Maraming ibon ang umaasa sa tree cavities, o mga butas ng puno, para mapugaran at magpalaki ng kanilang mga sisiw. Ang mga woodpecker, na kilala bilang pangunahing excavator, ay kayang lumikha ng kanilang sariling mga butas. Sa kabilang banda, ang mga non-excavator—tinatawag ding secondary cavity nesters—ay nakadepende sa mga likas o dati nang nahukay na mga butas para sa kanilang reproduksyon. Ang mga ibong ito ay nahaharap sa mga hamon tulad ng kakulangan ng angkop na mga butas, na maaaring maglimita sa paglaki ng kanilang populasyon.

Many birds rely on tree cavities, or tree holes, for nesting and raising their young. Woodpeckers, known as primary excavators, can create their own cavities. In contrast, non-excavators—also called secondary cavity nesters—depend on naturally occurring or previously excavated cavities for reproduction. These birds face challenges such as a scarcity of suitable cavities, which can limit their population growth. To protect them, it is important to understand the specific nesting needs of each bird species and how they interact with one another. In regions like the Philippines, where forests are vulnerable to change, there is still limited information on how these birds nest and how they are connected.