Rainfall and nest site competition delay Mountain Bluebird and Tree Swallow breeding but do not impact productivity

Optimizing breeding phenology, an important aspect of fitness, is complex
for migratory species as they must make key timing decisions early, and
remotely, from breeding sites. We examined the role of weather (locally
and cross-seasonally), cavity availability, and competitive exclusion in
determining among-year variation in breeding phenology over 17 years for
two migratory, cavity-nesting birds: Mountain Bluebirds (Sialia
currucoides; n = 462 nests) and Tree Swallows (Tachycineta bicolor;
n  = 572) using natural tree cavities in British Columbia,
Canada. We assessed weather effects within the winter and migratory range
and at our study sites. We quantified competition as the proportion of
cavities occupied by European Starlings (Sturnus vulgaris) (for both
species) and Mountain Bluebird (for Tree Swallow only) in each year. For
229 bluebird and 177 swallow nests with known fates, we tested whether
late years resulted in reduced productivity. Although the effects
were small, heavy rainfall and strong diurnal westerly winds during
migration were associated with breeding delays for Mountain Bluebirds.
However, cavity availability (earlier breeding with increases) had a 5-8X
greater effect on timing than migratory conditions. There was no evidence
that starling competition delayed bluebirds. In Tree Swallows, greater
local daily rainfall was associated with delayed breeding, as was starling
abundance (the effect of starlings was 1.4X times smaller than that of
rainfall). Neither bluebird abundance nor cavity availability changed
swallow phenology. Neither species showed reduced productivity in late
breeding years. In both species, individuals that bred late relative to
conspecifics within-year had smaller clutches and greater probability of
nest failure.  We conclude that breeding ground
conditions, particularly cavity limitation and local rainfall (for
swallows), are important drivers of breeding phenology for our focal
species, but that the productivity cost of late years, at least
for Tree Swallows, is minimal.