"Dear Enemy" Notes 



A neighbor's song means more than music to a hooded warbler's ears 

 by Renee Godard and Haven Wiley 



On a balmy. April morning, the bottom- 

 land hardwood forests near Chapel Hill in 

 central North Carolina are deceptively 

 peaceful. As we stand in the dense under- 

 story of arrowwood, with its pale new fo- 

 liage, two black-and-yellow sprites fly fu- 

 riously back and forth across an invisible 

 boundary. They are so oblivious to our 

 presence that they almost brush our legs in 

 passing. After about ten minutes of this 

 twisting through the arrowwood, the tiny 

 birds separate by some fifty yards. Each 

 takes a station just below the crowns of the 

 oaks and hickories and begins to belt out 

 its own version of a ringing song. The 

 black cowls over yellow faces reveal that 

 these rivals are male hooded warblers. 

 They have come to the forest to begin the 

 breeding season. 



One of the birds wears two lightweight, 

 red plastic bands on each leg. He was one 

 of the first males we banded for identifica- 

 tion here at our study site, the Mason Farm 

 Biological Reserve, and he has returned to 

 exactly the same location in this 370-acre 

 woodland for the fourth consecutive year, 

 an exceptional record. His rival, as yet un- 

 handed, is probably less than one year old 

 and is staking a territorial claim for the 

 first time. 



Like many migrating songbirds, 

 hooded warblers spend the winter in 

 warmer cUmes. They winter from Mexico 

 to Panama and begin to make their way 

 north in March. By April, they have 

 reached their summer quarters, which ex- 

 tend from the Gulf coast north to southern 

 Michigan and east to Connecticut. The 

 birds we have been observing have each 

 just returned from their Mexican and Cen- 

 tral American retreats and are now in seri- 

 ous competition for real estate. Their fe- 

 male counterparts will arrive in about five 

 to ten days. To attract a mate and eventu- 

 ally raise healthy young, each male needs 

 a territory of some twelve to twenty-five 

 acres of forest with a luxurious understory 

 of shrubs like arrowwood. The old-growth 

 bottomland forests in the Reserve are an 

 ideal habitat; each year five to ten hooded 

 warbler pairs nest here. Only about half of 

 these birds, however, survive the winter 



36 Natural History 5/94 



and round-trip migration from one year to 

 the next. The color-banded old-timers are 

 among the first to return, and each quickly 

 reclaims his former territory. In contrast, 

 newcomers ready to breed for the first 

 time must find an opening vacated by a 

 male that failed to return. This precise 

 "site-faithfulness" of returning males is 

 one of the remarkable features of migra- 

 tion for many songbirds. Why should 

 males not move from one year to the next? 

 After all, they might have settled for an in- 

 ferior territory the first year they bred; 

 surely some of them could upgrade their 

 location in a subsequent year. 



Part of the answer lies in the relation- 

 ships of neighbors. Male hooded warblers, 

 like many other male songbirds, have a 

 number of ways of dealing with rivals in 

 adjoining territories. The simplest interac- 

 tion of neighboring males is simply 

 singing within earshot of one another Our 

 systematic observations have shown that 

 the average male hooded warbler spends 

 55 percent of each early spring morning 

 just singing. When, on occasion, a male 

 meets a neighbor at a disputed boundary, 

 singing ceases and chasing begins, some- 

 times escalating to fighting. When the fe- 

 males arrive, aggression intensifies. Inter- 

 mittent chasing can last for two days 

 before both parties tentatively accept a 

 boundary. But once boundaries are estab- 

 lished, neighbors quickly develop a re- 

 spect for them. Males can then sing close 

 to the edge of their territory without pro- 

 voking an attack from a neighbor Such 

 apparent amicability does not, we have 

 noticed, prevent them from occasionally 

 venturing surreptitiously into one an- 

 other's territories. 



The birds have become what evolution- 

 ary biologists have termed dear enemies. 

 Instead of constantly battling, two individ- 

 uals appear to call a truce; while not be- 

 coming alUes, they can at least avoid con- 

 tinual contests. Our studies suggest that an 

 important factor of this detente is the 

 hooded warbler's ability to recognize a 

 neighbor's songs. Each male's repertoire 

 consists of five to ten stereotypical pat- 

 terns of notes. Each song is recognizable 



