Least Flycatcher, a locally common species of fairly mature deciduous forests in northern U.S. and Canada (except Alaska and the Pacific slope), was not well represented in the 1992-2006 MAPS database; although 1,070 adult individuals were banded at 37 stations located within 9 Bird Conservation Regions (BCRs), only 72 between-year recaptures were recorded.  As shown in the spatial display of results, about 53% of the adult individuals were banded in a single BCR, Prairie Potholes (BCR 11) and about 77% of the between-year recaptures were recorded in that same BCR.  The very low numbers of recaptures, particularly in the eastern portion of its range (where there were only 6 between-year recaptures of 356 banded adults), was remarkable, and may be related to Least Flycatcher’s habit of forming clumps of breeding territories that seem to be socially facilitated and may change locations to some extent from year to year.

Temporal and spatial analyses of 1992-2006 program-wide MAPS data produced weighted mean adult population density indices for Least Flycatcher of 2.4 and 4.1 adults per station, respectively, which, while lower than those for Acadian and “Traill’s” flycatchers, were higher than those for both wood-pewees and about 10% and 20% higher, respectively, than the corresponding means for all flycatcher species.  Annual variability in the index of adult population density (20.3%) was very similar to the analogous mean variability for all flycatcher species, while spatial variability in population density (58.6%) was high, among the highest of flycatcher species and 75% higher than the analogous mean variability for all flycatcher species.

The 1992-2006 geometric mean of the model-averaged annual estimates of MAPS lambda for Least Flycatcher (1.030, which was not significantly different from 1.0) suggested a non-significantly increasing population, while the analogous mean of the BCR-specific lambda estimates (0.987, which also was not significantly different from 1.0) suggested a non-significantly decreasing population.  These MAPS estimates differed from the population trend from the 1992-2006 program-wide North American Breeding Bird Survey (BBS; a mean lambda of 0.982, which was significantly different from 1.0) which indicated a significantly decreasing population.  The reason for this difference in population trends from MAPS and the BBS was because the majority of the MAPS data for Least Flycatcher came from the central BBS Region, where the species had a BBS lambda of 1.010, which was significantly different from 1.0 and indicated a significantly increasing population.  Both the annual (33.4%) and spatial (7.5%) variabilities in lambda for Least Flycatcher were rather high, about 50% and 35% higher, respectively, than the analogous mean variabilities for all flycatcher species.

Temporal and spatial estimates of adult apparent survival for Least Flycatcher were very similar to each other (0.378 and 0.375, respectively) and were very low, the lowest of all flycatcher species and, considering its body mass and the body masses of other species, among the lowest 15% of all 158 species on this website.  These very low estimates of adult apparent survival may reflect low site fidelity and a high emigration rate, perhaps related to the species’ socially-driven clumped distribution of breeding territories, rather than low true survival and a high mortality rate.  Thus, while overall adult apparent survival for Least Flycatcher appeared to be highly deficient, adult true survival may not have been deficient.  Both annual (30.4%) and spatial (13.9%) variabilities of adult apparent survival for Least Flycatcher were quite similar to the analogous mean variabilities for all flycatcher species (28.3% and 14.2%, respectively).

Temporal and spatial analyses also produced rather similar estimates for the index of productivity (0.287 and 0.255, respectively, from the selected models) for Least Flycatcher, which, in sharp contrast to estimates of adult apparent survival, were higher than the analogous indices for all other Empidonax and Contopus flycatchers except “Western” Flycatcher.  Perhaps the relatively high productivity index compensated to some extent for the very low adult apparent survival.  Annual variability in productivity (54.2%) was somewhat high, about 15% higher than the analogous mean variability for all flycatcher species, but spatial variability in productivity (35.4%) was low, only about 70% as high as the analogous mean for all flycatcher species.

