Good guardian, bad parent: tradeoffs between territory defense and parental care in Darwin’s finches

Although defending a territory may benefit individuals by allowing them to retain important resources, the time and energy costs associated with territory defense may lead territory owners to neglect other reproductively important behaviors. In this study, we assessed the potential trade-off between territory defense and parental care in four Darwin’s finch species on Floreana Island, Galápagos. First, using song playback, we simulated territory intrusions to measure male aggressiveness across multiple stages of the breeding cycle (unpaired, paired, incubating, and chick feeding). We conducted 168 playback trials at 98 nests (37 small ground finch, 28 medium tree finch, 27 small tree finch, 6 cactus finch). To quantify parental care at each nest, we conducted one-hour observations to record the frequency of male food deliveries and the duration of female incubation and brooding. We conducted 113 parental care observations at 59 nests (24 small ground finch, 18 small tree finch, 15 medium tree finch, 2 cactus finch). By breeding stage, we conducted 76 observations at 53 nests during the incubation period (mean ± SE observations per nest = 1.43 ± 0.07, range 1–3) and 37 observations at 28 nests during the chick feeding period (1.32 ± 0.12 observations per nest, range 1–3).

Methods
Study sites and species
We conducted our study from January to March 2024 on Floreana Island in the Galápagos archipelago, at two highland (humid zone) sites and one lowland (dry zone) site. The two highland sites, located at Asilo de la Paz (1°18’46” S 90°27’16 ”W) and the base of Cerro Pajas (1°17’46” S 90°27’06” W), consisted largely of remnant Scalesia pedunculata forest. The one lowland (dry zone) site comprised dry scrubland dominated by palo santo trees (Bursera graveolens) in and around the township of Puerto Velasco Ibarra (1°16’28” S 90°29’13” W). Our study included four of the five Darwin’s finch species considered extant on the island: the small ground finch (Geospiza fuliginosa), common cactus finch (Geospiza scandens), small tree finch (Camarhynchus parvulus), and medium tree finch (Camarhynchus pauper). Small ground finches and small tree finches were present at all three sites, medium tree finches only breed at the two highland sites, and cactus finches only at the lowland site. We did not identify any nests belonging to the island’s fifth extant species, the medium ground finch (Geospiza fortis).

Nest monitoring
We searched for Darwin’s finch nests at our three study sites and monitored the breeding activity of male finches whose territory defense behavior we had previously measured. Upon discovering an active display nest, we observed the male for ~20 min to determine if he was paired or unpaired. We then revisited the nest approximately every three days to confirm the male’s current status. If the male became paired, we continued to regularly monitor the breeding pair to determine the success of their nesting attempt. To establish the breeding stage (incubation or chick feeding), we either used a borescope to visually confirm the presence of eggs or nestlings or observed the parents’ incubation or provisioning behavior with binoculars, following a standardized protocol. When observing parental behavior, the observer was seated on the ground, approximately 10 m from the nest, with binoculars focused on the nest entrance. As Darwin’s finches do not noticeably alter their behavior in the presence of a human observer within 5 m, this observer distance was unlikely to disrupt parental behavior. All nest observations took place in the morning, between 06:00 and 12:00 GALT, to minimize time-of-day effects.

Territory defense trials
Our playback stimuli were constructed from Darwin’s finch songs recorded on Floreana Island in 2023. We created 25 unique playback tracks (8 small ground finch, 8 small tree finch, 7 medium tree finch, 2 cactus finch), each lasting 3 min (1 min of song playback, 1 min of silence, 1 min of song playback). Each 1-minute song playback period contained six repetitions of the same male song type, simulating a territory intrusion by a single unfamiliar conspecific.

Upon entering a Darwin’s finch territory, we placed the speaker in the branches of a shrub or tree about 5 m from the nest, at a height of 1–1.5 m. Playback tracks were randomly assigned to each subject, with the provision that each male received a conspecific song. We never started playback until the male was observed within 20 m of the speaker; in cases where the male was unbanded, we waited until he returned to the nest or interacted with the female to ensure we were observing the nest owner. After playback began, an observer (ACK or SK) narrated the male’s response into a digital audio recorder. For each trial, we quantified six response variables during the playback period: time (in secs) within 5 m of the speaker, time (in secs) within 1 m of the speaker, minimum distance (in m) from the speaker, number of flights, number of crosses (flights that crossed the speaker), and number of vocalizations (defined in Table 1). For a subset of trials (N = 24 of 168 trials), we also recorded the male’s behavior during a 1-minute baseline period prior to the beginning of playback.

Across 168 playback trials, we tested males at 98 nests (37 small ground finch, 28 medium tree finch, 27 small tree finch, 6 cactus finch). Of these, two small ground finch nests belonged to the same color-banded male over two consecutive breeding attempts. Territory defense trials were conducted across four stages of the breeding cycle: unpaired (male was unpaired and defending a display nest), paired (male was paired with a female), incubation (female was incubating eggs), and chick feeding (male and female were feeding chicks).

Parental care observations
During incubation and chick feeding, we conducted 1-hour parental care observations at Darwin’s finch nests between 06:00 and 12:00 GALT, following long-term standardized protocols for this study population. Observers were positioned about 10 m from the nest with binoculars focused on the nest entrance. During the incubation stage, we recorded: (1) the number of male food deliveries to the incubating female at or near the nest entrance, and (2) the total female incubation time (mins). During the chick feeding stage, we recorded: (3) number of male food deliveries to the chicks or females; and (4) total female brooding time (mins). In total, we conducted 113 parental care observations at 59 nests (24 small ground finch, 18 small tree finch, 15 medium tree finch, 2 cactus finch). For each observation, we estimated the within-stage timing of the breeding attempt based on our timeline of monitoring records. Given our monitoring schedule, this estimate was usually considered precise to within 3 days. Using this estimate, we then broadly assigned each protocol to the early (day 1–7) or late (day 8–14) stage of incubation or chick feeding.