Delayed dispersal in splendid fairywrens

Delayed dispersal is a critical first step in the formation of both cooperative breeding and family-living groups. However, disentangling the multiple, potentially co-occurring factors that influence the dispersal of young individuals is often difficult and can require multiple data sources. We combine data from a long-term mark-recapture study and field experiments to address the patterns, fitness consequences, and proximate causes of delayed dispersal in the splendid fairywren (Malurus splendens melanotus). This species is a cooperative breeder, with males remaining as helpers more often for more years and settling closer to their natal territories than females. We found that birds who were helpers in one or more years had greater first-year nest success, and the number of years spent helping had a positive impact on survival. By experimentally producing breeding vacancies for young males, we tested if male-biased sex ratio and breeding opportunities constrained dispersal in this system. Only half of the vacancies created were filled, and dispersal took, on average, approximately three days. These findings illustrate that ecological constraints, benefits-of-philopatry, and life history benefits may all be acting within the same system to shape dispersal patterns.

Methods
This data comes from a long-term study of color-banded splendid fairywrens (Malurus splendens melanotus) that has been conducted at Brookfield Conservation Park since 1992, and contains both an observational study of natural behavior and an experimental study.

Natural dispersal behavior

We assessed the natural dispersal behavior of splendid fairywrens hatched between 2004 and 2018, the longest uninterrupted stretch of monitoring in this population. Observations on the dispersal behavior of these individuals were conducted between 2005 and 2020. Only young banded as nestlings or as fledglings that were resighted in their first full breeding season (an age status which we refer to as first-year) were included. We categorized the first-year social status of each known-age offspring as either breeders or helpers. For each individual, we also categorized the first-year territory location. First-year territories could either be 1) near—in the same territory that they were hatched in or in an immediately adjacent territory, a proxy for natal philopatry or 2) distant—a territory that had one or more territories in between it and their natal territory. For females, the proportion of young who remain near their natal territory is likely an overestimate of the true values within the population as many females disperse out of the study population, and their final breeding location is never known (Rowley and Russell 1997; Pruett-Jones et al. 2010).

For a subset of individuals that persisted in the population beyond their first year, we also collected the following data: the number of years (if any) spent as a helper, the age at which they first bred (if applicable), the location of first breeding (near or distant, if applicable), their first-breeding-year success, and the number of years present in the population. First-breeding-year success was the success of a nest in the bird’s first season as a dominant individual and was determined by whether the bird’s social group produced a nest that progressed to nestlings of 6-8 days old, the age at which we banded nestlings. 

Experimental studies of male dispersal behavior

Breeding opportunities were created for 14 male splendid fairy-wrens (eight in 2017, six in 2018). For 13 of these removals, a single male was removed (group size of the focal territory of two), and for one experiment, two males were removed (a dominant male and a subordinate male). For each experimental group, we monitored the remaining female and the territory site for two hours immediately following the removal of the breeding male(s) and again in the afternoon on the day of removal. If no dispersal occurred on the day of the removal, we continued to monitor the territory twice each day until the vacancy was filled or two weeks had passed. After two weeks, we continued to check on the experimental territory every other day. There were three possible outcomes of each experiment: 1) a dispersal occurred, 2) a territory merger occurred in which an adjacent social group expanded its territory to include the focal territory and the remaining female, or 3) the remaining breeding female held the territory alone or left the area (collectively referred to as ‘solitary’). For each experiment, we recorded the outcome and the latency to dispersal if a dispersal occurred.  Latency to dispersal was documented as the number of hours till a dispersal was observed, rounded to the nearest half-day after the first day following removal. We also documented whether males from surrounding territories visited the experimental territory to establish if neighboring birds were aware of the breeding vacancy. No males who ultimately dispersed into breeding vacancies were included in the natural history data.