Concerning biological diversity and conservation, [1] highlights two non-overlapping scientific paradigms: small population paradigm and declining population paradigm. Of these two, this study focuses on the latter which seeks to identify the processes by which populations are driven to extinction by agents external to them. The declining population paradigm is rooted in empiricism and therefore focuses on detecting, diagnosing and halting population decline [1]. The tenet of this study is that there is a tangible cause for the decline in populations of biological species and that probing reveals this cause. Forest fragmentation is identified as a probable agent of species decline [2] and therefore forms a key part of this study.

Forest fragmentation not only affects population densities of forest bird species, but also affects their reproduction and dispersal [3]. Forest fragmentation can impact bird species negatively by reducing food resources available to birds as a result of a reduction in core forest patch sizes. Reduction in food resources for birds also has a consequent negative impact on their fecundity, causing a decline in their numbers within a particular area [4] [5]. Forest breeding birds such as Wood Thrush and Ovenbird species, with particular preference for core forest areas, are most negatively affected by forest fragmentation.

The Wood Thrush (Hylocichlamustelina) breeds in North America, migrating to Central America during the winter [6]. Recently, Wood Thrushes have become a species of conservation concern because of their narrow tolerance of certain microclimatic conditions and their preference for the interior of moist deciduous forests, a forest condition that is fast declining due to timber harvesting activities. Wood Thrushes are highly sensitive to forest fragmentation due to increased nest predation and brood parasitism by the Brown-headed Cowbird [7] in forest fragments.

The Ovenbird (Seiurusaurocapilla) is also a fragmentation-sensitive Neotropical migrant [8] with a preference for mature deciduous or mixed forests with closed canopies and sparse understory. Pairing success of male Ovenbirds is lower in forest edges and small forest patches, compared to areas with large core forest patches [9]. Numbers of Ovenbirds are also dependent on the surrounding vegetation patterns as small forest patches with surrounding agricultural lands have fewer Ovenbirds compared with large contiguous forest patches [9] [10]. Mechanisms of forest fragmentation effects on Ovenbirds are not completely understood as nest predation rates of Ovenbirds in large contiguous forests and in fragmented forests have been shown to be similar in certain studies [9] [11] [12].

There are specific threshold sizes for the patches formed by habitat fragmentation, beyond which particular species within the habitats cannot survive [13]. The relationship between habitat fragmentation and population density of the Wood Thrush and Ovenbird within Virginia can be used to identify impacts of forest fragmentation on the species in the state. These two migratory bird species were specifically chosen for this study because while they are both known to possess the ability to cross forest gaps [14], there have also been contrasting reports on how forest fragmentation driven by inter-patch distances, negatively affects their abundance [15] [16]. This presents a confusing theory on how both birds are influenced by forest fragmentation, driving a curiosity about the impact of forest patch sizes and patch connectivity on their population.

Given that human development is continuing, effective planning and management are needed to conserve biodiversity by determining minimum-area breeding requirements for Neotropical migrants such as Wood thrush and Ovenbird, and to identify landscape and vegetation features that support viable breeding populations [17] [18]. There is an opportunity to compare populations of these bird species with changing forested land areas over the years to identify correlation and subsequent impacts on the species. This research therefore aims to determine the impact that changes in forested land areas in Virginia, have had on Wood Thrush and Ovenbird populations.

Between 2001 and 2011, populations of both Wood Thrush and Ovenbirds species have declined. On the average, the lowest numbers of Wood Thrush and Ovenbird species detected in each survey route have not gone any lower than previous years. However, the highest numbers detected in each survey route have varied considerably between 2001 and 2011 (Figure 4). The average number of Ovenbirds recorded per survey route in 2001 was approximately 13, compared to 12 in 2011. There is a more a drastic change among Wood Thrushes as the average number recorded per route in 2001 was 13 but declined to 7 by 2011.

Table 2 shows the significance of changes in forest fragmentation in Virginia between 2001 and 2011 in the various buffers. Whereas, changes in connected and fragmented forests are significant in the 15, 20 and 25 mile buffer radius, only connected forests are significant in the 5 mile buffer radius. Changes in core forests are not significant between 2001 and 2011. Generally, the trends are similar at all buffer sizes, with core forest declining and both connected and fragmented forest increasing (Figures 5-9), although these trends were statistically significant only for the latter two forest types at larger scale buffer sizes.

Using the 1 mile buffer radius, Ovenbird numbers were significantly lower in fragmented forest compared to other forest types, and significantly higher in the Coastal Plain physiographic region than in all the other regions (Table 3). Also, slope categories 2, 5 and 8 have negative relationships with Ovenbird numbers, whereas 1, 3, 4, 6 and 7 have positive relationships. Only the negative relationship with slope category 5 (9% - 15%) was significant. Ovenbird numbers did not differ significantly between time periods (Table 3). Wood Thrush abundance was significantly lower in 2011 than in 2001 (Table 4). Within the 1 mile buffer radius, the only environmental variable that had a sizeable effect on Wood Thrush numbers was the slope category 4 variable (Table 4). Results using the 5 mile buffer were identical to those for the 1 mile buffer (Table 5 and Table 6).

The Coastal Plain region had significantly higher numbers of Ovenbirds using the 15 mile buffer (Table 7), and again, Wood Thrush numbers were lower in year 2011 (Table 8). None of the variables analyzed had a significant relationship to Ovenbird numbers using the 20 mile buffer, and for Wood Thrush, the only significant variable again was the year, 2011 having significantly lower numbers than 2001 (Table 9 and Table 10). Using a 25 mile buffer radius around the survey routes, Wood Thrush numbers were significantly lower in year 2011 compared to 2001 (Table 11 and Table 12).

