We report the first data on the breeding biology of the five most abundant open-cup forest-dwelling birds in their breeding grounds at the southernmost forests of the world. Although most of these species are facultative cavity nesters in other localities [34, 35], we found they exclusively built open cup nests on Navarino Island. It is noteworthy that we found differences in nesting strategies of T. falcklandii breeding at different latitudes.
In general, the relative nest abundances for each species parallels their frequency of capture in the study area during the breeding season [36]. The main exception is P. patagonicus, which was the most frequently captured bird [36], but represented only 17% of the nests we found. A possible explanation for this difference could be due to the difficulties in locating nests for this species. One cue we used to find nests was observing birds flushing from a nest. Unlike the other species, P. patagonicus did not flush from nests, even when we were less than 1 m away.
The observed nest characteristics for each species in our study generally agreed with those described in other locations [37,38,39,40]. However, farther north, four of our species have also been described nesting in cavities [34, 35]. Specifically, T. falcklandii has been described as a facultative cavity nester (10.1–90.0% of nests in tree cavities), P. patagonicus as a marginal cavity nester (1–10% of nests in cavities), and both E. albiceps and Z. capensis as incidental cavity nesters (< 1% of nests in cavities) [34, 35]. We did not find any active cavity nests for these species. This finding could be a response to the relatively low number of potential predators in the study area, as it is traditionally accepted that cavities confer protection from predators [41, 42].
The fastest egg laying rhythm was for Z. capensis that laid daily, and the slowest was A. parulus, which separated eggs by more than 2 days. Only two previous studies reported this breeding trait, both referring to Z. capensis, indicating the same daily laying rhythm that we report here [43, 44]. If a nest was located after the first egg had been laid, then egg laying rhythm was taken into consideration when we estimated nest initiation date. In general, when this information is not available, authors assume one egg is laid per day (as we did in this study) [26, 43, 45]. However, as our records suggest, this does not necessarily apply to every passerine species. Given that some species, such as A. parulus and E. albiceps, might lay less than one egg per day, this assumption may bias estimation of egg laying date towards later in the season.
The present study is the first that reports developmental periods (i.e., incubation and nestling periods) for P. patagonicus, which lasted approximately 1 month from the start of incubation until chicks fledged (Table 2). Duration of these periods for T. falcklandii and Z. capensis were similar to those that have been described previously [38, 43, 44, 46, 47]. The only records for A. parulus are from [38] who reports, without specifying the region, that incubation lasts 12 days and nestling 13 days. For this species, we observed longer developmental periods on Navarino Island (incubation = 15.5 days [n = 2], nestling = 16.1 [n = 4]). Based on only one observation in an unknown location, [38] also described the incubation and nestling period of E. albiceps as 12 and 13 days, respectively. On Navarino Island we observed somewhat longer developmental periods for this species (incubation = 14.5 days [n = 3], nestling =16.1 days [n = 6]). Our observations agree with the developmental periods of E. albiceps on Chiloé Island, Chile, where incubation lasts 14–15 days and nestling 14–17 days [47].
As predicted, we found that E. albiceps breeds later compared to most of the other species. This is the only long-distance forest migratory species that breeds on Navarino Island [48, 49] and is the last to arrive on the breeding ground (in October). The other migratory species, Z. capensis, arrives in August from central Chile [36]. The same pattern occurs in central Chile, where E. albiceps breeds later than other sympatric species [5]; it arrives in September but it starts breeding in November, with its peak of breeding activity in December [4]. Interestingly, over 3500 km south on Navarino Island, E. albiceps also starts breeding in November, with its peak of breeding activity also in December, even though it arrives to the island in mid-October, a month later than the northern population. Therefore, on Navarino Island E. albiceps starts breeding considerably sooner (1 month) after arrival compared to lower latitude populations. This raises evolutionary questions about adaptive behaviors for a small bird worth studying along the latitudinal breeding range.
Both E. albiceps and Z. capensis started breeding earlier during the last breeding season. Coincidently, spring temperatures during the 2016–2017 breeding season were higher than the previous season. In September, the mean temperature in 2015 was 1.83 °C (SE = 0.51) and in 2016 it was 5.33 °C (SE = 0.34) [15]. Given our limited data, we were not able to test whether there is an effect of spring temperature on egg-laying date. However, we urge scientists to implement a long-term monitoring program on the breeding phenology of these species on Navarino Island to assess whether climate change may be affecting local species. Contrastingly to these two species, we did not find annual differences in laying dates for the resident species; nevertheless, this result should be taken with caution given the limited sample size. In addition, there seems to be only one brood per species per season (Fig. 2).
As predicted, species differed in substrate used for nesting, but overall, they were generalists. The substrates used by the five species on Navarino Island coincided with reports from other locations [38, 39, 43]; however, no previous study has reported a substrate diversity index. We found that E. albiceps had one of the highest diversity indexes, suggesting that this species may be more adaptable to potential changes in its environment. Noteworthy, E. albiceps uses exotic pine plantations as a substrate for nesting in central Chile [4]. In contrast, a less substrate-generalist species such as A. parulus, might not be able to respond to the rapid landscape changes that are currently taking place on Navarino Island.
As predicted, and possibly associated with the lack of terrestrial predators on Navarino Island [18], we found that two of the five birds in our study area were primarily ground nesters, and the other three placed nests on average less than 1.5 m from the ground. Interestingly, E. albiceps and T. falcklandii on Navarino Island nested closer to the ground than populations farther north. In our study area, E. albiceps nested between 0.4 m and 3.6 m from the ground, with 50% of them under 1 m. This only partially corresponds to what has been described for this species in a study conducted in central Chile (35°S), where this species nested between 0.5 and 15 m above the ground [4]. Additionally, [38] described three nests for this species, all at least 2 m above the ground. Turdus falcklandii also nested very close to the ground (median = 10 cm) on Navarino Island, which differs from what we found in La Araucanía Region (median = 319 cm) and with previous studies that have reported this species building its nests at least a few meters from the ground. In our study area, the average canopy height was about 15 m, but we did not find nests near that height for either E. albiceps or T. falcklandii, even though these birds often feed in the canopy. Unlike the forest community in La Araucanía Region, which includes ground predators such as wild cats and foxes [20, 21], on Navarino Island birds evolved in the absence of ground mammalian predators. It could be that by placing their nests closer to the ground on this island, birds avoid nest depredation by native raptors [50, 51]. Today, this behavior could, however, put Navarino birds at a higher risk of predation by recently introduced ground predators, particularly the American mink (N. vison) [18, 52]. Similar naivety to mink predation given the lack of coevolution was suggested for small rodents on this island [53]. However, because previous reports [38, 39], as well as our analysis, are based on a limited sample size, these comparisons should be taken with caution. Furthermore, placing nests closer to the ground could be a response to different biotic and abiotic factors, such as understory structure, temperature, and/or wind speed.
We found no major differences when comparing clutch sizes for the five passerines from Navarino Island to the descriptions available in the literature [2, 3, 5, 37,38,39, 43,44,45,46, 54,55,56,57,58,59,60,61]. However, when we made this comparison based on our data the difference in clutch size was clear. As predicted, the clutch size for T. falcklandii on Navarino Island (3–4 eggs) was statistically higher than La Araucanía Region (2–3 eggs). This corresponds with the frequently described pattern of larger clutch size at higher latitudes [22, 23, 41, 62]. It also corresponds with the pattern of larger clutch size for species nesting on islands compared to continents [63]. Finally, it could also be that a larger clutch size of T. falcklandii on Navarino Island is an interaction between latitude and island habitat [63].