Research priorities outlined in 1996 by Murphy (200) included increasing our knowledge of (1) the distribution, ecology and mortality of Eastern Kingbirds in the nonbreeding season, (2) molt and plumages, (3) extra-pair mating systems, (4) timing of copulations, (5) proportion of birds to breed in their first breeding system, (6) whether or not pairs migrate or even possibly overwinter together, and (7) why clutch size and egg size vary geographically.
Many of these questions have been at least partially answered in the succeeding 20 years. Most Eastern Kingbirds make trans-Gulf migratory flights and most individuals are intratropical migrants, a better (but not perfect) understanding of molts and plumages exist, extra-pair paternity (but not intraspecific brood parasitism) is rampant, copulations almost certainly occur in the predawn darkness and females most likely initiate most extra-pair copulations. It appears common, in at least one population, that a high proportion of individuals do not breed at their first potential breeding season, and that floater populations of both sexes exist. Clutch size varies strongly with longitude, but unlike many species, is independent of latitude. Whether or not individuals, principally pairs, associate after breeding remains unknown.
Future research needs are many. For instance, although we have a much better grasp of migratory routes and destinations of Eastern Kingbirds, females have not been studied and therefore we do not know their paths and destinations and whether their departure dates and migratory speeds differ from males. Regardless of sex, current archival geolocator data suggest that birds along the east coast move through Florida whereas birds from populations in Nebraska and Oklahoma and further west enter and leave North America between Louisiana and east Texas; what of the birds west of the Appalachian Mountains and east of the Mississippi River? Preliminary data suggest low connectivity (high mixing) of overwintering individuals from different breeding populations, but more data are needed.
The winter ecology of Eastern Kingbirds is barely known. Other than switching to being a flocking frugivore that exhibits intratropical migration and roosts overnight in large flocks, we know nothing of their day-to-day lives. Why do most, but not all, individuals occupy two sites? Of those that move, migration within South America is not synchronous; what determines when an individual moves to its second tropical wintering site? Are they driven by external factors such as timing of rainfall or fruiting or individual factors such as age, sex or body condition? Do individuals exhibit site fidelity to wintering locations? Do individuals, when resident in 1 of their 2 semi-permanent overwintering sites, leave roosts to reclaim foraging territories, or do they remain as foraging flocks that move across the landscape in directions that change daily? Does most nonbreeding season mortality occur during migration or is the time in South America also a period of high mortality?
While our grasp of their mating system has grown immensely, we still do not know why extra-pair paternity is so rampant. This is not a question unique to Eastern Kingbirds, but kingbirds would be an excellent system for attempting to determine whether frequent extra-pair copulatory behavior by female passerines is a trait expressed because of genetic covariance in male-female behavior and strong sexual selection on males to seek copulations, or, is truly an adaptive behavior that benefits female fitness. This question can probably only be settled by determining whether extra-pair young are more likely to recruit than within-pair young, and it will require use of a study site much like Malheur National Wildlife Refuge; an ecological island to which young show high site fidelity. Furthermore, it is still an assumption that female Eastern Kingbirds move to males in the dawn song period to initiate copulations; confirmation is needed through the use of radio telemetry. Radio telemetry would also be applied usefully to the question of whether territory prospecting occurs among failed breeders late in the breeding season. Breeding dispersal data indicate that individuals move to better locations in the year following reproductive failure (15), but it is unclear exactly when the assessment of potential new territories is made.
Although patterns of geographic variation in clutch and egg size are now well described, satisfactory explanations remain elusive. The hypothesis that more frequent rainfall in the east favors smaller clutches remains the most likely explanation, but assessment of daily variation in food availability across their breeding distribution is needed. Geographic variation in egg size is very strong and is largely unrelated to body size, nor does it appear to reflect trade-offs with clutch size. This leaves the possibility that the physical environment favors different sized eggs across the geographic range. For instance, large eggs are laid at high latitudes where the threat of lethal chilling of eggs is greatest; large eggs may reduce rates of heat loss while females are off the nest during incubation. But differential rates of water loss may also be a factor. The largest eggs are generally produced in the northwest part of the range where elevations are greatest and atmospheric pressure is consequently lowest. The west is also dry compared to the east and the combination of low atmospheric pressure and low vapor pressure density (i.e., high aridity) would generate high rates of water loss that may favor large eggs with extra albumen (and thus water) to avoid embryo death. Examination of geographic variation in egg shell structure and egg composition, along with well-designed experiments, are needed to resolve these possibilities.
Lastly, and probably most important, why is the Eastern Kingbird declining across virtually its entire breeding geographic range? Answers will not be easily obtained, and must start with whether limitation occurs on the breeding or wintering ground, or possibly from events occurring during migration. Comparatively speaking, Eastern Kingbirds exhibit high adult survival for a socially monogamous and migratory passerine (148; MTM, unpublished data). Likewise, first year survival is relatively high (148). These facts suggest that the principal cause may lie in events happening on the breeding ground. In the east it seems in part related to farm abandonment and habitat succession, but this seems less likely in the middle of their range. Habitat loss to intensive agriculture may be the driver, but in both locations low annual offspring production resulting from high rates of nest loss to predators remains a possibility. Further research is needed in additional sites to establish whether nest predation rates are uniformly high across the range of the Eastern Kingbird, and whether or not reduction of nest predator populations would reverse local population declines as seen in central New York (190) and eastern Oregon (MTM, unpublished data). On the other hand, populations of aerial insectivores of many species are declining in North America, especially in the northeastern U.S. and southeastern Canada (28). Eastern Kingbirds are exhibiting some of their sharpest declines in that same reason. Concerns over declining insect populations (201, 202, 203) and therefore food supplies for insectivorous birds need further attention as a possible contributor to population declines of Eastern Kingbirds.