"In species with a long period of parental investement after birth of young, one might expect biases in parental behavior toward offspring of different sex, according to the parental condition; parents in better condition would be expected to show a bias toward male offspring."

Males produce more offspring than females:
In favorable settings, mothers produce
more male offspring than female offspring.

Trivers, R.L., and Willard, D.E., 1973. Natural selection of parental ability to vary the sex ratio of offspring. Science 179: 90-92.
Elephant Seals SST
Mother's Health Hypothesis
Testosterone Hypothesis
The Red Deer
Crafoord Prize in Biosciences 2007 Trivers himself
Adams, J, 1989. Natural selection of parental ability to vary resource partitioning among offspring? Oikos 56: 135-137.
Lee, D.E. and Sydeman, W.J. 2009. North Pacific climate mediates offspring sex ratio in northern elephant seals. Journal of Mammalogy 90: 1-8.
Dugdale, H.L., Macdonald, D.W., Newman, C. 2003.Offspring sex ratio variation in the European badger, Meles meles. Ecology 84: 40-45.