A Month-by-Month Nesting Calendar for Birds on Montana Buildings

Where do these dates come from, and how far can you trust them?

Every date below is from a Montana Field Guide species account, read on 11 October 2026. Most of the migration periods it gives are for the Bozeman area, and most of the egg and nestling dates are from records near Fortine, in the northwest corner of the state. None are Missoula or Bitterroot Valley records, and some go back decades. We did not find month-level dates for our two valleys on the pages we loaded.

So treat this as a guide to the order of events and roughly when they happen. A low valley floor can run ahead of a mountain town, and a cold spring can push everything back. The calendar tells you when to look. It does not replace looking.

Which birds are on the building all winter?

The three introduced species. The Field Guide lists rock pigeons, house sparrows and European starlings as non-native and gives no migration period for any of them. It says house sparrows use the same habitat in winter as in the breeding season, and that pigeons are found chiefly in cities, towns and farms with livestock.

Pigeons are the species with no real off-season. The Field Guide says pair formation runs from February through September with no peaks, that overlapping clutches and broods are common in summer, and that the gap between the first eggs of one clutch and the next averages 46 days. It gives no month-level Montana nesting dates for pigeons, so we are not offering any. FWP simply says they have multiple broods during the year. Our guide to pigeons on Montana buildings covers what that means for a roofline.

What happens in March and April?

The year-round birds start first. The Field Guide says house sparrow nesting typically begins in early April. Male starlings choose a nest site and begin building before they have a mate, so a starling carrying grass into a dryer vent is an early warning. Robins move through from early March, and flickers from late March through April by the Bozeman dates. FWP notes that woodpecker drumming on siding, gutters and chimney caps each spring is birds establishing territories and attracting mates.

The first swallows show up in April. The Field Guide's Bozeman dates put tree swallows from April 9 and violet-green swallows from April 25. By the end of the month there are eggs: its records near Fortine start on April 29 for both flickers and robins. For a building, March is the last comfortable month to close openings before something moves in.

What happens in May?

Everything arrives and most things lay. Barn swallows come through in the first half of May and cliff swallows from May 5 into early June, on the Bozeman dates. Near Fortine, starling egg dates are May 1 to 15, tree swallow eggs run from May 15 to June 12, and the first barn swallow eggs are recorded from May 20. Statewide, the Field Guide says robin eggs first appear in mid-May and cliff swallow nesting runs from mid-May to mid-July.

This is the month the phone rings, and for native birds it is already too late to exclude. FWP states that once woodpeckers have eggs or young, it is illegal to remove them from the nest or to scare the adults away. The same caution applies to the swallows and robins, all of which the Field Guide lists under the Migratory Bird Treaty Act.

What is going on in June and July?

Nests are full. The Field Guide's Fortine records have young starlings in the nest from June 2 to July 8, young flickers from May 6 to July 9, and tree swallow young leaving the nest between June 23 and July 10. Cliff swallow eggs there run from June 9 to July 17. Violet-green swallow nests statewide are reported from early June into early July.

Second broods start while the first are still being fed. The Field Guide records the earliest second robin brood on June 1 and second barn swallow broods with eggs from July 4. That is why a nest that looked finished in June is busy again in July, and why a single quiet visit proves little. For swallows in particular, see swallows nesting on a Montana building.

When does it wind down in August and September?

Later than people expect. Near Fortine the Field Guide records second-brood barn swallow eggs as late as August 27 and robin fledging into early August, and it says robin nesting statewide runs into mid-August. House sparrows are the long tail: the Field Guide says successful birds may raise up to four clutches in a season and that nesting may last to mid-September.

Then the migrants go. The Bozeman dates have tree swallows leaving between August 10 and September 1, barn and cliff swallows between August 25 and September 10, and violet-green swallows in the first ten days of September. Flickers move from late August to mid-October and robins from September into the start of November. A nest with August eggs still has young in it in September, so check before assuming a mud nest is empty.

October through February: the working window

Once the swallows have gone and the last broods have flown, the building is as empty as it gets. This is when exclusion goes in. A 2025 Fish and Wildlife Service memorandum treats a nest as inactive once the nestlings have fledged and no longer depend on it, and recommends doing work outside the nesting season because that reduces the risk of harming birds, eggs or chicks.

In practice, old nests and debris come out, droppings are cleaned up, and then the openings are closed: netting and exclusion under eaves and in open bays, screens on vents, and covers on ledges. Two cautions apply even in winter. FWP notes that woodpeckers excavate cavities for roosting outside the breeding season, so a flicker hole may be occupied on a cold night. And pigeons, sparrows and starlings are still there, so every opening is checked for birds before it is sealed.

What does the calendar mean for tree work?

Robins and flickers nest in trees as well as on buildings. The Field Guide says flickers usually excavate nest cavities in dead or diseased trunks and large branches, and its records put eggs or young in those cavities from late April into July. Robins have nests going from late April into mid-August. A summer pruning or removal job is where a nest check matters most.

Most pruning is better done in the dormant season anyway, which sits inside the same October to February window. When a hazard tree cannot wait, our tree services crew looks before climbing and works around what it finds. Large raptor nests are a separate subject with their own rules, covered in osprey and eagle nests in Montana.

How should you use this calendar?

Work backward from spring. If birds were a problem this year, the time to book an assessment is late summer or fall, so the work can be done over winter and finished before April. If you are reading this in June with a nest overhead, the useful steps now are to identify the bird, photograph the site, and get on the schedule for fall.

Identification comes first because the three introduced species are treated differently from the native ones. FWP says pigeons are not protected in Montana and that starlings are not protected by federal or state law, and it lists house sparrows among non-protected species. That changes what can be done and when, which is what our bird mitigation assessment sorts out on site. The rest of the series is in the Montana Bird Control Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a bird assessment and we will identify the species, the access points, and what will actually stop them.

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