32 ILLINOIS AUDUBON BULLETIN 
territories were honored. Craighead and Craighead (1956), however, stated 
that range peripheries overlapped to some extent, forming mutual hunting 
grounds. Hamerstrom and De La Ronde Wilde (1973) also reported some 
overlap. 
The two pairs of 1976 birds exhibited interesting social behavior. When 
| observed the nests, the males rarely were in the vicinity, approaching only 
to deliver food to their respective mates. On one occasion, however, the 
male from the east nest helped to defend it against my intrusion, then 
followed me and also defended the west nest site. The male from the second 
nest was absent at this time, but the female from the west nest joined in the 
defense. Breckenridge (1935) stated that the male hunts while the female 
guards the nest, but of ten nests observed by Hecht (1951), six were 
defended by the female, three largely by the male, and one only by the male. 
Yokum (1941) in providing evidence for polygamy, reported one male 
defending two nests 100 yards apart. The females defended only their own 
nest site. 
In 1976, each female usually defended only its respective nest site. On 
two occasions, however, the two females each helped to defend each 
other’s nest. Once the female from the west nest continued to defend into 
the area of the east nest, where it was joined by the other female. Once the 
east female followed me into the area of the west nest where the two females 
simultaneously defended the same nest site. Such combined defense by the 
females has not been reported previously. 
Whether the close proximity of the nests has a detrimental effect upon 
reproduction is unknown. The regularity of the habit would indicate that it 
does not. The two 1976 nests, however, probably fledged only a maximum 
of three young, far below the three or five from each of the previous, isolated 
nestings. 
Conclusions 
From 1971 to 1976, Northern Harriers at Goose Lake Prairie have 
nested erratically and have become a regular but tenuous part of the 
avifauna. The nestings have been confined to two general areas, and there 
may be some alternate year nesting. The two nests in 1976 were in close 
proximity to each other. This type of sociality has been reported previously 
ok this species, but combined defense of adjacent nest sites by the females 
as not. 
Acknowledgment 
| appreciate the help of Vince Mathrews, interpreter at Goose Lake 
Prairie State Park, in notifying me of the locations of the 1976 nests. 
