Local conspecific density does not influence reproductive output in a secondary cavity-nesting songbird

Abstract

Density dependence is a conceptual cornerstone of avian population biology and, in territorial songbirds, past research has emphasized interactions among food limitation, density, and reproduction. Documenting the importance of density effects is central to understanding how selective forces shape life histories and population dynamics. During the 2008–2011 breeding seasons, we nearly doubled overall conspecific breeding densities on study sites, and manipulated nest box spacing to increase local breeding densities (defined as the number of pairs breeding within 200 m of a pair’s nest) of a secondary cavity-nesting songbird, the Prothonotary Warbler (Protonotaria citrea). Our primary objective was to test for effects of food limitation, as mediated by conspecific local densities, on measures of productivity. We monitored breeding pairs and recorded the total number of fledglings produced along with several components of reproductive output (clutch size, hatching success, nestling survival, and probability of attempting a second brood), rates of nestling provisioning, and nestling body condition prior to fledging. We predicted that if the availability of food were affected by local densities, then one or more of these parameters measuring reproduction would be affected negatively. We did not detect an effect of local density on total reproductive output or its components despite our vast range of local densities (1–27 pairs; i.e. 0.16–2.23 pairs ha–1). Further, we also did not detect differences in nestling provisioning rates and nestling body condition relative to local density. By breeding in a productive ecosystem rich in food resources, these warblers appear to avoid reduced reproductive output when breeding in high densities. Whereas density-dependent food limitation may commonly reduce reproductive output in many species, the ecological circumstances underlying when it does not occur merit further investigation and may provide new insights into what is driving territoriality and what are the primary factors affecting individual fitness.

Warmer April Temperatures on Breeding Grounds Promote Earlier Nesting in a Long-Distance Migratory Bird, the Prothonotary Warbler

“…The likely reason for this is that earlier breeding associated with warmer temperatures resulted in an increased probability of fledging two broods within a given breeding season. An increase in attempting second broods previously had been documented in our study system (Hoover et al, 2020), as well as in a study of prothonotary warblers in Virginia, United States (Bulluck et al, 2013). Similar patterns of increased double brooding, or increased chances of successfully producing a replacement brood, as a byproduct of nesting earlier in warmer years has also been found in other species (Halupka et al, 2008;Monroe et al, 2008;Verhulst and Nilsson, 2008;Townsend et al, 2013).…”

“…Previous research in our study system demonstrated that females that nest earlier are more likely to attempt second broods (Hoover et al, 2020). Thus, local warmer April temperatures could also increase the probability of females successfully fledging two broods and explain any increases in productivity we observed.…”

“…The warblers are insectivorous but not highly specialized on one guild of insect (Petit et al, 1990a,b;Dodson et al, 2016;Petit, 2020), and they occur in bottomland and swamp forests where there is an abundant and diverse invertebrate biomass continuously emerging during the warbler's breeding season (Petit and Petit, 1996;Heinrich et al, 2013;Batzer et al, 2016;Dodson et al, 2016). Previous work on these warblers in our study system found no support for food to limit reproductive output, even when densities were manipulated to increase substantially (Hoover et al, 2020). One previous study of prothonotary warblers in the eastern part of their breeding range found changes in local spring temperatures not to have an effect on mean nest initiation dates, and for warmer spring temperatures to increase mean probability of attempting a second brood when older females were more abundant in the population being studied (Bulluck et al, 2013).…”

“…Food resources (insects) in the forested wetlands and swamps where the warblers prefer to nest can even be supplemented by the terrestrial habitats found in bottomland forest ecosystems (Nakano and Murakami, 2001;Baxter et al, 2005). Differences in warbler clutch sizes likely contributed little to the trend in annual productivity we report here because the vast majority of warbler first clutches contain five warbler eggs in any given year (Hoover, 2001(Hoover, , 2003bHoover et al, 2020). In terms of age effects, even though the young female warblers on average started later and produced fewer total fledglings than the older females, the general effects of local April temperatures on first egg dates and annual productivity in young females were similar to the older females (Figures 2-4).…”

“…We monitored warblers that used artificial nest boxes placed on trees or metal conduit, located 50 to 100 m apart, within the 14 study sites (Hoover, 2003a). Prothonotary warblers used nest boxes almost exclusively (Hoover, 2003a;Hoover et al, 2020) and the nest boxes mimicked natural cavities in terms of nest predation and cowbird parasitism (Hoover, 2001). Nest boxes were monitored every 3-5 days from mid-April to early August each year.…”