and hatching order. 
We used polymerase chain reaction amplifications to 
identify the gender of 66 nestlings from 32 clutches 
-panning 7 years. There was a tendency to produce 
more female offspring in years of high nestling survival 
following high rainfall with a slight female-bias in 
third-hatched nestlings. We found no significant 
associations between brood sex ratio and rainfall, 
hatching date, or hatching order within clutches. Our 
results suggest the examined factors provide insufficient 
differential costs or benefits of offspring gender to 
promote sex ratio bias in this monomorphic species. 
Received 17 August 2011, Accepted 29 November 2011. 
Animals that have the ability to alter the .sex 
ratio of their offspring are predicted to do so as an 
adaptive response to external factors (Trivers and 
Willard 1973). Parents are expected to bias 
offspring sex ratio toward the gender that will 
yield the greatest fitness benefits (Trivers and 
Willard 1973, Addison et al. 2008; but see 
Fawcett et al. 2011). Factors shown to affect 
brood sex ratio in birds include resource avail¬ 
ability' (Budden and Beissinger 2004, Addison 
ei al. 2008). date of hatching (Dijkstra et al. 1990, 
Radford and Blakey 2000). and sequence of 
hatching (Genovart et al. 2003). 
Strong dimorphism and non-monogamous mat¬ 
ing systems are usually an indication of strong 
sexual selection, which may promote a bias in 
hrood sex ratio at hatching (Hcinsohn et al. 1997. 
Trewick 1997. Genovart et al. 2003), or second¬ 
1 Department of Biology. Mail Stop Code 3AF. New 
Mexico State University. Las Cruces, NM 88003. USA. 
Estacidn dc Biologta Chamela. Instituto de Biologfa. 
Uiversidad Nacional Autonoma dc Mexico. Apartndo Postal 
21. San Patricio. Jalisco, Codigo Postal 48980. Mexico. 
Departamenio dc Zoologia. Institute dc Biologta. Uni- 
versidad Nacional Autonoma dc Mexico, Mexico D.F. 
4 Corresponding author; e-mail: wright@nmsu.edu 
arily through differential mortality of young of a 
specific gender (Pike and Petrie 2003. Addison 
et al. 2008. Hcinsohn et al. 2011). It remains less 
clear whether monogamous or monomorphic 
species should exhibit similar control over hrood 
sex ratios. 
Among parrots, the Eclectus Parrot ( Eclectus 
roratus) and the Kakapo ( Stngops Itabroptila) 
have shown extreme bias in brood sex ratio 
(Hcinsohn et al. 1997, Trewick 1997. Hcinsohn et 
al. 2011); both species are sexually dimorphic 
with non-monogamous mating systems. In con¬ 
trast. no sex ratio bias was found in the sexually 
monomorphic Yellow-napcd Amazon (Amazona 
auropaUiata) (South and Wright 2002). 
We studied the Lilac-crow ned Parrot (Amazona 
finschi). a socially monogamous and sexually 
monomorphic species endemic to the tropical dry 
forest of western Mexico. Clutches are usually 
small (mean = 2.6. range = 1-4 eggs), and eggs 
hatch asynchronously (Renton and Salinas- 
Melgoza 1999, 2004). There is no difference in 
nestling growth rate between first- and second- 
hatched chicks; third-hatched chicks demonstrate 
significantly slower growth and lower probabili¬ 
ties of survival, while fourth-hatched chicks 
occurred only in 1 year and all chicks died within 
a few days of hatching (Renton 2002. Renton and 
Salinas-Melgoza 2004). Previous work has shown 
marked inter-annual variation in reproductive 
success with fluctuations in rainfall resulting from 
the El Nino-La Nina cycle of the Southern 
Oscillation (Renton and Salinas-Melgoza 2004). 
Thus, there is potential for inter-annual fluctua¬ 
tions in rainfall and resulting differential parental 
investment to affect brood sex ratio in this 
species. 
We measured nestling sex ratio to test three 
hypotheses that predict an association between 
external factors and sex ratio. (1) We used annual 
rainfall to test the local resource hypothesis, which 
predicts a brood sex ratio bias in years of relative 
resource abundance (Trewick 1997. Sasvari and 
Nishiumi 2005), (2) We examined distribution of 
males and females across the breeding season for 
