Balsam fir (Abies balsamea) is the most vulnerable species to the spruce budworm (Choristoneura fumiferana), one of the most devastating defoliators in the world. For decades, pest managers have advocated for reducing its abundance in the landscape to minimize losses to the spruce budworm (SBW). Although reduction of fir occurred during the endemic phase of the SBW cycle, there is little information about the extent to which this general principle (reduction of fir) was applied during an outbreak and whether it occurs at both stand and landscape levels. The objective of this paper is to compare the effect of insect and harvest disturbances on forest structure during the 1970–80s outbreak in Québec. We evaluate whether, (i) forest management activities targeted fir forests and whether patch size of host species influences management or SBW disturbance, (ii) SBW outbreaks and logging have similar or divergent effects on forest composition. Although data are from an earlier outbreak, they are at a scale rarely studied and will be useful in guiding decisions made at larger scales in the current and future outbreaks. Our results show that spruce was targeted preferentially by harvesting (up to 69% of plots) during the outbreak period, while it represented less than one third of plots defoliated by the SBW. On the other hand, fir stands represented up to 75% of plots that were defoliated by the SBW but less than 35% of plots that underwent harvesting. Harvesting targeted large blocks of spruce forest more than large blocks of fir-dominated forest while the opposite was observed for the SBW. In terms of regeneration, SBW tends to reduce fir and favor spruce recruitment, along with non-host species, whereas the opposite tendency was observed following harvesting. In terms of spatial organization of stands, our results support the suggestion that small stands of fir and large stands of spruce undergo the least SBW damage. Thus, in order to attenuate SBW impacts in the future, efforts should be made to ensure that spruce recruitment is favored and that its abundance increases at both the stand and landscape scale.

Most songbird species show some degree of fidelity to their previous breeding location, especially after successful reproduction. However, species associated with highly dynamic food sources (e.g., outbreaking insects) may have to adopt more flexible strategies. Three species (Tennessee Warbler, Leiothlypis peregrina; Cape May Warbler, Setophaga tigrina; and Bay-breasted Warbler, S. castanea ) show strong numerical responses to spruce budworm (Choristoneura fumiferana-SBW) outbreaks. These species, referred to as "budworm-linked warblers", might track SBW larvae through extensive natal dispersal. Then, the superabundance of food during outbreaks would be expected to lead to high breeding productivity which, in turn, should promote breeding site fidelity. Here, we aimed to determine whether budworm-linked warblers were faithful to their previous year's breeding season location and, if so, whether their probability of return was influenced by habitat characteristics such as the density of SBW larvae, stand structure, or landscape structure. We hypothesized that return rate of budworm-linked warblers will be high, as reported in other species of New World warblers, and we predicted that among habitat characteristics, return rate will increase with the density of SBW larvae. We banded 117 budworm-linked warblers (94 % being males) in 75 study plots distributed along a gradient of SBW density and searched for returning individuals within 50 m of their capture sites using song playbacks. Contrary to our hypothesis, resighting rate was very low (0-10.5%). This relative "infidelity" suggests that breeding dispersal of budworm-linked warblers was relatively extensive. Only habitat proportion within an 8-km radius had an important (negative) effect on the probability of resighting Bay-breasted Warbler. Budworm-linked warblers did not exhibit strong site fidelity as adults, but instead performed breeding dispersal movements, presumably to track SBW outbreaks. This strategy may reflect strong spatiotemporal variations in the density SBW larvae. © 2021 by the author(s). Published here under license by the Resilience Alliance.

Outbreaks of eastern spruce budworm (Choristoneura fumiferana; hereafter SBW) are a major natural disturbance in coniferous forests of eastern North America. These outbreaks provide a superabundant source of food for insectivorous birds. Three species, referred to as budworm-linked warblers, exhibit strong positive numerical responses to early increases of SBW density: Tennessee Warbler (Leiothlypis peregrina), Cape May Warbler (Setophaga tigrina), and Bay-breasted Warbler (S. castanea). Their abundance increases even before defoliation is visible from aerial surveys. Budworm-linked warblers may detect new epicenters of SBW outbreaks through natal dispersal, as this movement is typically much more extensive than subsequent (breeding dispersal) movements. Our main objectives were, thus, (1) to determine whether sudden increases in the abundance of budworm-linked warblers could be used to detect early stages of SBW outbreaks, and (2) to examine age-specific responses of budworm-linked warblers to local and landscape-level habitat characteristics, in order to investigate the potential role of natal dispersal in the detection of new epicenters. To do so, we estimated the abundance of each species of budworm-linked warbler in 75 study plots sampling a gradient of SBW density and related them to 7 stand variables and landscape metrics with generalized additive mixed models. We also compared the responses of yearling (second-year; SY) and older (after-second-year; ASY) individuals to the density of SBW larvae and habitat variables at different spatial scales. We captured 31 Tennessee Warblers, 27 Cape May Warblers, and 57 Bay-breasted Warblers. The abundance of all three species of budworm-linked warblers increased with SBW larval density, but the numerical response of Bay-breasted Warbler was initiated earlier and it varied with age. SY individuals tended to be associated with stands supporting lower larval densities than ASY individuals and, as suggested by other authors, Bay-breasted Warbler appeared to be more efficient at exploiting SBW larvae at low density. For that reason, this species represents an early indicator of stands undergoing SBW outbreaks and we propose to use its abundance as an indicator to orient labour-intensive ground surveys of SBW larvae.