Many studies such as [3] [21] hypothesized that population declines in migratory specialist species such as the Ovenbird and Wood Thrush, were caused by reduction and isolation of their breeding habitats. The three forest classes in this analysis, Core, Connected and Fragmented were categorized based on surrounding pixels and the connectivity of those pixels and as such, capture the forest conditions important to the Wood Thrush and Ovenbird.

As our results show a significant increase in fragmented forests across all survey routes from 2001 to 2011, the decline in populations of Wood Thrush in Virginia over that period parallels an increase in forest fragmentation. Forest fragmentation leads to the creation of small forest patch sizes which limit the food resources available for the bird species. Thus, the link between the decline in Wood Thrush populations and forest fragmentation in Virginia is consistent with the findings of [22] who found a direct relationship between forest patch sizes, reproductive success, and return rates for Wood Thrushes in Delaware.

Our results indicate a modest decline in Ovenbird populations between 2006 and 2011, whereas Wood Thrush declined significantly and consistently between 2001 and 2011. Since forest fragmentation significantly affected Ovenbirds in the 1 and 5 mile buffers, our results are consistent with the results of [9] study suggesting that pairing success of Ovenbirds can be influenced by habitat fragmentation. Ovenbirds avoid forest fragments which might be the reason for their modest decline compared to the significant decline in Wood Thrush populations.

Core forest areas are important for buffering Wood Thrush and Ovenbird species from the incursions of predators as a result of the shield provided by the high and closed canopy structure with small canopy gaps [23]. However, our results show that the decline in core forest in Virginia is not significant. This means that the decline in Wood Thrush is caused by other factors such as increased forest fragmentation leading to increased predation.

For many migrating interior forest specialists like the Ovenbird and Wood Thrush, although small habitat spaces are not appropriate for breeding, they serve as stopover habitats and corridors that link core forest patches together. Our results show that the change in connected forests in all the buffer regions is significant which is important for ensuring the persistence of healthy Wood Thrush and Ovenbird populations.

Reductions in populations of migrant habitat specialists such as the Ovenbird and Wood Thrush could be a result of increasing urban development which directly leads to increased rates of nest predation in forest fragments close to development sites, changes in habitat structure and composition, and consequently, avoidance of developed areas by birds due to human habitation [24] [25]. Among effects of urban development, are increases in numbers of domestic pets from nearby homes in the area, which are likely to disturb nesting birds [26] [27].

Our results show that at local scales (1, 5 and 15 mile buffer radius), fragmentation, slope and physiographic regions have significant effects on Wood Thrush and Ovenbird populations. At the regional scale (20 and 25 mile buffer radius), forest fragmentation has no effect either due to the fact that forests no longer dominate the matrix or the species do not respond to the landscape at these large scales. There was a similar trend in changes in core, connected and fragmented forest in every buffer area considered (Figures 5-9). Core forest areas declined from 2001 to 2011 while fragmented forest areas increased, demonstrating a general increase in forest fragmentation across Virginia. This means that a singular policy aimed at decreasing forest fragmentation can be effective across the entire state of Virginia. For instance, [28] showed that economic research can rely on a model of net benefits that reflects or emphasizes the fragmentation or pattern of the landscape, rather than individual species or specific locations within the landscape. However, given that effects of all variables considered in each buffer zone on the populations of wood thrush and ovenbird differ, it is important that more specific deliberations are made, with particular species in mind. For example in the 5 mile buffer, fragmented forests, the Coastal Plain region and slope category 5 have significant effects on Ovenbird populations but none of these factors have significant effects on Wood thrush populations in the same buffer. Hence, results are dependent on the species under focus and policy makers should be cautious in the application of generalized concepts to all species.

The study shows that populations of Wood Thrush and Ovenbird in Virginia decreased between 2001 and 2011, forest fragmentation increased over this period, and that Ovenbirds are less common in forest fragments than in core forests. Increased fragmentation could account for the significant decline in Wood Thrush since our results show that they do not avoid fragments whereas Ovenbirds avoid forest fragments and have therefore only declined moderately. This study also demonstrates how habitat conditions affect populations of Wood Thrush and Ovenbird. For example, slope has a significant effect on Wood Thrush and Ovenbird populations because it determines the intensity of development in an area. The Coastal Plain region harbors larger numbers of Wood Thrush and Ovenbird than other physiographic regions in Virginia as its mixed forested lands with high pine concentrations provide an ideal habitat for both the Wood Thrush and Ovenbird.

The study identifies factors whose consideration may allow policy makers to make more effective decisions to increase Ovenbird and Wood Thrush populations in Virginia. In particular, use of differing buffer zones showed that these species respond to their environment at relatively small (1 -5 miles) spatial scales. The trace plots of our results show effectiveness of our model in identifying the effects of multiple variables over a time gradient. Thus, our model can be replicated and improved by other studies to ensure that effective decisions are made to improve bird populations.

The author declares no conflicts of interest regarding the publication of this paper.

Fynn, I.E.M. (2022) Understanding Changes in Wood Thrush and Ovenbird Populations in Virginia—The Role of Forest Fragmentation and Connectivity. Journal of Environmental Protection, 13, 797-818. https://doi.org/10.4236/jep.2022.1311051