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Variability in Nest Survival Rates and Implications
to Nesting Studies

Appendix


Method Used to Calculate Mortality Rates by Age Class and Calendar Period

We calculated daily mortality rates for nests in each 5-day period of age (1-5 days, 6-10 days, etc.) and for each 10-day calendar period (Days 110-119, 120-129, etc.). To do so, we calculated the exposure of all nests in each category and the number of such nests destroyed. The daily mortality rate is the number destroyed divided by the number of nest-days exposure. As an example, consider a nest found at age 3 days (laying with 3 eggs) on Day 118 (28 April). It was checked on Day 125 (5 May) and found to be still viable. Thus, there were 0 destructions occurring during 7 nest-days of exposure. The exposure was distributed over the AGE-DATE categories as follows:

AGE: 3 4 5 6 7 8 9
DATE: 118 119 120 121 122 123 124
Exposure: 1 1 1 1 1 1 1
Losses: 0 0 0 0 0 0 0

Hence the following AGE-DATE categories would receive the indicated contributions to exposure:

  AGE
DATE 1-5 6-10

110-119

2 0

120-129

1 4

Had the nest been destroyed by the visit on Day 125, then one loss would be attributed to the nest and the exposure reduced, because the expectation of the number of days the nest was at risk is less than 7. The single loss would be distributed over the 7 days in the interval according to the probability that the nest was destroyed on that day. To evaluate that probability, we assumed that nest mortality occurred at a constant rate of 5%/day. Then the probability that a nest was destroyed, for example, by the next day (Day 119), if it was destroyed when next visited on Day 125, is

    Pr{ survive exactly 0 days | not survive 7 days}

       =    Pr{survive exactly 0 days}
          1 - Pr{survive at least 7 days}

     

       = 0.1657.

The distribution of the single loss over the 7-day period between visits to the nest is as shown in the following table:

AGE: 3 4 5 6 7 8 9
DATE: 118 119 120 121 122 123 124
Exposure: 1.0000 0.8343 0.6768 0.5272 0.3851 0.2501 0.1218
Losses: 0.1657 0.1575 0.1496 0.1421 0.1350 0.1282 0.1218

The exposure is also distributed over the interval. The exposure for Day 118 will be 1, because the nest was exposed to risk on that day. The exposure accruing to Day 119 will be

    E{exposure on Day 119 | not survive to Day 125}

       = 1 × Pr{survive at least 1 day | not survive 7 days}

       = Pr{survive exactly j days | not survive 7 days}

       =

       =

       = 0.8343.

The distribution of exposure over the 7 days between visits is as shown in the preceding table. From that table we can determine the contribution of the single nest to the losses and exposure for AGE-DATE categories. The following table shows losses/exposure for each category.

  AGE
DATE 1-5 6-10
110-119 0.3232/1.8343 0/0
120-129 0.1496/0.6768 0.5271/1.2842

After adding nest losses and exposure in each category, daily mortality was calculated by the Mayfield Method.


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