Primary And Secondary Cavity Nesting Birds And Why They Fight Over Birdhouses

Birds fight for birdhouses because there are not enough cavities suitable for cavity-nesting birds. Several studies have found that the primary reason birds fight for cavities, including birdhouses, is habitat loss, resulting in fewer trees and snags for woodpeckers to excavate cavities and other natural cavities to form. Removing dead or decaying trees worsens the problem.

This article explains how birds create and use nesting cavities to demonstrate why birds fight over cavities and birdhouses.

What is a cavity-nesting bird?

A cavity-nesting bird builds a nest, lays eggs, and raises young inside a cavity, a sheltered chamber, or another structure resembling a cavity. Some species are exclusively cavity nesters, meaning they only nest in cavities with a small entrance hole. Other species are more flexible and nest in cavities with small entrance holes and structures that resemble cavities.

Red-bellied Woodpecker at nesting cavity. Photo: Stan Lupo.

Primary and secondary cavity nesters

 There are two types of cavity-nesting birds:

  1. Primary cavity nesters are birds that excavate or make their cavities. These include woodpeckers, which are considered strong excavators, and nuthatches and chickadees, considered weak excavators.

    Weak and strong excavators differ mainly in the substrate they dig their cavities. 

Primary cavity nesters excavate cavities in relatively soft wood. However, woodpeckers do so in harder wood, and chickadees and nuthatches need decayed wood to do so.
 

  1. Secondary cavity nesters do not excavate their cavities but use existing cavities either excavated by primary cavity nesters, cavities created by broken branches or decaying wood, or artificial structures that resemble a sheltered chamber or cavity, such as birdhouses.

    Secondary cavity nesters comprise the majority of birds that use birdhouses or nest boxes. 

The key role of primary cavity-nesting birds

tufted titmouse in tree cavity
Tufted titmouse at cavity excavated by a primary cavity-nesting bird. Photo: Brian Henderson.

Woodpeckers can take between 10 and 16 days to excavate a cavity, and they often excavate multiple cavities for nesting and roosting. 

Chickadees and nuthatches may take a week to 12 days to excavate a cavity, depending on how soft the wood is.

After woodpeckers stop using a cavity for breeding or roosting, it becomes a nesting site for secondary cavity-nesting birds and small mammals like mice, bats, and flying squirrels.

It is common for these cavities to be used by secondary cavity nesters repeatedly over time, either by the same or different breeding pairs of the same or multiple bird species. 

Primary cavity-nesting birds as regulators of bird diversity

Woodpeckers, in combination with standing forest and dead trees, maintain a high bird diversity.

According to a recent analysis of multiple world regions, areas with a high diversity of tree-excavating birds showed a positive correlation with secondary cavity nesters. 

Researchers concluded that cavity excavators help monitor biodiversity across a broad range of spatial scales and geographical regions, especially in the tropics.

The valuable role that tree excavating birds play makes them an effective management target. 

Cavity-nesting birds and snags

Standing dead trees, commonly known as snags, present a unique structure and potential food source for birds. Wood-boring invertebrates, including ants and beetles, excavate and live within snags, feeding many birds, primarily woodpeckers. 

Woodpeckers and chickadees, and to a lesser extent, nuthatches, use snags as food sources and as a place to nest.

The reproductive cycle of cavity-nesting birds depends on dead and dying trees in the landscape.

The main reasons why birds fight over birdhouses

According to multiple studies, habitat loss, which results in a lack of substrates to excavate cavities, is the main reason secondary nesting birds fight for birdhouses and nest boxes. The scarcity of nesting cavities may limit their reproductive success.

The high competition over nesting cavities is mainly due to four reasons: including

  • Destruction of native woodlands

As woodlands are cleared, woodpeckers are less likely to find dead trees in which to excavate cavities. Furthermore, cavities created by broken-off branches and trunks do not exist without the forest.

  • Introduction of invasive bird species

The introduced European starlings and house sparrows, both secondary cavity nesters, are abundant and breed in natural cavities and nesting boxes in every state. 

These two aggressive competitors evict nesting birds, destroy the eggs, and even kill the young inside to take over the nesting box or other cavities. The displaced pair must find another nesting box to nest in and may need to fight over the next available birdhouse, and the cycle of competition for birdhouses continues.

Starlings have been known to evict even woodpeckers that have chisel-like bills and vigorously defend their cavities. 

However, it is important to note that while European starlings and house sparrows evict and take over native birds from birdhouses, which leads to competition for birdhouses, the effect on native birds’ reproductive success is still poorly understood.

  • Removal of dead or decadent trees 

Cavity nesting birds excavate their cavities or use existing cavities in dead or deteriorating trees.  Such trees are commonly called snags.

Forest managers consider snag trees undesirable for various reasons, including their unattractive appearance, interference with other management practices, the potential for pest infestations, or even fire or safety hazards.

It (is) was once common to remove dead trees from the forest during a timber harvest. Therefore, cavity-nesting birds were left with few nesting sites. 

Several ecologically and aesthetically important species, such as woodpeckers, swallows, wrens, nuthatches, and owls, would be further harmed if the current pressures to harvest large dead or dying trees continues.

If a dead tree and its branches are left to decay, woodpeckers and chickadees will dig cavities. When the woodpeckers leave, secondary cavity-nesting songbirds move in and nest in the abandoned cavities.

That’s the natural way things work for cavity-nesting birds. 

People have developed a dislike for the not-so-neat look of dead and decaying trees on their property and eliminate them, making potential nesting sites for cavity-nesting birds even more scarce. 

  • Preference over nesting boxes
tufted titmouse in nesting box
Tufted titmouse at a nest box. Photo: Rain0975.

Studies on secondary cavity-nesting birds have found that some species of secondary cavity-nesting birds prefer nesting boxes when natural cavities and nesting boxes are available. Such preference over nest boxes leads to competition.

The main reason for the preference for nest boxes is that nesting birds are often more successful in raising young than in cavities in the forest or other natural habitats.

Nest boxes generally have entrance hole sizes that preclude larger birds from taking over the cavities of smaller birds. Also, nest boxes often have predator guards and are better protected from raids by predators.

Moreover, nesting boxes are generally placed close to people’s houses, where snakes and other predators may be less likely to venture in search of eggs and baby birds.

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