Even birds follow Pareto's 80–20 rule



Vilfredo Pareto noted in 1906 that 80% of the land in Italy was owned by 20% of the population – and that the ratio seemed to apply to many countries, many time periods, and many economies. But is it more than a useful rule of thumb? Pareto also observed that 20% of the pea pods in his garden contained 80% of the peas. His ratio seems to pop up everywhere. Fred J. Rispoli, Suhua Zeng, Tim Green and Jennifer Higbie try telling it to the birds.

Bird watching is massively popular throughout the world. According to the Cornell Lab of Ornithology, there are over 800 species of birds just in the United States and Canada1. In New York State there are at least 185 species of birds present. Bird sampling, a formal form of bird watching, is an important technique used by environmentalists to help assess environmental activity. In a 13-year ongoing survey at Brookhaven National Laboratory, researchers have so far noted roughly 125 different species. However, only a relatively small percentage of the species accounted for a large percentage of the total observed bird population. Does that sound familiar?

In an entirely different intellectual domain, analysts often point to the “Pareto principle” to get across a similar notion in a competitive economic environment. Vilfredo Pareto (1848–1923), an Italian economist, made the observation that 80% of the wealth in Italy is owned by 20% of the population. This principle has also been noted in much older economies. A paper in Nature bears the title “Few had wealth in ancient Egypt”2. The author studied the remains of the city of Akhetaten, which briefly flourished in the 14th century bc. They gauged the wealth of its people from the floor area of their houses. And – guess what? The 80–20 ratio turned up there as well. The biggest houses – the top 20% of them – took up 80% of the housing area of the city

And it even seems to apply to the wealth and poverty distribution of the world as a whole – see Visualisation on page 39 of this issue.

The phenomenon occurs so often it is now known either as the “Pareto principle” or “Pareto's 80–20 rule” or, even, with a philosophical bent, “the law of the vital few”. You could say that 80% of wealth is often owned by 20% of the population; but it seems more general than that. One economist has even said that 80% of complaints to a company come from 20% of its customers. (He added that companies should ditch such customers, but we need not go there.) Stated in simple terms, Pareto's heuristic states that 80% of the outcomes of interest are often due to 20% of the possible causes.

Although the Pareto principle (or, more generally, the Pareto distribution) has been observed numerous times in management and economics (a quick Google search will convince you), more recently it is also being used in scientific studies. Ecologists Seuront and Mitchell3 have used Pareto laws to study marine ecology. They list phytoplankton growth processes and differential grazing by zooplankton among the interactive systems to which Pareto applies.

Is it a fundamental law of competition, or just an oddity that crops up? Either way, bird-watchers could take note

Which is where our small case study comes in. We examined the results of the Brookhaven National Laboratory bird data set that we mentioned at the start. The Brookhaven survey site is a small one. It occupies only 8.23 square miles. We also examined the comprehensive North American Breeding Bird Survey4, to look at the much larger area of New York State, which covers 54 000 square miles. We studied the yearly data, which covered the 13 years from 2000 to 2012. And we found the Pareto 80–20 rule holding true: 80% of the birds that were seen belonged to 20% of the species that were seen. It held on a year-by-year basis, and for the whole time period; and it held on the small scale of the Brookhaven site, and on a much larger scale in the New York State data set. A summary of our data set is given in Table 1.

Table 1. We looked at data each year during the period 2000–2012, identified the 20% of the species with the largest populations at the Brookhaven National Laboratory (BNL) and New York State (NYS) sites, and then determined what percentage of the total population is due to the top 20% of the species
YearPercentage of BNL population from top 20% of speciesPercentage of NYS population from top 20% of species

A blue jay (Cyanocitta cristata), one of the 20% of common species that make up 80% of bird sightings. Photo: Saforrest

Whether it is people competing for financial resources or birds competing for natural resources, an unequal, skewed distribution is inevitable. What is so surprising is how often the top 20% of the species, or causes, is responsible for very close to 80% of the population, or outcomes. It appears that the Pareto principle may be a fundamental law of competition.