An as yet unpublished study has added to mounting evidence that the majestic bald eagle, the US national symbol, is making a comeback.
But at the same time President Reagan has rescinded an executive order banning the use of Compound 1080 - a poison that has killed several eagles. The move has enabled the US Environmental Protection Agency to open hearings to reregister the chemical as a means to combat coyote attacks on sheep herds.
The encouraging news about the growth in the eagle population comes in a new study entitled ''The Ban of DDT and Recovery of Bald Eagle Production'' by Dr. James Grier, associate professor of zoology at North Dakota State University in Fargo.
Dr. Grier's study takes three significant conclusions:
* There is a firm relationship between the ban on the pesticide DDT and improved reproduction rates in bald eagles. While this relationship has long been suspected by ornithologists, Grier says his study provides the conclusive evidence lacking until now.
* The ban on DDT is valid and effective. The study shows that as the presence of DDT in eggs dropped, the number of young hatching in each nest increased.
* The recovery of bald eagles from the effects of DDT is much more rapid than expected. A study released in 1980 showed that DDT in soil and earthworms did not diminish over an 11-year period, meaning scientists could not begin to calculate how long it take to break down. Grier says his findings show that in some way eagles are reducing their DDT levels significantly in only a few years.
This year the US is celebrating the 200th anniversary of this mighty bird of prey as its national symbol. President Reagan has declared this the ''Year of the Eagle'' and June 20 as ''National Bald Eagle Day.''
On that day in 1782, the Continental Congress chose an eagle clutching the arrows of war in his left talon and the olive branch of peace in his right for the Great Seal of the United States. Today, the Great Seal is perhaps the most recognizable symbol of the US with the exception of the American flag.
Grier's findings confirm the results of the latest National Wildlife Federation (NWF) bald eagle census, released earlier this year. The NWF counted some 3,000 of the birds in 1976 and 13,709 in 1981.
Grier, who serves as team leader for the Northern States Bald Eagle Recovery Plan, checked about 200 bald eagle nests in Ontario, Canada over a 16-year period. He found that since the banning of DDT on Dec. 31, 1972, the number of eaglets born in each breeding area, or nesting territory, has rising steadily - from a low of 0.46 young per breeding pair in 1974 to 1.12 in 1981.
He also was able collect 19 unhatched eggs that showed diminishing amount of DDE, the most harmful residue of DDT. DDE does not kill eagles outright, but weakens eggshells, which then break before the young can develop and hatch. The rise in births and the decrease in DDE show ''a definite inverse relationship when plotted as graphs,'' Grier says. No such relationship was shown with PCBs, mercury, lead, or several other substances for which he tested.
The findings do not mean, Grier says, that the bald eagle, an endangered species in 43 states, is no longer threatened. Loss of habitat, shootings by man , and accidental losses in beaver or fox leg traps, are continuing problems. Even acid rain, he says, could be just as important a factor as DDT as in reducing eagle populations. ''If acid rain wipes out fish in a certain area, it would remove the bald eagle's source of food,'' he says.
Grier is also concerned by the EPA hearings now taking place to reregister Compound 1080 as a predator control. The poison ''obviously would pose a threat to the birds,'' he says. ''It's possible that it could offset the gains we've seen in reproduction.''
Before being banned by President Nixon in the early 1970s, Compound 1080 was used in meat bait to control coyotes. An Jagle eating such bait could receive a lethal dose. Grier says little is known of the survival rate of adult eagles, but ''some computer modeling I've done indicates that changes in (adult) survival are more important than reproduction'' in determining whether the number of eagles increases.
Many environmentalists are concerned that President Reagan's decision earlier this year to rescind the executive order will have an adverse effect on wildlife , including ferrets, badgers, bobcats - and golden and bald eagles. ''There's no question that if an eagle consumes bait with 1080, it will die,'' says Robert Davison, legislative representative for the NWF. While he admits there is no conclusive evidence as to the degree of threat 1080 poses to wildlife, he says the burden of proof must fall with those who want it rfturned to use.
An EPA spokesman says ''a volume of new information'' has presented by wool growers and others at informal hearings prompted EPA administrator Anne Gorsuch to launch formal hearings on 1080. The hearings, conducted at seVeral sites around the country this spring and summer, are expected to conclude in August. A recommendation for or against reregistering 1080 will then be made to the EPA administrator. She is expected to announce her decision before the end of the year.