Egg size in an important measure of how parents divide their investment in offspring. It has fitness consequences for both parent and offspring. Yet, significant variation in egg size is found in many avian populations and the causes and consequences of this variation are not well understood. This study examines egg size variation in a wild population of North Island brown kiwi, from Rarewarewa Reserve, central Northland, New Zealand.

My results demonstrate that in North Island brown kiwi, egg weight is a good predictor of chick hatch weight (R² = 0.80). The range of hatch weight in my study was large (254-431 g) and predicted signficant differences in both the number of days that hatchling kiwi were at risk from introduced mammalian predators and in estimated probability of survival to safe weight (>800 g). Considering both of these results, I suggest that egg weight is an important predictor of lifetime fitness in North Island brown kiwi. Assortative mating is a pattern of mating where certain individuals, with particular traits, pair more often than would be expected under random pairing. Using measures of adult bill length and body weight, I could find no evidence of assortative mating in North Island brown kiwi.

Eggs in my study population ranged in estimated fresh weight from 317-551 g (n = 496), with a mean of 445 ± 45 g (± SD). Egg weight had a high degree of repeatability (R = 0.67) for individuals. Factors that did a good job of explaining fresh egg weight, and were included in all 6 top models, were male body weight, female body weight, and site. Also included in some top models was clutch size (3 models), male bill length (2 models), and sequence of egg in year (2 models). Male and female body weight had a strong positive association with egg weight, whilst the effect of male bill length and clutch size was weak and negative.

Egg weight did vary across metrics such as clutch size, sequence of egg in a year, or nest in year. This has relevance to conservation breeding programmes such as Operation Nest Egg, which takes wild kiwi eggs away from parents for incubation in captivity thus encouraging birds to lay replacement clutches, as it provides evidence that female kiwi do not respond to the pressure of having to lay more eggs by making later ones smaller.