In my last post, I talked about the impacts of forest loss and fragmentation on boreal birds. These issues impact birds whether they are year-round residents or summer visitors, and the impacts can be pretty intense- Sauer et al. (2005) estimated that the boreal chickadee population experienced an annual decline of about 3.6% between 1966 and 2004 in eastern North America. Unfortunately, boreal birds face more than deforestation due to logging or land development.
Greater Scaup breed in boreal habitats, but spend the winter elsewhere- thus far, they don’t seem to be adjusting the timing of migration to earlier springs. Photo by D Dewhurst and courtesy of USFWS.
The world’s boreal forests lie at the latitudes expected to experience the greatest temperature increases as the climate warms, and that means they will also experience changing fire- and water-regimes, changing plant ranges and growth rates, and changing yearly calendars (Ruckstuhl et al. 2008). That’s a lot of change, and we are still figuring out exactly what it all means for the plants and animals that live there. Rather than be overwhelmed by the sheer scale of change expected, let’s talk about changes in timing (or phenology) and how that interacts with changes in range. It should come as no surprise that warmer global temperatures can translate into earlier springs, and not just in boreal landscapes- I grew up in rural New Hampshire and saw that lilacs (my favorite flowers) bloomed in late May/early June when I was little, but now bloom in early May. You might think that earlier springs help birds out by expanding the period when plants, insects, and other food sources are more plentiful, but some birds initiate migration to breeding grounds not on the basis of weather conditions, but on day-length. Research into scooter and scaup populations in western North America suggests that earlier springs mean the birds may miss the peak abundance of their main food sources because they aren’t migrating earlier for breeding (Drever et al. 2012). Other birds are able to respond to changing temperatures, but that may not fix everything over the long-term. Willow tits in Finland, for example, are breeding earlier each year, and luckily the caterpillar which is their main food during nesting is also hatching earlier (Vatka et al. 2011). The rate of change for the two species is different, however- for the moment, that means that the period when the birds need the most food is a better match with peak caterpillar abundance. But the time will come when the two species start drifting apart, and that will make it more difficult for the birds to successfully raise young.
And the mismatch is not just about whether or not the right insects are available at the right time during a summer season- we’re also talking about whether habitat can shift as quickly as bird ranges can. Researchers in Sweden found that the limit of certain tree species had shifted upward in altitude by 70-90 m in the 20th century (Kullman and Öberg 2009). While that suggests that plants can respond to climate change, think about the timescale here and how mobile birds are- birds can disperse faster than plants can, so there are concerns that suitable temperatures and weather patterns for birds will shift more quickly than the plants which provide food and shelter (Stralberg et al. 2015).
So, boreal birds are facing a number of big challenges- do we have options for mitigating some of these effects? Yes, but it requires some changes in management and some planning at a large scale. To lessen habitat loss, researchers have suggested that we limit thinning in managed forests to both encourage the development of a shrub layer (and the fruits eaten by birds before migration) (Major and Desrochers 2012) and maintain snags for nesting (Vatka et al. 2014). Cardinal et al. (2012) also suggested limiting deer populations just after timber harvest to enable the forest to regenerate more effectively (and there is a whole body of literature on how moose and deer browsing impacts birds, if you’re interested in that relationship).
One of the most important things we can do right now is to protect large areas of different habitats (Stralberg et al. 2015), especially mature forest because it takes so long to develop, and then provide birds with intermediate patches of habitat to facilitate movement between locations (Glennon 2014)- admittedly, that’s a tall order and, as you’ll see in my next post, that’s exactly where all of us come into play.
Works cited:
Cardinal, E., J.-L. Martin, J.-P. Tremblay, and S.D. Cote. 2012. An experimental study of how variation in deer density affects vegetation and songbird assemblages of recently harvested boreal forests. Canadian Journal of Zoology 90:704–713.
Drever, M.C., R.G. Clark, C. Derksen, S.M. Slattery, P. Toose, and T.D. Nudds. 2012. Population vulnerability to climate change linked to timing of breeding in boreal ducks. Global Change Biology 18:480–492.
Glennon, M.J. 2014. Dynamics of Boreal Birds at the Edge of their Range in the Adirondack Park, NY. Northeastern Naturalist 21:NENHC-51.
Kullman, L., and L. Öberg. 2009. Post-Little Ice Age tree line rise and climate warming in the Swedish Scandes: a landscape ecological perspective. Journal of Ecology 97:415–429.
Major, M., and A. Desrochers. 2012. Avian use of early-successional boreal forests in the postbreeding period – Utilisation des jeunes peuplements d’une forêt boréale par les oiseaux après la période de nidification. The Auk 129:419–426.
Ruckstuhl, K.E., E.A. Johnson, and K. Miyanishi. 2008. Introduction. The boreal forest and global change. Philosophical Transactions of the Royal Society B: Biological Sciences 363:2243–2247.
Sauer, J.R., J.E. Hines, and J. Fallon. 2005. The North American breeding bird survey, results and analysis 1966-2004. USGS, Patuxent Wildlife Research Center, Laurel, Maryland, USA.
Stralberg, D., E.M. Bayne, S.G. Cumming, P. Sólymos, S.J. Song, and F.K.A. Schmiegelow. 2015. Conservation of future boreal forest bird communities considering lags in vegetation response to climate change: a modified refugia approach. Diversity and Distributions 21:1112–1128.
Vatka, E., K. Kangas, M. Orell, S. Lampila, A. Nikula, and V. Nivala. 2014. Nest site selection of a primary hole-nesting passerine reveals means to developing sustainable forestry. Journal of Avian Biology 45:187–196.
Vatka, E., M. Orell, and S. Rytkönen. 2011. Warming climate advances breeding and improves synchrony of food demand and food availability in a boreal passerine. Global Change Biology 17:3002–3009.