SHORT COMMUNICATIONS 
395 
Sex ratios are presented as mean ± SD and 
significance was set at P < 0.05. 
RESULTS 
The overall sex ratio was 50% male with 33 
males and 33 females of 66 nestlings (Table 1). 
The annual sex ratio was 48.3 ± 10.6% males, and 
ranged from 28.6% males in 2009 to 58.8% males 
in 2002 (Table 1). The number of males produced 
per year did not differ {X : 6 = \A,P> 0.05) from 
the number of males expected assuming a 1:1 ratio 
(Table 1). There w as a tendency to produce more 
females following periods of high rainfall (Fig. I), 
particularly in 2004 and 2009. when nestling 
survival was high (Table 1). However, there was 
a 50:50 sex ratio in 2007 following the highest 
annual rainfall of 1.065 mm. Overall, the annual 
nestling sex ratio w-as not related < R : = 0.413: F\ 5 
- 3.5, P = 0.12) to environmental quality as 
indicated by rainfall (Fig. 1). 
The frequency of offspring gender was not 
associated with date of hatching (X\ = 1.8. P = 
0.88). Twenty-two nestlings hatched in the early 
period of which 54.5% were male; 28 nestlings 
hatched in the middle period with 39.3% males, 
and 14 nestlings hatched in the late period with 
57.1% males. There was no association of nestling 
sex ratio with hatching order (X‘\ - 0.68, P = 
0 . 61 ). However, 51.7% of first-hatched nestlings 
were males and 53.6% of second-hatched nestlings 
were males whereas third-hatched nestlings were 
only 33.3% males (Table 2). A nominal logistic 
regression showed no effect of rainfall prior to egg- 
laying. hatch date or hatch order on nestling sex 
(Whole Model X\> = 3.7. P = 0.92) 
DISCUSSION 
We found no evidence of modification of brood 
sex ratio by the Lilac-crowned Parrot for the 
variables evaluated. Our results did not support 
the local resource hypothesis, although we 
observed a tendency to produce more females 
following periods of high rainfall when there was 
high nestling survival. The date of hatching 
hypothesis was also not supported by our results. 
However. Lilac-crowned Parrots exhibit high 
synchrony in nest initiation (Renton and Salinas- 
Melgoza 1999), which may limit the influence of 
hatching date on brood sex ratio. We found no 
significant association of sex ratio with hatching 
order, although our data indicate a slight female- 
bias in third-hatched nestlings. The sexually 
dimorphic Common Kestrel {Falco tinnunculus ) 
and Audouin’s Gull (Ichthyaetus audouinii ) both 
produce more males in early clutches and fewer 
males later in the season (Dijkstra el al. 1990, 
Genovart el al. 2003). There may be a tendency for 
Lilac-crowned Panels to produce more female 
offspring in years of high nestling survival by 
producing larger clutches with a slight female-bias 
in third-hatched nestlings; a larger sample size may 
be required to detect significant associations. 
One possible explanation for the lack of 
evidence in our study is that the factors evaluated 
do not affect the costs or benefits associated with 
rearing either males or females (Radford and 
Blakey 2000). The factors we evaluated have been 
associated w ith sex ratio manipulation in strongly 
dimorphic parrot species (Trew ick 1997, Heinsohn 
et al. 2011). but they may not result in differential 
costs or benefits in monomorphic species like the 
Lilac-crowned Parrot or the Yellow-naped Ama¬ 
zon (South and Wright 2002). In addition, 
restrictions imposed by chromosomal sex-determi¬ 
nation could prevent females from altering the 
primary sex ratio of their offspring (Pike and Petrie 
2003; but see Heinsohn et al. J997. Genovart et al. 
2003). However, some studies suggest this con¬ 
straint can he overcome, although the mechanisms 
are not well understood (West and Sheldon 2002. 
Korsten el al. 2006). 
ACKNOWLEDGMENTS 
Research and CITES export permits were provided by 
the Secretaria del Medio Ambicntc (SF.MARNAT) in 
Mexico and CITES import permits by the U.S. Fish and 
Wildlife Service. S. M. Pease conducted this project while 
participating in the NMSU-Howard Hughes Medical 
Institute Research Scholar Undergraduate Research Schol¬ 
ars Program. We are grateful lor logistical and financial 
support from the Fundacion Ecologies de Cuixmala, 
Denver Zoological Foundation, World Parrot Trust (to K. 
Renton), and NSF grant IOS-0725032 and REU supple¬ 
ment IOS-0940689 (to T. F. Wright). We thank Elizabeth 
Hobson and Erie Pease for helpful comments on this 
manuscript. 
LITERATURE CITED 
Addison, B.. A. S. Kitaysky. and J. M. Hipfner. 2008. 
Sex allocation in a monomorphic seabird with a single- 
egg clutch: test of the environment, mate quality, and 
female condition hypotheses. Behavioral Ecology and 
Sociobiology 63-135-141. 
Bidden. A. E. and S. R. Beissinger. 2004. Against the 
odds? Nestling sex ratio variation in Green-rumpcd 
Parrotlets. Behavioral Ecology 15:607-613. 
BlELOCK, S. H. 1986. Climate of Chamela, Jalisco, and 
trends in the south coastal region of Mexico. Archives 
