Sexual Dimorphism and Weight Analysis of Kiwi Birds

Introduction to Kiwi Bird Weight Research

Purpose: This investigation could be useful in scientific research regarding the weight distribution of kiwi birds while considering their genders. For example, the Department of Conservation (DOC) might be interested in finding alternative ways to distinguish female from male kiwi birds. By identifying patterns in weight relative to gender, it may become easier to identify individuals during scientific studies.

Investigation Question

I wonder if the median weight (kg) for female kiwi birds is higher than the median weight (kg) of male kiwi birds from the population of 700 kiwi birds observed in New Zealand in 2020.

Hypothesis and Biological Context

According to the Whakatane Kiwi Trust, "the female is 20 to 30 percent bigger than the male and will lay eggs for the males to incubate." Biologically, this sexual dimorphism occurs because a female kiwi must produce and carry an exceptionally large egg, which can weigh up to 20% of her own body mass, requiring significant energy and fat reserves. Based on this information, I expect female kiwi birds to have a higher median weight than male kiwi birds.

Variables and Sampling Method

Variable: The numerical value is the weight of the kiwi birds (kg). The groups are female and male kiwi birds observed in NZ in 2020.

Sample: I chose to take a simple random sample of 280 kiwi birds from the total population of 700 (representing a 40% sample size). This unbiased selection process resulted in an almost equal split of 141 female and 139 male kiwi birds. Because both genders ended up with nearly identical representation, this sample size is sufficiently large and balanced to make a fair, accurate, and reliable comparison.

Analysis of Weight Distribution

Centre

In my sample, the median weight of female kiwi birds is 2.9309 kg, compared to the median weight of male kiwi birds, which is 2.2458 kg. This shows that, on average, female kiwi birds have a higher weight. This is likely because females are the biological egg carriers. Conversely, male kiwi birds use their strong legs and claws to dig burrows; they must be fit and lean for this physical labor, contributing to their lower body mass.

Shape

The female kiwi weight distribution is unimodal and highly symmetrical. It features a broad, flat central peak spanning from approximately 2.6 kg to 3.3 kg. The male distribution is also unimodal and largely symmetrical, with a prominent peak at around 2.2 kg. The tighter peak in the male dataset suggests more consistent body mass, as they do not experience the drastic weight fluctuations associated with egg production.

Spread

The Interquartile Range (IQR) for female kiwi birds is 0.608 kg (3.23 - 2.622). The IQR for male kiwi birds is 0.358 kg (2.417 - 2.059). There is a 0.250 kg difference between the two groups, indicating that female kiwi birds have a larger spread in weight distribution. This variation is likely due to some females carrying large eggs during data collection while others were not.

Shift and Overlap

There is a massive visual shift between the two datasets, and the middle 50% boxes do not overlap. The entire middle 50% of female weights sits completely to the right of the male weights. In fact, the lower quartile of females (2.62 kg) is greater than the upper quartile of males (2.42 kg), meaning the heaviest 75% of females are entirely heavier than the lightest 75% of males.

Unusual Observations

I identified one clear outlier in the female dataset at just above 1.6 kg. This could be a juvenile kiwi recorded before reaching adult size, or a malnourished bird suffering from illness. This is unusual, as female kiwis typically require a significant baseline mass to produce eggs.

Informal Confidence Interval Graph

Inference and Statistical Significance

Female Kiwi Bird Weight (kg):
N = 141 | Median = 2.94 | IQR = 0.608
Calculation: 2.94 +/- (1.5 x 0.608 / √141) = 2.8632 kg to 3.0168 kg

Male Kiwi Bird Weight (kg):
N = 139 | Median = 2.246 | IQR = 0.358
Calculation: 2.246 +/- (1.5 x 0.358 / √139) = 2.2005 kg to 2.2915 kg

The confidence interval for female weight does not overlap with the interval for male weight. Therefore, I can confidently claim that in the population, the median weight for female kiwi birds is greater than the median weight of male kiwi birds.

Sampling Variability Statement

If I took another stratified sample of the same size, I would expect different medians and IQRs. However, I would still expect the statistics and displays to be similar. I would likely be able to make the same call—that there is enough evidence to claim a difference in the population. A larger sample size would provide even more reliable and accurate results.

Conclusion and Reflections

I hypothesized that female kiwi birds would have a higher median weight than males, and the data supports this claim. One limitation is that the data was collected exclusively in 2020. Environmental factors like pest control cycles or droughts change over time, so these weights might not perfectly represent the population in 2026.

A major confounding variable is the specific subspecies of the kiwi birds. Different species, such as the Great Spotted Kiwi versus the Little Spotted Kiwi, have different baseline sizes. If the sample was unintentionally skewed toward a larger species for females, it could artificially inflate the difference. Future studies should record and balance the specific subspecies to ensure higher accuracy.