Estimating Migratory Bird Arrivals and Departures

A White-crowned Sparrow en-route to feed nestlings. The arrival and breeding timing of some species appear to show insufficient tracking of seasonal food availability, and the decoupling of phenology and seasonal food availability has been linked to population declines.

NPS/Jared Hughey

When birds nest, lay eggs, and fledge (life events) strongly influences their ability to successfully reproduce. Knowing when migratory birds arrive and depart their nesting grounds can help us understand how their populations change over time and across their breeding areas. The timing of nesting and other life events (phenology) of some species can change rapidly. It is well documented that, due to climate change, timing of life events has already changed in a number of species. However, the arrival and breeding timing of some species doesn’t match when food is available; this mis-match has been linked to population declines. Thus, monitoring the phenology of important life events may provide insight into how species can cope with changing environmental conditions and, potentially, how they adjust to these changes. In this paper, we developed a model to assess how populations are changing in combination with the timing and location of life events by estimating bird abundance and arrival and departure times on the breeding grounds from bird surveys. We expect that our model will help us understand how the timing of arrival and departure is changing and what this means for changes in population size and geographic distribution.

A binomial N‐mixture model for estimating arrival and departure timing

Abstract

1. The timing of life cycle events has strong fitness consequences, suggesting that monitoring of arrival and departure timing may help understand spatial and population dynamics. Several existing models with inference to arrival and departure in unmarked populations are applicable to detection/non‐detection data which is a reduced information summary of the underlying population and phenological dynamics. These models also do not directly address the dependence of seasonal variation in availability (e.g. song rate) on arrival timing, often treating the seasonal distribution of availability as fixed across years despite allowing variation in arrival. Model development in an abundance framework has largely occurred in the context of stopover populations, rather than populations that exhibit some period of closure between arrival and departure phases.
2. We developed an N‐mixture model that accommodates the dependence of seasonal availability on arrival timing, providing inference about abundance and both arrival and departure timing based on repeated count data. The model is applicable to populations in which there exists some period of closure between the arrival and departure phases. We developed two general formulations of the model, both of which include a model for the arrival process, but differ in the model for seasonal availability. The first formulation is applicable to cases in which seasonal availability is a function of cue production. The other is applicable to situations where seasonal availability is a function of departure of individuals or their transition to a state in which they remain unavailable for detection.
3. We demonstrated through simulation that both versions provide unbiased and precise estimates of phenology and abundance and illustrated the cue production formulation using data collected in Denali National Park, Alaska for three passerine species.
4. We expect that our inference framework will be broadly applicable in studies of unmarked populations where joint assessment of population, spatial and phenological dynamics is of interest.

Mizel, J. D., Schmidt, J. H., Phillips, L. M. and Mcintyre, C.L., 2019. A binomial N‐mixture model for estimating arrival and departure timing. Methods in Ecology and Evolution 10(7): 1062-1071.