Overview of nesting periods

Introduction

We estimate the period when nesting is most likely to occur for most species of migratory birds. You should use this information when planning your activities so you can avoid disturbing or harming birds, nests and eggs.

In Canada, the general nesting period may:

• start as early as mid-March
• extend until late August

These nesting periods:

• begin with the time of first egg-laying
• end when the young have naturally left the vicinity of the nest

These periods do not include the nest building period before egg-laying. Building the nest can take anywhere from a few days up to about 2 weeks. Use caution at the beginning of the general nesting period to avoid harming birds, nests or eggs.

The nesting period varies across Canada, mainly due to differences in:

• species present in a region
• climate
• elevation
• habitat type

Generally, the nesting period occurs later in more northerly latitudes, corresponding to vegetation development and food availability.

How the general nesting period is estimated

The regional nesting period table and calendars are based on 640,000 nest observations gathered from 180,000 nests by volunteers and others who contributed their data to Project NestWatch. This project is managed by Bird Studies Canada in collaboration with provincial and regional nest record schemes across Canada.

Among the 364 federally protected species known to breed in Canada:

• 264 species (72%) were included for the determination of the regional nesting period table and calendars
• the remaining 100 species were excluded because not enough nest records were available

Consult the technical summary for more a detailed account on the methodology.

Accuracy of the nesting predictions

The accuracy of the estimated nesting dates could vary by 10 days or more, due to:

• natural variations between:
  • regions
  • individuals within a species
  • years
• factors associated with the method used, such as sampling or data constraints

The actual nesting periods in a given location could have different start dates and/or durations due to:

• micro-climatic conditions in specific areas, such as:
  • high elevation sites
  • coastal sites
• inter-annual variation due to factors such as:
  • early spring
  • cold, wet summer

It is possible that birds will nest before or after the estimated nesting dates. The probability of encountering an active nest (with eggs or with young) is much lower, but not zero.

Exceptions to the general nesting period

The nesting period of some species may fall outside of the general nesting periods presented in this section. These include:

• birds that are not under federal jurisdiction, such as:
  • jays, owls, hawks, blackbirds, grouse, cormorants, pelicans
• species that can breed anytime when conditions are right, such as:
  • Red Crossbills and White-winged Crossbills that nest in winter when cone crops are available
• species that may nest earlier in March, such as:
  • Great Blue Heron and American woodcock
• species that may nest later such as:
  • Cedar Waxwing, Bohemian Waxwing, Pine Siskin, American Goldfinch, Common Murre and Great Blue Heron until the end of September
  • Leach’s Storm-petrel, Fork-tailed Storm-petrel and Northern Gannet in October
• species that nest in the south of British Columbia, on the coastal area especially, where:
  • some species can start nesting in February, such as:
    • Song Sparrow
    • Anna’s Hummingbird
    • Great Blue Heron
    • Mallard
  • some species can start nesting in early March, such as:
    • Canada Goose
    • Common Merganser
    • Killdeer
    • Bushtit
  • some species may nest later in September such as:
    • swallows
    • wrens
    • chickadees
    • bushtit
    • Swainson’s Thrush
    • Dark-eyed Junco
• mountainous landscapes where the nesting period can start later on mountain tops or earlier in valleys, such as:
  • the Rockies
  • the Torngats

Estimates of nesting periods for species under provincial and territorial jurisdictions are available from Project NestWatch at Bird Studies Canada.