Related paper: Postnatal dispersal and drivers of successful recruitment in resident Mountain Chickadees (Poecile gambeli) by Lauren E. Whitenack, Ai Ana Richmond, Benjamin R. Sonnenberg, Joseph F. Welklin, Virginia K. Heinen, Angela M. Pitera, Sofia M. Haley, Carrie L. Branch, and Vladimir V. Pravosudov. Ornithology.
Being a fledgling is difficult—and dangerous. The vast majority (~80 percent) of young birds do not survive this stage, succumbing to predation or starvation. What might give a vulnerable fledgling a head start when leaving the nest? Once they leave, how far do they move between their hatch sites and first breeding sites, and why? These questions are critical pieces of natural history knowledge that are answered with carefully collected long-term data. Members of Vladimir Pravosudov’s Lab have studied Poecile gambeli (Mountain Chickadee) reproduction in nest boxes since 2013 as part of a long-term study on tagged individuals. In our latest paper, we used 12 years of data to uncover patterns of juvenile dispersal and recruitment.
Our Long-Term System
P. gambeli are highly resident songbirds with specialized spatial cognitive abilities that allow them to survive harsh winters in montane coniferous forests of the North American West. They use these abilities to recover tens of thousands of food caches hidden in bark crevices, between bits of lichen, or in clusters of evergreen needles.
In chickadees, juvenile recruitment occurs in two stages: recruitment into winter flocks and recruitment into the breeding population. Both stages are filled with obstacles. To recruit into a winter flock, a bird must survive the fledging stage, and to recruit into the breeding population, a bird must also survive its first winter. After dispersing in late summer, chickadees settle in permanent locations and do not make additional dispersal movements.
The Pravosudov Lab studies P. gambeli in the northern Sierra Nevada mountains, California, United States, at a high elevation site (2,380–2,590m; 7,800–8,500ft) and a low elevation site (1,965–2,070m; 6,500-6,800ft). These sites are located across an approximate snow line, where precipitation tends to fall as snow above this line and as rain below this line during transitional months.
We track reproduction in nest boxes starting at the nest-building stage. In most years, this involves hiking for miles in deep snow at high elevations beginning in late April. We record laying and hatching dates, and band and weigh all nestlings when they are 16 days old (immediately before fledging at 20–24 days).
During the autumn months, we maintain a series of bird feeders across our sites which we use for target mist-netting of chickadees. All birds captured as adults (either in mist nets or breeding in boxes) are fitted with a unique combination of color bands and a passive integrated transponder (PIT) tag, which allows birds to interact with our radio frequency identification equipped feeders that we use in the winter for cognitive and behavioral testing in the wild.
Our dispersal and recruitment records come from recaptures of adult birds that were initially banded as nestlings from our nest boxes. These birds were then recaptured as adults either at feeders in autumn and winter, indicating recruitment into winter flocks; or breeding in nest boxes, indicating recruitment into the breeding population.
Early Breeding is Better
P. gambeli experience harsh conditions in the early breeding season, including high snowpack, cold temperatures, and limited insect food. Despite these harsh conditions, we previously found that chickadees that laid eggs earlier tended to have higher reproductive output at both higher and lower elevations.
Why are birds pushed to breed early? Does early fledging also give young individuals an extra “boost,” increasing their chances of surviving the fledging period?
Over the last 12 years, we have banded >5,200 nestlings and recaptured 326. This has allowed us to compare the characteristics of birds that survived the fledging stage to those of individuals that were never recaptured (and presumed to have not survived). We found that birds that hatched earlier were more likely to survive and recruit, both into winter flocks and into the breeding population. We also showed that birds that hatched earlier tended to disperse shorter distances to their breeding territories compared to birds from later nests.
Earlier fledging may give young birds a leg up for a couple reasons. Juveniles that fledge early have more time to settle, molt, and prepare for winter. Overwinter mortality of juveniles is high in this system, and any extra time for preparation may be critical. Additionally, early birds may benefit from claiming territories closer to their natal site with more familiar ecological and environmental conditions.
Nestling Condition and Survival
While fledging earlier may give fledglings a boost, slight differences in body mass may also be critical for surviving the fledging period. We found that heavier fledglings were more likely to recruit, both into winter flocks and into the breeding population. Individuals that survived and recruited into winter flocks weighed just 2–3% more (on day 16) on average compared to those that were never recaptured.
Our findings echo previous studies demonstrating that heavier fledglings are more likely to survive and recruit, likely because they leave the nest in better physical condition. Therefore, parental investment (via supplying food to their young) is a major factor affecting offspring survival and reproductive fitness.
Dispersal Distances and Sex
P. gambeli in our study generally dispersed very short distances (median: 644m; 2,100ft). Males dispersed shorter distances than females, based on both feeder data and nest box recapture data. This pattern is common among Paridae (tits, chickadees, and titmice) species and other songbirds that tend to settle close to their hatching sites. In chickadees, males are socially dominant over females, which may allow them to claim closer sites with more familiar conditions. Additionally, it is possible that males and females have evolved to disperse slightly different distances to avoid inbreeding.
Final Take-Aways
Surviving as a young chickadee is a gamble—but your chances are better if your parents invest in raising you earlier in the season and provide you with plenty of insect food. Once you leave the nest, you may want to try to claim a territory closer to your hatching site, since dispersing across and settling in unfamiliar habitats can be risky. In the future, with more samples, we hope to identify whether birds that disperse closer to the natal site have higher reproductive output, resulting in a fitness advantage. Our results provide a natural history foundation for future studies on chickadee dispersal, recruitment, and evolutionary processes.