Population dynamics of a little brown bird on the move

March 3, 2019


Animal migrations are fascinating and awe-inspiring phenomena. Typically, these movements en masse cycle seasonally between breeding and wintering areas. From the point of view of biodiversity resilience and conservation efforts, migratory animals pose special problems due to their dependence on more than one habitat area. Woodworth et al. (2017) sought to clarify how a migratory species’ population growth rate may be affected by demographic and environmental factors throughout its annual cycle and habitat range.

For their study, they chose a population of migratory songbirds living on Kent Island in New Brunswick.  The Savannah sparrow (Passerculus sandwichensis), a so-called “little brown bird”, can be distinguished from other birds carrying that label by the splash of yellow above its eyes. Savannah sparrow is a philopatric migratory bird - at the close of winter, it will migrate back to the area of its birth, and in this case, that area is Kent Island.

The population of Savannah sparrows at Kent Island has been surveyed for 26 years. On an annual basis, new members of the population are sexed and colour-banded for ease of identification and tracking purposes, and breeding territories are mapped.

In order to determine the population’s wintering grounds, Woodworth et al. (2017) attached geolocators to 163 birds between 2011 and 2013.Geolocators Geolocator_2  are akin to very small battery-powered backpacks that can be used to estimate latitude and longitude. Data, which was successfully retrieved for 38 individual birds, indicated that the primary wintering grounds include Virginia, North and South Carolina, and Georgia.  

Daily measures of temperature and precipitation, environmental factors that can affect songbird populations, were obtained from weather stations located at the breeding area on Kent Island and the wintering areas. As far as demographic factors that could affect population growth, population density estimates were modelled using the long-term dataset at Kent Island and data from 127 Christmas Bird Count routes at the wintering areas between 1986 and 2015. Woodworth et al. (2017) also estimated age- and sex-specific survival probability, fecundity, and sex-specific immigration rate and evaluated the relative contribution of each to population growth.

The results of the study provided clear evidence that population density regulates population growth at the breeding grounds on Kent Island thru decreased survival of adult males and reduced fecundity. However, population growth was not affected by density at the wintering grounds. One reason for the difference may be that within the limited area of an island, individuals could be forced to breed at higher densities thereby intensifying competition for resources. At the wintering grounds, the above-average temperatures observed promoted survival which had a positive effect on population growth.

This study and others like it can help direct research and management activities towards habitat areas and demographic variables that would yield the greatest benefits for the conservation of migratory species.


Read the article here (open access):



Woodworth BK, Wheelwright NT, Newman AE, Schaub M, and Norris DR. 2017. Winter temperatures limit population growth rate of a migratory songbird. Nature Communications. 8:14812. doi: 10.1038/ncomms14812.