by Robert McFarlane, Houston Bird Survey Coordinator

Every birder knows that the longer you look, and the more places you look, the more birds you will see. Some of our surveys are necessarily of rather brief duration. How many repeat surveys are required to get a near-total list? (There may be no such thing as a total list because you can conceivably add species forever) The Houston Bird Survey has provided one answer.

Garrett Stewart conducted 12 surveys at his residence in Kingwood over an 18-day period during June, 2006. He recorded a total of 20 species, with individual surveys ranging from 8 to 14 species each, averaging 11.6 species per survey. He began with 10 species, added 3 species with the 2nd survey, 3 more with the 3rd, and two in the 4th. The total species count held at 18 until the 9th survey when it rose to 20 species and held steady once more. While the total species recorded in the final survey was double that recorded in the initial survey, most of the information had been gathered by the fourth survey. There was clearly a point of diminishing returns.

As seen in the bar graph, only five species were observed in every survey (Carolina chickadee, house sparrow, northern cardinal, red-bellied woodpecker, and tufted titmouse) and two others (blue jay, Carolina wren) were observed in all but one survey. Three species were observed only once and half of the species were observed in less than half of the surveys. The 19th and 20th species added during the 9th survey were seen only once.