In an era of concern about the environment, the National Audubon Society has developed an investment tool that permits individuals to make donations for the Audubon's cause but still earn income for life.

Moreover, Audubon's Life Income Fund also may generate substantial tax savings for the investors involved.

Started several years ago, the fund has received little publicity. But nearly three-quarters of a million dollars have been invested to date, mostly by members of the non-profit society.

Founded in 1905, the non-profit National Audubon Society seeks to protect wild birds and animals and operates wildlife refuges across the country.

The new Life Income Fund was designed to help the society build a future endownment with funds earmarked to help acquire and maintain sanctuaries.

According to Frances Breed, vice president of Audubon, the income fund receives gifts of money and securities from donors - irrevocable gifts under the Internal Revenue Code.

In return, individuals receive "units of participation" in an investment pool managed by Morgan Guaranty Trust Co., of New York as fund trustee. The gifts are invested with all other gifts in the pooled fund by Morgan.

The number of units an individual receives depends on the value of the gift and the value of the fund at that point. Part of the fund is invested in stocks and the value of the fund may appreciate; annual income from the investments is distributed to the donors as long as they live.

During the year in which a gift is made, donors also may claim a charitable deduction on their federal income tax returns with the amount determined by Internal Revenue Service tables that discount the amount of the gift by the value of retained interest income. If the income is payable to one individual, there will be no federal estate tax on the pooled, income units.

Breed said the fund seeks an annual return of between 7 percent and 8 percent and was earning 7.57 percent at the end of December. In addition to common stocks, the fund invests in fixed-income securities and convertible stocks.

A minimum contribution is $1.000 and Breed said some persons have donated more than $100,000. A doctor on the West Coast is giving $10,000 a year and one secretary is giving $1,000 a year, she said.

The Audubon fund was one of the first of its type set up by a non-education institution and Breed described it "as an ideal vehicle for persons with middle-wealth and for singles." The fund's promotional emphasis is one of giving to the Audubon Society, "and not as a tax-loop-hole or hedge against inflation," she stated.