To determine the threshold of diminishing returns in detection of new species, record the cumulative number of birds you observe in your yard over a selected time period at a selected time interval.

For example, record the cumulative number of species every minute for 20 consecutive minutes (Fig. 1).  Plot the values as in the graph below. You will likely notice that most species were recorded in the first 10 minutes.

Figure 1. An example of the total number of species detected over time. In this case, half of the species were detected in 5 minutes and over 90% detected within 10 minutes. In this case, the optimal time for an observation session is somewhere between 6 to10 minutes.

Figure 1 illustrate that the rate at which new species are detected gets to a point of diminishing returns.  In other words, at the start of a count session, an observer is likely to detect several new species each minute whereas after some threshold point, it may take an hour or more before a new species is detected.  At the point of diminishing returns, it is more efficient to undertake multiple shorter counts rather than longer counts.