Habitat loss has been implicated as the single most important factor that has contributed to declining Cerulean Warbler populations across their range. A thorough assessment of the status of Cerulean Warbler populations in North America by the U.S. Fish and Wildlife Service determined that habitat destruction, modification, and fragmentation, the division of continuous, connected habitat into smaller patches, are believed to be largely responsible for the decline of the species on both the breeding and non-breeding grounds. For instance, over 90% of the northern Andes, important wintering ground habitat for Cerulean Warblers, has been deforested for agriculture. Cerulean Warblers inhabit dense mature forests and may therefore be particularly susceptible to anthropogenic disturbances such as logging and mining activity that cause habitat loss and fragmentation.
Part of the problem with Cerulean Warblers seems to be their specific habitat requirements. Cerulean Warblers primarily live in the forest interior and require, large, relatively undisturbed forested areas in order to successfully breed. According to Dr. Paul Martin, a biology professor at Queens University who has worked on these birds, Cerulean Warblers are unusual among most local songbirds because of their need for large expansions of mature, undisturbed forest habitat on their breeding grounds, This requirement, he says, makes Cerulean Warblers a particular vulnerable species in a rapidly developing world where habitat fragmentation is becoming more commonplace. When Cerulean Warblers are exposed to almost any degree of fragmentation, their breeding success and population numbers can fall off rapidly.
A study by Dr. Frank Thompson and his colleagues from the University of Missouri provided experimental evidence for these ideas. Surveying forested habitat along rivers in Missouri and Northern Arkansas, these researches showed that Cerulean Warbler abundance greatly increased with increasing forest cover. Based on statistical modelling, they were able to predict that Cerulean Warblers could not exist at all in areas where there was less than 50% forest cover. The authors concluded that Cerulean Warblers require extensive forest cover for suitable habitat and highlighted how habitat fragmentation can lead to population declines. Similarly, Dr. Cathy Weakland and Petra Wood from the University of West Virginia found similar patterns in West Virginia, where there are large populations of Cerulean Warblers. They found that Cerulean Warblers were much more abundant in intact forests compared to fragmented ones, and that Cerulean Warbler population densities increased with increasing tree canopy cover, canopy height, and distance from human mining activities in the area.
Protecting important forest habitat and restoring abandoned agricultural land may both help to conserve Cerulean Warblers. Alternatively, sustainable forestry practices which log only a small % of active forest each year could help balance economical human benefits with habitat preservation. This type of forestry, known as silviculture, has been shown to provide Cerulean Warblers with suitable habitat. A study by Sarah Register and Kamal Islam found that Cerulean Warblers were just as abundant in forest plots that had been selectively cut using silviculture as they were in undisturbed forest. This suggests that Cerulean Warblers may be able to persist in areas where logging pressures aren't as intense, although more work is needed to see whether or not Cerulean Warblers maintain breeding success in these areas.
Perhaps the greatest threat facing Cerulean Warblers is the fact that we still don't have a great understanding of where they need to be protected. Being a migratory species, Cerulean Warbler split their time each year between their wintering and breeding grounds and also spend several weeks between these areas on intermediate migratory stops. Though progress is being made in this area, more studies are needed to address these issues and determine Cerulean Warblers need to be more protected.
We asked Dr. Martin the one piece of information he could hypothetically have in order to better tackle the question of how to conserve Cerulean Warblers. His answer was simple: where do these birds die? Knowing where mortality is occurring (i.e. breeding grounds vs. wintering grounds) would help researchers in two ways. First, it would establish baseline data for mortality rates that could be compared to future data in order to better understand how mortality rates are increasing through time. Secondly, in the short term, it could help researchers to focus their effort and resources to protect birds in areas where they are most vulnerable. As you can probably imagine, data of this kind is difficult to collect. However, according to Dr. Martin, the recent development of small satellite trackers may offer a promising avenue to pursue in Cerulean Warbler conservation to enable researchers, for the first time, to be able to locate where these birds are dying. Simply put, we just don't have all the information we need yet to properly conserve Cerulean Warblers, and until we do, this in itself may pose the greatest threat to their populations.
Though habitat loss seems to be the major concern, there has been some evidence to suggest that brood parasitism may also be negatively affecting Cerulean Warbler populations. Brood parasitism refers to when a bird from one species lays its eggs in the nest of another. When this happens, the latter species cares for the eggs of the former at the cost of its own reproductive success. In the case of Cerulean Warblers, one of the major species that is known to parasitize their nests is the Brown-headed Cowbird.
The Brown-headed cowbird is a common brood parasite that can be found all across North America. It is known to parasitize nests of many different songbird species across its range, including Cerulean Warblers. When a Brown-headed cowbird lays an egg in a Cerulean Warbler nest, the female will often unknowingly incubate the cowbird egg until it hatches. Brown-headed cowbird eggs typically mature faster and hatch sooner than cerulean eggs can, and it has been documented that the faster developing chick(s) will sometimes push cerulean warbler eggs out of the nest in order to receive more food from the female Cerulean Warbler. Even if this doesn't occur, the breeding Cerulean Warbler pair must spend more time and energy to nourish the cowbird chick(s) at the expense of their own offspring. Most of the time, Cerulean Warbler nests infected by Brown-headed Cowbirds produce fewer than normal or no chicks at all.
Studies of Brown-headed Cowbird brood parasitism in Cerulean Warblers have shown that it occurs to varying degrees across its range, but most agree that it does occur and has the potential to reduce population sizes. Interestingly, habitat loss and fragmentation could actually work together to increase the effect of brood parasitism on Cerulean Warblers since Brown-headed Cowbirds prefer open habitats such as fields and pastures. A study by Margaret Brittingham and Stanley Temple found that Brown-headed cowbird brood parasitism in a variety of songbird species increased in more fragmented forests; forested areas that were closer to and more surrounded by open areas had higher amounts of brood parasitism associated with them. Cerulean Warblers are forest interior nesters that are normally more protected from Brown-headed cowbird brood parasitism. However, increased brood parasitism may be one reason why this species can't tolerate habitat fragmentation, a definitive area where more research is needed.