Millions of waterfowl (水禽) were killed at the hands of market hunters and a handful of overly ambitious sportsmen.

本句抄自 2021 年新高考英语全国乙卷（新全国卷Ⅰ）阅读理解C。

我不太明白其中的 market hunters 是什么意思，另外 ambitious sportsmen 好像用在这里也比较突兀。请专家老师解释一下，谢谢！

附原文（方便从上下文理解）：

When the explorers first set foot upon the continent of North America, the skies and lands were alive with an astonishing variety of wildlife. Native Americans had taken care of these precious natural resources wisely.

Unfortunately, it took the explorers and the settlers who followed only a few decades to decimate a large part of these resources. Millions of waterfowl (水禽) were killed at the hands of market hunters and a handful of overly ambitious sportsmen. Millions of acres of wetlands were dried to feed and house the ever-increasing populations, greatly reducing waterfowl habitat (栖息地).

In 1934, with the passage of the Migratory Bird Hunting Stamp Act (Act), an increasingly concerned nation took firm action to stop the destruction of migratory (迁徙的) waterfowl and the wetlands so vital to their survival. Under this Act, all waterfowl hunters 16 years of age and over must annually purchase and carry a Federal Duck Stamp. The very first Federal Duck Stamp was designed by J.N. “Ding” Darling, a political cartoonist from Des Moines, Iowa, who at that time was appointed by President Franklin Roosevelt as Director of the Bureau of Biological Survey.

Hunters willingly pay the stamp price to ensure the survival of our natural resources.

About 98 cents of every duck stamp dollar goes directly into the Migratory Bird Conservation Fund to purchase wetlands and wildlife habitat for inclusion into the National Wildlife Refuge System—a fact that ensures this land will be protected and available for all generations to come.

Since 1934, better than half a billion dollars has gone into that Fund to purchase more than 5 million acres of habitat. Little wonder the Federal Duck Stamp Program has been called one of the most successful conservation programs ever initiated.