Justification of Red List category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km² combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size has not been quantified, but it is not believed to approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
The population size is not known. The species is considered to be scarce, but widespread and locally common or numerous; it may be easily overlooked (Martínez-Vilalta et al. 2020).
The species is suspected to be in decline as a consequence of habitat loss and local impacts of hunting. Over three generations (11.6 years), 6% of tree cover is lost within the range (Global Forest Watch 2021, using Hansen et al.  data and methods disclosed therein). Given that the species is strictly dependent on forests near watercourses, the rate of population decline may be larger than the rate of tree cover loss due to the impacts of habitat fragmentation and degradation; moreover, hunting may additionally aggravate the rate of population decline. Therefore, the rate of population decline is tentatively placed in the band 10-19% over three generations.
Zebrilus undulatus is a poorly known species of the Orinoco, Negro and Amazon basins. It occurs from east Colombia and Ecuador, where it is considered rare and local (del Hoyo et al. 1992). In bordering Venezuela there have been an increasing number of records, and the species may be locally common (Hilty 2003). From here, its range extends eastwards through Guyana to Suriname, French Guiana and north-central Brazil, and south-west to east Peru and north-west Bolivia (del Hoyo et al. 1992). It appears generally very rare, but has probably been significantly under-recorded due to its elusive nature and is likely to have a wider distribution than is currently known (Restall et al. 2006, A. Lees in litt. 2011).
This species occurs in streams, slow-moving rivers, pools, marshes and swamps of tropical evergreen forests, as well as in mangroves and gallery forests in savanna habitat (Martínez-Vilalta et al. 2020). Its diet is not well documented, but is thought to mainly consist of small fish and flying insects (del Hoyo et al. 1992).
The primary threat to this species is accelerating deforestation in the Amazon basin, as land is cleared for cattle ranching and soy production, facilitated by expansion of the road network (Soares-Filho et al. 2006, Bird et al. 2011). Changes to the Brazilian Forest Code reduced the percentage of land a private landowner is legally required to maintain as forest (including, critically, a reduction in the width of forest buffers alongside perennial steams) and included an amnesty for landowners who deforested before July 2008 (Bird et al. 2011). Locally, the species is susceptible to hunting, for example in Venezuela (del Hoyo et al. 1992, A. Lees in litt. 2011).
Conservation Actions Underway
The species occurs in several protected areas across its range, including Pacaya-Samiria National Reserve (Peru) and Cantão State Park (Brazil). It is listed as Vulnerable at the national level in Ecuador (Freile et al. 2019).
Conservation Actions Proposed
Quantify the population size. Monitor the population trend. Expand the protected area network to effectively protect key sites. Effectively manage protected areas, utilising emerging opportunities to finance protected area management with the joint aims of reducing carbon emissions and maximizing biodiversity conservation. Incentivise conservation on private lands through expanding market pressures for sound land management and preventing forest clearance on lands unsuitable for agriculture (Soares-Filho et al. 2006).
28-33 cm. Smallish, dark heron. Cryptic, brown and black vermiculated plumage. Variable leg colour.
Text account compilers
Butchart, S., Ekstrom, J., Khwaja, N., Lees, A. & Symes, A.
BirdLife International (2023) Species factsheet: Zebrilus undulatus. Downloaded from http://datazone.birdlife.org/species/factsheet/zigzag-heron-zebrilus-undulatus on 04/12/2023.
Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://datazone.birdlife.org on 04/12/2023.