Wild bird feeding is a popular and growing activity, with approximately half of households participating in nations including the United States, Canada, the United Kingdom, New Zealand, and Australia. Supplemental feeding can increase survival and reproductive success of birds (which is often a goal of supplemental feeding efforts), but it raises concerns about phenological mismatch and possible increased risk of nest predation. Our objectives were to test whether access to supplemental food during the breeding season was correlated with (1) advanced egg-laying phenology, (2) increased clutch size, or (3) improved nest survival of 3 cavity-nesting species (Carolina Chickadee [Poecile carolinensis], Black-capped Chickadee [P. atricapillus], and Eastern Bluebird [Sialia sialis]) at a large spatial scale (the United States and Canada; spanning 22° latitude and 55° longitude). We examined data from 24,528 nest attempts submitted to NestWatch from 2014 to 2019. For Eastern Bluebirds, birds with access to supplemental insects initiated clutches 5.83 ± 0.89 (mean ± SE) days earlier than birds without access to food subsidies, whereas predicted nest survival was 5% greater for birds with access to supplemental insects (probability of nest success = 0.79, 95% CI: 0.77–0.81) than for birds without (0.74, 95% CI: 0.72–0.75). Clutch size of Eastern Bluebirds did not differ between birds with and without access to supplemental insects. For chickadees, supplemental feeding (of seeds, suet, insects, or fruit) was not correlated with phenology, clutch size, or nest survival. Our results suggest that supplemental feeding of insects can be an effective tool for increasing nest survival in Eastern Bluebirds and potentially other food-limited insectivores. However, the efficacy of supplemental feeding for improving reproductive success varies across taxa, likely related to diet. Despite widespread feeding of Black-capped and Carolina Chickadees, supplemental food had no discernable effect on timing of egg-laying, clutch size, or nest survival.
LAY SUMMARY
Feeding wild birds is a popular household activity in North America, but it can have positive and negative consequences for birds.
We looked for effects of wild bird feeding during the breeding season on three key aspects of reproduction: timing of egg-laying, clutch size, and survival of nests.
Our 6-year study focused on three species commonly fed across most of their breeding range: Black-capped Chickadee, Carolina Chickadee, and Eastern Bluebird.
We found that Eastern Bluebirds with access to human-provided food started laying eggs earlier and had better nest survival than those without, but clutch size was unaffected. Access to food was unrelated to the three reproductive measures for either chickadee species in our study.
Our results suggest that reproductive impacts of feeding birds vary by species, likely due to dietary differences.
Supplemental feeding during the nesting season may only be a viable conservation strategy for food-limited populations.
