Disclaimer: This is a homework problem

A School of Ornithology researcher wants to estimate the number of red-tailed hawks in Ithaca. She radio tags 10 birds, and then sets up a feeding station with automatic camera

The researcher believes that each individual bird's visits to the feeder can be modeled as a Poisson process with some unknown rate $\lambda$. Over the first five weeks, she observes an average of 28.8 birds (tagged and untagged) visiting the feeder, with an average of 6 tagged birds per week. Use the method of moments to obtain an estimate for the total population.

The method of moments is a way to estimate the parameters by gathering a system of equations using empirical moments and setting them equal to the actual moments. Here, we only need one equation for one unknown: $\lambda$.

The way I understand this problem, we have $m_1 = \frac{X_1+X_2+X_3+X_4+X_5}{5} = 6$. Then we want $\mu_1 = E[X] = \frac{1}{\lambda} = m_1 = 6 \implies \lambda = \tfrac 16$. However, I am unsure of my reasoning here as this does not take into account the untagged birds. I am also not sure if I am understanding the empirical moment correctly from this problem.


1 Answer 1


Roughly, assuming a constant population and its random mixing between the time of tagging and the time of observing hawks at the feeder, the proportion $10/N$ of tagged hawks in the population should be estimated by the proportion $6/28.8$ at the feeder. So we estimate $\hat N = 288/6 = 48$ hawks in the population.

Note: This 'Lincoln-Peterson' method fails if no tagged hawks are seen at the feeder. See Wikipedia and other references on mark-recapture or capture-recapture estimation` for somewhat more satisfactory methods.

  • $\begingroup$ Can you elaborate why it should be estimated by $6/28.8$? $\endgroup$
    – hkj447
    Oct 18, 2020 at 23:18
  • $\begingroup$ You said on average 28.8 hawks per week were observed, of which on avg 6 were tagged (and presumably on avg 22.8 not tagged). $\endgroup$
    – BruceET
    Oct 18, 2020 at 23:25
  • $\begingroup$ I guess I am confused on how you used the knowledge that it is a Poisson process and moments to solve it. I understand your answer intuitively, but in a strict mathematical sense I do not see how the pieces come together. $\endgroup$
    – hkj447
    Oct 18, 2020 at 23:29
  • 1
    $\begingroup$ I believe I have figured it out: $m_1=E[X_{tagged}] = \sum_{i=1}^{10} E[X_i] = 10\lambda = 6 \implies \lambda = \frac{6}{10}$. Now we estimate the population of hawks by assuming we have a sum of $N$ Poisson processes. Using $\lambda = \frac{6}{10}$, we get $E[X_{pop}] = N\lambda = 28.8 \implies N = \frac{288}{6} = 48$ hawks $\endgroup$
    – hkj447
    Oct 19, 2020 at 0:16

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