Nesting periods of migratory birds: technical background

The science behind the nesting periods of migratory birds. (Technical report: The nesting phenology of birds in Canada [PDF; 38.57 MB])


This technical report is the first attempt to produce a nationally consistent and unified description of bird nesting phenology across the different provinces and territories of Canada. It provides species-specific predictions of the nesting phenology of 311 bird species (71% of those breeding in Canada), outlining the timing and intensity of nesting. The use of a standardized method applied to all regions enables predictions to be made for boreal and Arctic regions.

First objective

The first objective was to program a set of algorithms that would automatically process the hundreds of thousands of field observations contained in the nests records within the Project NestWatch database (Bird Studies Canada). A procedure was developed to reconstruct the most likely nest chronology with the greatest possible accuracy. The nest chronology was considered to start with the laying of the first egg and to end with the departure of the last young from the nest. The backcalculation algorithms were applied to 85% of the original nest records (n = 202,407), representing 478,419 nest visits.

Second objective

The second objective was to make use of the estimated nest chronologies to develop models that would predict the nesting phenology of birds across Canada. The mean annual temperature (MAT) was the main predictor variable for the timing of nesting. The slope of the relationship between the MAT and nesting phenology was determined by the migratory strategy, the tendency to breed early or late, and the capacity to lay multiple clutches, while the intercept (or height) of prediction curves was determined by species. Quantile regression was used to describe the beginning, the midpoint and the end of the nesting period for each species. Predictions were derived from the three models and were restricted to the range of MAT experienced by a species within its normal breeding distribution. For most species, model predictions supported the two main initial assumptions, namely that the beginning of the nesting season should be earlier in warmer regions and that the length of the nesting season should be equal or progressively shorter with decreasing temperature. The coding for the backcalculation algorithms is available through the Internet as a package of R-language functions, named rNest.

Third objective

The third objective was to determine whether the use of the backcalculation procedure was justified and whether it leads to biases when predicting nesting phenology, and to provide estimates of uncertainty concerning the predictions. In general, the bias in predictions associated with using backcalculation was small for most species, and negligible compared to the uncertainty in the nesting phenology predictions, even for those species with a long active nest period. For a given species, the variable amount of nest records between regions, the quality of data and the constraints associated with the method used, coupled with the variability in the timing of nesting events between regions, individuals and years, contributed to different levels of uncertainty in the estimations. For several MAT intervals, there were sufficient nest records to allow satisfactory predictions of the nesting phenology based on local observations, but for other MAT intervals, the uncertainty around predictions was probably higher due to the fact that fewer data were available. In general, the uncertainty surrounding the estimated nesting dates can vary by up to about 10 days, or perhaps more in certain cases, due to natural variability in the timing of nesting events between regions, individuals and years.

Fourth objective

The fourth objective was to consolidate the results of the first two objectives within a multi-species analysis to propose regional nesting calendars that would help determine regionally relevant periods during which nesting is likely to occur, and to provide a general portrait of the nesting phenology of federally protected bird species across Canada. To describe the general nesting period of migratory birds in a given region, the proportion of species actively nesting was calculated for each day from early March to the end of August. The regional nesting calendars were associated with broad geographical areas distributed across Canada, referred to as nesting zones.

Fifth objective

The fifth and final objective was to provide a compendium of all the information that was used for, and generated by, the analysis of the Project NestWatch database. This is presented in the form of species-specific accounts. The aim of these is to provide easy access to practical information allowing a rapid assessment of the amount and quality of basic nesting information that was available for a particular species of interest, together with estimations of the nesting periods proposed for that species, and the uncertainty surrounding the predictions. The nesting period estimates for each species and for each of the 1,021 ecodistricts are available on the Bird Studies Canada website as an interactive online tool that offers the possibility of creating customized nesting calendars by selecting species and regions of interest.

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