Eight persons — A, B, C, D, E, F, G and H were studying about different birds in different months of the same year, viz. January, March, May, June, July, October, November and December, but not necessarily in the same order. Each of them is studying about different birds — Kingfisher, Toucans, Penguin, Ostrich, Woodpecker, Loon, Sparrow and Peacock.

G is studying in the month having odd number of days. Either two persons were studying before G or after G. Two persons were studying between G and the one who is studying about loon. H was studying three persons after the person who is studying about ostrich and four months before A. Three persons were studying between A and the one who is studying about Woodpecker. B is studying about Sparrow two persons before F. F is studying before E. The one who is studying about Penguin is studying immediately after the one studying about Kingfisher. Two persons were studying between the ones studying about Kingfisher and Peacock. D is not the first one to study.

How many persons were studying between D and the one studying about Kingfisher?

To solve this puzzle, let us first list the months in chronological order along with the number of days in each:
1. January (31 days)
2. March (31 days)
3. May (31 days)
4. June (30 days)
5. July (31 days)
6. October (31 days)
7. November (30 days)
8. December (31 days)

Now, let us analyze the clues step-by-step to place the persons and the birds they are studying:
- G's position: G is studying in a month with an odd number of days (31 days), and either two persons study before G or two persons study after G. This means G is either 3rd (May) or 6th (October).
- Loon's position: Two persons study between G and the one studying Loon.
- H and A's position: H studies three persons after Ostrich (Index of H = Index of Ostrich + 3) and four months before A.
Looking at the months, the pairs with a difference of exactly 4 months are:
• January (1st month) and May (5th month)
• March (3rd month) and July (7th month)
• June (6th month) and October (10th month)
• July (7th month) and November (11th month)

Let us test the case where G is 3rd (May):
Since G is 3rd (May), the person studying Loon must be 6th (October).
Since G is in May, A cannot be in May, so H cannot be in January.
If H is in June (4th position) and A is in October (6th position):
• Ostrich must be 3 positions before H, which is 1st (January).
• A (6th) is studying Loon.
• Three persons study between A (6th) and the one studying Woodpecker, placing Woodpecker at 2nd (March).

- B and F's position: B is studying Sparrow two persons before F (Index of F = Index of B + 2). F is studying before E.
Let's place B at 5th (July) studying Sparrow, which puts F at 7th (November). Since F is before E, E must be 8th (December).
- D and C's position: Since D is not the first one to study, D must be 2nd (March), leaving C to be 1st (January).

- Remaining Birds:
• The one studying Penguin studies immediately after Kingfisher.
• Two persons study between Kingfisher and Peacock.
If we place Kingfisher at 7th (November) and Penguin at 8th (December), then Peacock must be at 4th (June). This satisfies the condition of having two persons (5th and 6th) between Kingfisher (7th) and Peacock (4th).
The remaining bird, Toucans, is studied by G at 3rd (May).

The final arrangement is as follows:
1. January: C (Ostrich)
2. March: D (Woodpecker)
3. May: G (Toucans)
4. June: H (Peacock)
5. July: B (Sparrow)
6. October: A (Loon)
7. November: F (Kingfisher)
8. December: E (Penguin)

We need to find the number of persons studying between D (2nd / March) and the one studying Kingfisher (7th / November / F).
The persons studying between them are G, H, B, and A.
Therefore, the number of persons studying between D and the one studying Kingfisher is Four.