Black-throated blue warbler

The black-throated blue warbler forages actively in low vegetation, sometimes hovering or catching insects in flight. It often forages in one area for a while before moving on to the next. It mainly eats invertebrates such as caterpillars, crane flies, and spiders. It may supplement its diet with seeds, berries, and fruit in the winter.[9]

Males and females prefer different foraging sites. While males usually hover among the higher shrub foliage between 3 and 9 m (9.8 and 29.5 ft), females tend to forage at lower strata.[12] The time within a breeding season influences where the males forage. When it is time to feed the fledglings, males come down to the same foraging strata as females. The black-throated blue warbler mostly forages in the understory instead of the canopy.[14] The large leaves and long branches in the understory affect its foraging behaviors. The black-throated blue warbler more often hovers rather than gleans its prey because it is more difficult to glean among thick understory foliage.[14]

Typical nest site

The black-throated blue warbler is a monogamous species.[15] Its breeding season usually begins in May and ends in July.[16] As a songbird, the male black-throated blue warbler attracts a female's attention by singing a soft melody. He then follows the female while she is foraging or searching for nesting sites. As soon as the female stops to rest, the male droops his wings slightly, stretches his head forward and up, opens his bill, and faces the female. The female also makes displays to the male by vibrating her wings. In response, the male mounts the female for 2–3 seconds and then flies off.[11]

A 1996, researchers showed that the black-throated blue warbler prefers to reside in hardwood forests with higher shrub densities where food is more abundant compared to lower shrub density plots. Within these high shrub density habitats, not only is there a higher density of warblers, but the population age average is also older, being composed of males and females who are at least two years of age.[17]

The black-throated blue warbler uses social cues in its evaluation and choice of nesting sites.[16] In particular, it listens to the post-breeding songs given out by other males. These songs have strong temporal dependencies. Males sing at the beginning and the peak of breeding season, but songs are not indicative of reproductive success. Near the end of a breeding season, a male that has successfully mated continues to sing while a male that has failed to reproduce abandons the habitat. Therefore, post-breeding songs are reliable indicators of reproductive success within the particular habitat and convey essential information to the natal and breeding dispersers. In comparison to the traditional idea of direct assessment of the vegetation structure, the vocal cue is much more efficient and easier to obtain, hence revealing the advantage of social communication in survival and reproduction. A female, however, does not respond to post-breeding songs directly. Instead, she is likely to rely on the presence of males in deciding nesting sites.[16]

Although the black-throated blue warbler is a socially monogamous species, males are frequently observed in territories of other males, suggesting the occurrence of extra-pair matings.[15][18] Nestling parentage is identified by microsatellites in a study plot at the Hubbard Brook Experimental Forest in New Hampshire.[19] The results show that extra-pair fertilization occurs and that the majority of the extra-pair sires come from males in neighboring territories. Only very few extra-pair sires are from distant territories. This local reproductive interaction is also supported by another study conducted earlier, which finds that extra pair fertilizations are strongly and positively correlated with local synchrony but there is no significant association with population level synchrony.[15]

Males engage in mate guarding during the period females are most vulnerable to successful extra pair copulations. They usually stay close to their social mate, singing slowly on the side and following the mate while she is foraging or searching for a nesting site.[11] The guarding behavior, though, may conflict with males' pursuit of extra-pair fertilizations. It is not yet clear to what extent a male will prefer mate guarding over extra-pair fertilizations.[15] Male retention studies have shown that removal of a male increases the chance of extra-pair offspring in the brood, suggesting that mate guarding reduces extra-pair fertilization attempts.[18] The extra-pair fertilization rate nonetheless cannot be eliminated even if males are allowed to stay near their social mates during fertility risk period. Several hypotheses try to explain this phenomenon: females may be able to manage extra-pair mating even while its social mate is guarding it, or females may reject extra-pair copulation attempts by other males in the absence of male guarding.[18]

Females who participate in extra-pair fertilizations may incorporate better genes in their offspring than they could get with their social mate, but they are likely to receive less help with parental care from their social mates because of cheating. Extra-pair fertilization, therefore, can be costly to females as well.[20] A possible theory why extra-pair fertilization occurs is that female organisms select males with overall high heterozygosity or dissimilar genetics from themselves.[21][22] A microsatellite study suggests an alternative to heterozygosity selection. Because no correlation is found between female extra-pair fertilization frequencies and the overall heterozygosity of their social mates, it is suggested that females may choose only a selective set of heterozygous genes, particularly the MHC locus, which can affect the immunocompetence of offspring.[23]

Males' differential recognition of local and nonlocal songs has been studied in two populations: one in the northern United States (New Hampshire) and the other in the southern United States (North Carolina).[24] An asymmetry of response has been found between the two populations. The northern black-throated blue warbler responds strongly to local songs but relatively weakly to the song of southern warblers. In contrast, a warbler from the south responds equally to songs from both the north and the south. A potential explanation of this asymmetry is the difference in female preference between the northern and southern black-throated blue warblers. Females from the north are less likely to mate with a "heterospecific" male from the south; therefore it is not necessary for a northern male to respond strongly to the song of a southern challenger. It is possible that a barrier to gene flow from south to north exists while a barrier to the reverse does not. Therefore, female choice of male songs is likely to play a role in gene flow and reproductive isolation, which may eventually lead to diversification.[24]

It has long been believed that a male black-throated blue warbler achieves reproductive maturation well into its first breeding season.[25][26][27] A yearling participates in extra-pair mating and cuckoldry as much as or even more than older males. However, research by Graves has found opposing evidence in terms of testicular size and sperm production.[28] Directional asymmetry is present in many passerine birds. The left testicle is often larger in size than the right one, and this holds true for both yearling and older male warblers. However, the testicle to body mass ratio nonetheless is much lower in yearlings than in older males. Moreover, older males have a greater degree of directional asymmetry than do yearlings. Because the size of testes in birds is correlated to the ejaculate quality, it is likely that females employ age-dependent choice in favor of older males who can be distinguished by their definitive age-specific plumage.[28]