Do Falcons Mate for Life?

A male peregrine falcon returns to the same Chicago bridge ledge in late February, performs a series of power dives in the cold air above the river, and within days a female has joined him. It looks like reunion. It may only be real estate.

Peregrine falcons (Falco peregrinus) are widely described as birds that mate for life. The short answer is roughly true: most bonded pairs stay together across multiple breeding seasons, and if you watch a pair long enough you will likely see the same two birds at the same nest each spring. The longer answer, drawn from a 12-year study of urban peregrines across nine Midwestern cities, is more unsettling and more interesting than the romance version.

What the data actually show

Between 1997 and 2009, researchers tracked 350 birds across 20 nest sites in Chicago and eight other cities, analyzing parentage via DNA and logging every breeding attempt. Of 122 nesting attempts by long-term breeders at six Chicago-area ledges, mate changes occurred in only 12 - a fidelity rate of roughly 90 per cent. That sounds like strong evidence for lifelong pairing. But when the researchers examined causes, six of those 12 changes followed presumed mate death, and three more followed a mate’s disappearance after territorial fights. Only three changes appeared to be voluntary switches.

The study, published in PLOS ONE in 2016, found something even more revealing: nest-site fidelity was higher than mate fidelity. Pairs changed their ledge in only six of 122 attempts - just 4.9 per cent. When a mate disappeared and was replaced, the surviving bird almost always stayed at the same address. The Animal Diversity Web entry for the species states it plainly: “both males and females have a strong attachment to previous nesting sites, which may explain monogamy over multiple breeding seasons, rather than attachment between individuals.”

The peregrine’s pair bond is real. But the research suggests what is truly fixed is the ledge, not the relationship.

This is not a small distinction. It changes the nature of what fidelity means for this species. One Chicago male in the study, tracked across 12 consecutive breeding years, produced 26 offspring with a single mate - by any reasonable definition, a lifelong bond. But if that female had died and a new female had settled on the same ledge, the evidence suggests he would have paired with her just as readily.

How courtship actually works

The male - called a tiercel, roughly one-third smaller than the female - begins pair formation by displaying at the ledge: arriving, calling, perching close. The Animal Diversity Web account describes a progression from roosting nearby to sitting side by side, then short “peep” vocalizations, mutual preening, toe-nibbling, and billing. Before egg-laying, Audubon’s field guide records aerial sequences of high circling flight, dives, and chases by both sexes. The male delivers prey to the female throughout these weeks, establishing the provisioning pattern that carries through incubation.

Extra-pair paternity is exceptionally rare. The same Midwestern study found only two of 126 chicks carried a different father’s genes - 1.58 per cent of young, 2.85 per cent of broods tested. Both cases involved a male who had recently acquired a new mate. Peregrines are monogamous genetically as well as socially.

Breeding facts

Clutch size: three to four eggs
Incubation: 33 to 35 days, primarily by the female
Fledging: 39 to 49 days after hatching
First breeding: two to three years old
Lifespan: average around 13 years; oldest recorded wild individual at least 19 years and 9 months, according to Cornell Lab of Ornithology banding records (a bird banded in Minnesota in 1992, identified again in 2012)

Peregrine falcons do not build nests. They scrape a shallow hollow in whatever substrate the ledge provides - cliff gravel, bridge girder, building parapet - and lay directly into it. North Carolina Wildlife notes that a disturbed pair will often abandon a scrape for the season if adults are repeatedly flushed during incubation. The attachment to the site is strong; the physical investment in it is minimal.

Why the ledge is the real bond

The peregrine evolved on sea cliffs and rocky crags where good nest sites are genuinely scarce: high enough to exclude ground predators, exposed enough for wide sightlines, sheltered enough to buffer eggs from wind. A city building at 30 storeys satisfies all four conditions and adds something a sea cliff rarely provides - a limitless pigeon and starling supply directly below. Wildlife agencies in New York, Chicago, and other major cities have logged the same ledges producing young across decades, across a sequence of individual birds.

A proven ledge is among the most valuable pieces of raptor real estate available. The bird that holds one has strong reason to stay. Any bird looking for a first territory has strong reason to pair with whoever holds one. The bond that results can look, from outside, like two individuals choosing each other. NC Wildlife’s peregrine profile confirms the male hunts throughout incubation and delivers prey to the female on the nest while she covers eggs; both birds incubate, but the hunting falls heavily on the tiercel for the first weeks after hatching. The cooperation inside the bond is genuine. The question is only whether it is organized around the partner or around the place.

Given that the place often outlasts the individuals, and that both birds accept a new partner at the same address without any apparent searching period, the address appears to be the more durable commitment. Most falcons that people describe as mating for life are, more precisely, ledge-holders who pair with whoever else commits to the same ledge.