Eastern Kingbird Breeding Ecology

I spent three years at Portland State University working on my M.S. research. I originally came to PSU to study Prairie Warbler wintering ecology in the Bahamas. However, funding never materialized and therefore I chose somewhere closer for my field work. I ended up spending three breeding seasons at Malheur National Wildlife Refuge, one of the most beautiful places in the country. Because of my strong interest in wintering ecology and annual cycle ecology, I wanted to select a topic that involved thinking outside of the breeding season alone. The main goal of my M.S. research was to uncover the reproductive consequences of spring migration timing for both male and female kingbirds.

I have also always been fascinated by the idea that non-territorial birds or "floaters" might represent an alternative breeding strategy. I first sought to determine if there was a floater population of kingbirds at Malheur, through the use of removal experiments. If one existed, I also wanted to figure out why it existed and whether floaters were able to successfully reproduce through extra-pair copulations.

Age- and sex-dependent spring arrival dates of Eastern Kingbirds

In nearly all species of migratory birds, males arrive to the breeding grounds before females, and older birds before younger birds. Early arrival by older birds may be driven by experience, age-dependent changes in body condition, or age-dependent access to resources on the wintering grounds. Earlier male than female arrival (i.e., protandry) has typically been explained by male competition for high-quality territories. However, females also reap benefits from high-quality territories and therefore I sought out other more complete explanations. One hypothesis that seemed likely in kingbirds, because of their very high extra-pair copulation rate, is the mate opportunity hypothesis. This hypothesis suggests that male-male competition for mates favors earlier male arrival because it provides males with priority access to females and more time to pursue extra-pair copulations later in the season.

I used four years of observational data on kingbird migration timing to describe basic age- and sex-based patterns and also to begin to explain why these patterns exist. I found older birds arrived 5-6 days before younger birds, and males arrived 4-5 days before females.

Ultimately, I think that older birds may have less to gain from early arrival because they are likely to be competitively subordinate in obtaining high-quality territories upon arrival. I also argue against several common hypotheses and conclude that the mate opportunity hypothesis is most likely to explain protandry in kingbirds. This research was published in the Journal of Field Ornithology in 2009.
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Male and female standardized arrival dates for second year (SY) and after-second year (ASY) Eastern Kingbirds from 2004-2007.

Reproductive correlates of spring arrival date in the Eastern Kingbird

Harsh weather in spring presents energetic challenges to birds during migration and upon reaching the breeding grounds, and yet, birds of most species arrive well before breeding begins. Evolutionary theory predicts that benefits should exist to outweigh these costs, and indeed, early-arriving birds often experience the highest reproductive success. Early-arriving males may be more likely to acquire a mate, acquire higher quality mates, gain priority access to the best territories, breed early, have more opportunities for extra-pair copulation, and have more time to re-nest should early nesting attempts fail.

I studied a breeding population of Eastern Kingbirds from 2004 to 2007 to determine what the reproductive consequences of early arrival were. I found that early arriving kingbirds were more likely to acquire a high-quality territory and to replace nests after failure. Early-arriving birds also bred earlier and early breeding led to larger clutches and greater production of young. Early-arriving males also sired more extra-pair young than later arriving males.

My data suggests that arrival date is in part influenced by individual quality, and that arrival date has reproductive consequences, with the primary benefits of early arrival being the acquisition of high-quality territories, early breeding, and increased probability of replacing failed initial nests. This research was published in the Journal of Ornithology in 2011.
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Male Eastern Kingbird. Photo taken by Karen Williams

Density-dependent age at first reproduction in the Eastern Kingbird

Theory predicts that maximal fitness is obtained by individuals who begin to breed immediately upon reaching sexual maturity. However, delayed breeding occurs regularly in some species of birds and mammals, especially those with long lifespans or those that have limited access to suitable habitat. In general, delayed breeding is not expected in short-lived migratory passerines, but this assumption is largely untested.

I used a combination of observational and experimental approaches to document the prevalence and investigate the causes of delayed breeding in Eastern Kingbirds. Due to its extreme geographic isolation, we have observed a large number (~30%) of young return to Malheur National Wildlife Refuge each year. Nearly half of the fledged nestlings that returned to the breeding grounds did not breed in their first potential breeding season. Parentage analysis failed to show any reproductive success for female floaters and only limited success for male floaters, indicating that floating was not a successful breeding tactic. On the other hand, we did find a strong negative relationship between population size and the proportion of young birds that bred in their first year. Moreover, floaters quickly filled territory vacancies created when I experimentally removed adults.

Together this indicates that limited breeding habitat and the territorial behavior of conspecifics appears to be the main cause of delayed breeding in Eastern Kingbirds. The prevalence of delayed breeding in most species is unknown, but nonetheless potentially important because failure to incorporate accurate estimates of age at first reproduction in population models may lead to flawed population projections. This research was published in Oikos in 2009.
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Relationship between population size (# breeding pairs) and proportion of the second-year male (SY) population to that bred in their first year.