Hunting pheasants in Ipswich, South Dakota over the years has given me the opportunity to hunt in all kinds of weather and with all kinds of hunters. I've seen a lot of good shooters come through OO Hunting and I've seen a lot of hunters shoot a box of shells only to come up empty handed. One thing the good shooters consistently say is "you have to lead the pheasant and keep swinging". Usually during these conversations you will hear one of good shots say "you're at least 5 foot behind" or "in the wind, lead them the length of a barn". It's hard to believe you would ever be 5+ feet behind a pheasant or need to lead them the length of a barn, so I thought I'd try to prove this.
There are a lot of assumptions built into this to make the calculations simple but I think even with these assumptions it's easy to get an understanding of how important the lead is.
I started by looking up the average speed of a pheasant from wikipedia (http://en.wikipedia.org/wiki/Common_pheasant), it turns out on average pheasants will fly 27 - 38mph but can reach upwards of 56mph if they are being chased. For this exercise I picked the top range of the normal speed, 38mph.
I then looked up the shot speed of Remington Nitro Mags. 2 and 3/4 inch 1.5oz #4 shot, it travels at 1,260fps (feet per second). This can be found on any box or online ammo stores: http://www.cheaperthandirt.com/product/6-0301897
With that I had to convert the pheasant flying speed to fps to have the same units as the ammunition. It turns out 38mph is 55.733fps.
Now we have to work in distance to the pheasant. To keep it simple I assume 30 yards or 90 feet.
Under this scenario, if you shot directly at a pheasant it would take .07 seconds (90 feet / 1,260fps) for your pellets to reach the pheasant at 90 feet (assuming the pellets maintain muzzle velocity speed). If the pheasant was traveling at the 55.733 feet per second, you would end up 3.98 feet (55.733 fps * .07 seconds) behind the pheasant.
Now lets look at a pheasant flying with the wind and having the sense of being chased. Under this scenario we can assume the pheasant is flying 56mph. This works out to 82.133 feet per second. In the .07 seconds it takes to reach the pheasant, the pheasant will have travelled 5.74 feet, leaving you well behind.
So while the length of a barn may be a bit of an exaggeration, it's very easy to see how you are likely to need to lead a pheasant by at least 3 - 6 feet. Hopefully this will help you understand the importance of the lead and bring down a few more roosters.