Temporal analyses for Least Flycatcher showed that lambda was moderately but not significantly positively correlated with post-breeding effects, weakly negatively correlated with productivity, and not correlated at all with adult apparent survival.  This suggests that annual variation in lambda was essentially driven only by annual variation in post-breeding effects.  While annual variation in post-breeding effects likely reflect annual variation in first-year survival of young, it also could reflect annual variation in recruitment of the surviving young and of emigrating adults associated with the aforementioned breeding territory clumps that seems to characterize at least eastern populations of this species.  Post-breeding effects were strongly and highly significantly negatively correlated with productivity and rather strongly and near-significantly negatively correlated with adult apparent survival, suggesting possible competitive interactions among young birds and between young and adult birds affecting first-year survival of young birds on the non-breeding grounds and perhaps also affecting recruitment of young birds on the breeding grounds.  As perhaps expected from those two correlations, adult apparent survival was moderately but non-significantly positively correlated with productivity.

Temporal analyses also showed a rather strong and significant temporal correlation between lambda and the index of adult population density, suggesting a substantial degree of density-dependent population regulation for Least Flycatcher.  The moderate but non-significant negative correlation between productivity and adult population density was stronger than both the weak negative correlation between post-breeding effects and population density and the weak positive correlation between adult apparent survival and population density.  These results suggest that density dependence was primarily effected on the breeding range through productivity, but possibly also effected on both the non-breeding and breeding ranges through post-breeding effects that could have reflected both first-year survival of young birds and subsequent recruitment of the surviving young birds.

Spatial correlations between lambda and other vital rates for Least Flycatcher differed from analogous temporal correlations primarily in that lambda was moderately but non-significantly positively spatially correlated with adult apparent survival, rather than showing no temporal correlation whatever with adult apparent survival.  Spatial correlations between lambda and post-breeding effects (rather weak and non-significantly positive) and between lambda and productivity (rather weak and non-significantly negative) were very similar to the analogous temporal correlations.  These results suggest that spatial variation in lambda for Least Flycatcher was driven primarily by adult apparent survival and secondarily by post-breeding effects.  As with temporal correlations, these spatial correlations could have reflected survival of adult and young birds on the non-breeding grounds, and/or recruitment of emigrating adults and surviving young bird on the breeding grounds.  Post-breeding effects were also strongly and significantly negatively spatially correlated with productivity, as was the temporal correlation between these two vital rates.  The signs of the spatial correlations between adult apparent survival and both productivity and post-breeding effects, however, were opposite to the signs of the analogous temporal correlations, and the strength of the two spatial correlations were weaker than the strength of the two corresponding temporal correlations.

Summary of research and management hypotheses – Research and management efforts to reverse population declines and maintain stable or increasing populations of Least Flycatchers should first be directed toward determining and then enhancing habitat conditions that promote the successful recruitment of young birds and emigrating adults into breeding populations.  Research should also be directed toward determining and enhancing the formation and stability of the apparently socially-facilitated clumps of breeding territories, at least within the eastern portion of the species’ breeding range.  Considerable additional research may be needed in this regard, because apparently no habitat types or habitat characteristics associated with the formation and maintenance of these clumps of territories have yet been identified.  Second, research and management efforts should be directed toward determining and enhancing habitat types and characteristics associated with high first-year survival of young, especially during years in which first-year survival of young is low and appears to drive population declines.  Considerations of weather and climate change will likely need to be included in all these efforts to maximize their effectiveness.  Third, research and management efforts should be directed toward determining and enhancing habitat types and characteristics associated with high survival of both adult and young birds, especially within regions where first-year survival of young birds and, especially, adults is low and appears to drive population declines.  It is likely that considerably more information on migratory connectivity for Least Flycatcher will be necessary to insure the effectiveness of these latter research and management suggestions.  Interestingly, results from analyses of MAPS data provide no indication that productivity for Least Flycatcher was deficient or positively correlated, either temporally or spatially, with lambda, although there is a suggestion that density-dependence in Least Flycatcher was effected primarily through productivity.  Thus we provide no direct suggestions for research or management aimed toward enhancing Least Flycatcher productivity, but we do suggest that productivity of the species should continue to be monitored.

Please cite this narrative as:  DeSante, D. F., D. R. Kaschube, and J. F. Saracco.  2015.  Vital Rates of North American Landbirds.  www.VitalRatesOfNorthAmericanLandbirds.org: The Institute for Bird Populations.