Townsend P, Dickinson 



A female hooded warbler, left, arrives at the species' breeding 

 grounds about a week later than the first males. For an early spring 

 male migrant, below, a still-bare branch in New York City 's Central 

 Park provides a perch fi^om which to dart out and catch insects. If 

 males return too early, cold and scarcity of insects can be deadly. But 

 if they arrive too late, all the best territories will be taken. 



as a hooded warbler's — although some do 

 not come very close to the descriptions in 

 standard field guides — yet each has at 

 least a few details that make it characteris- 

 tic also of the individual. 



The ability of male songbirds to dis- 

 criminate the fine, individual differences 

 in the songs of rivals, both known and 

 new, was established through experiments 

 several decades ago. Our experiments 

 with hooded warblers in the Mason Re- 

 serve since 1987 have demonstrated that 

 these birds are even more discerning. A 

 male hooded warbler can recognize the 

 songs of each one of his neighbors and can 

 also learn their usual locations in relation 

 to his territory. To demonstrate this abiUty, 

 we chose twelve hooded warblers from the 

 Mason Reserve and adjoining woodlands 

 as study subjects. First, we played a tape 

 recording of a neighboring warbler's 

 songs just inside a subject's territory near 

 the boundary shared with that neighbor 

 Then we broadcast the same tape, also just 

 inside but now on the opposite side of the 

 subject's territory, near a boundary shared 

 with a different neighbor (Because in an 

 experiment of this sort, the order of pre- 

 senting the two playbacks might influence 



the results, we played neighbors' songs to 

 half of the subjects in reverse order) Sub- 

 jects often quickly approached the speaker 

 and searched frantically for the apparent 

 invader However, our subjects responded 

 much less vigorously to neighbors' songs 

 coming from the expected direction than 

 to the same songs emanating from the op- 

 posite direction. Hooded warblers, then, 

 know each neighbor's songs, and know 

 just where they should come from. To our 

 subjects, a playback of a neighbor singing 

 on the wrong boundary signaled a serious 

 territorial invasion. 



Many ornithologists have noticed that 

 former neighbors returning from winter 

 quarters act like dear enemies right from 

 the start. As with our red-banded male that 

 early spring morning, returning males are 

 more likely to dispute boundaries with 

 new birds. Do returning neighbors just re- 

 member old boundaries, or are they ca- 

 pable of remembering one another's 

 songs? The latter feat would be remark- 

 able: the birds have had no chance to hear 

 the songs for more than six months. They 

 do not sing for most of the winter We also 

 know that hooded warblers from the 

 Mason Reserve do not migrate together 



because they do not arrive at the breeding 

 grounds together Nor, presumably, do 

 they winter together in Mexico and Cen- 

 tral America. 



To test song memory, we duplicated the 

 experiments just described, with an added 

 element. We started our tests on the very 

 day a male appeared in April on his previ- 

 ously occupied territory. Familiar songs of 

 neighbors from the year before, played 

 near the old boundary, elicited little re- 

 sponse; to our subjects they must have 

 sounded like an old friend back in his 

 usual place. In contrast, the same songs 

 played near the "wrong" boundary evoked 

 a strong response — a quick approach and 

 frenetic searching. Male hooded warblers 

 do, in fact, remember each neighbor's 

 songs from one year to the next. These 

 birds provide one of the few demonstrated 

 cases of long-term memory in a nonhu- 

 man vertebrate. This abiUty has important 

 practical consequences for a hooded war- 

 bler By returning to precisely the same 

 territory year after year, a male can expect 

 to avoid "bargaining" for boundaries with 

 about half of its neighbors. The time and 

 energy thus saved can be used to deal with 

 other neighbors and to attract and court a 

 female. 



A male reacts strongly to a trespass into 

 its territory, a transgression that amounts 

 to an abrogation of a mutually accepted 

 treaty. Does such a trespass have conse- 

 quences beyond a chase by the subject 

 male? Evolutionary theory predicts that it 

 should. A dear enemy relationship in- 

 volves reciprocal respect for an arbitrary 

 boundary. Such reciprocity in a potentially 

 exploitative relationship can persist when 

 rivals play tit-for-tat. Rivals must recog- 

 nize each other individually, so they can 

 keep track of each other They also must 

 interact repeatedly over an indefinite pe- 

 riod of time, so neither can take advantage 

 of the other on their last interaction. Fi- 

 nally, each must retaliate whenever the 

 pact is broken. Our warblers met the first 

 two conditions, and we devised another 

 test to determine if trespass provoked re- 

 taliation by the offended male. 



We first presented a neighbor's songs 



39 



