I have hunted groundhogs since 1970...the start of my long range passion. Like others here, I always forced myself to wait until June to shoot any so that I would not leave starving babies in the hole.
I went to Ohio to shoot groundhogs with a commercial guide service (Mike at North Central Ohio Varmint Express) and learned a LOT about groundhog biology and behavior. We also shot quite a few groundhogs.
One thing I learned is that nursing females stay in the den with the babies until they are weaned. When she finally comes out, they come out with her and within a few days, she will run them off, forcing them to leave her territory.
I never knew any of that.
The process is that in late summer, the female groundhog will fatten herself up and will also put away dried grass ("hay") in a spare chamber in the multi-chambered burrow. When the female goes down for the winter, she has not yet been bred.
In February and early March, males come up and look for females (hence 'Groundhog Day'), visiting burrows and sniff-checking for eligible females. When the male finds a female, he will mate with her in her den, then leave her, moving on to look for other females.
The fertilized female will sleep during gestation and when the babies are born, she will live on accumulated fat reserves and the dried grass she has put away. She stays underground with the babies until they are weaned.
This means that the groundhogs one sees early in the season are most likely either males or dry females. Killing either of them then has no effect on the young of the year or nursing females.
In late May and early June around here (MD/PA/VA/WV), the mothers and babies appear and the little guys are fully weaned and independent.
Mike books trips almost year-round and when setting up dates, I asked him about shooting them too early, which led to him explaining all this. He does this for a living and would probably like to conserve his resources by not killing nursing females.
I didn't fully believe this at first, but after my trips with Mike, I started shooting groundhogs here as soon as they started appearing in the spring and found them to be either males or dry females. I never got a female with signs of nursing (swollen mammaries/no hair around nipples) until I started seeing young ones out too.
I really regretted my previous habit of waiting, because by the time June rolls around, a lot of the grass and vegetation is getting tall enough to make spotting them difficult. I had passed on MANY opportunities to hunt them early.
