We caught in total four cats (two females and two males)—though we used baited traps set from mid-April, all captures occurred within 1 week in early May. Of the four cats, the last cat collared (a female) was younger and smaller than the other three, and since the data collected for this cat was very short-term (24 days) and showed significantly shorter-range movements than the others, we chose to disregard these data on the grounds that they appear not to be representative for adult feral cats. A summary of the GPS data collected for the three cats is given in Table 1.
Home ranges and general habitat use
The home ranges of the cats ranged from 1.12 to 1.46 km2 with 1.27 km2 being the mean value (Table 2). Cats C2417 (female) and C2418 (male) had larger home ranges than C2419 (male), but the core range for C2419 was larger than that of the other two cats, implying increased consistency in its use of the area available to it. There was very little difference between the nocturnal and diurnal ranges of each cat, with the home ranges overlapping by an average of 86% of the area. Only one cat, C2419, showed high variation in its diurnal and nocturnal area use, but only at the core range level, which overlapped by only 35.1% between day and night. The home ranges of all three cats overlapped and the range of the smallest (therefore possibly the youngest) cat, C2419, existed in the middle of those of the other two (Fig. 2). Cats C2418 and C2419 showed the largest overlap of 36% of the total area covered by their home ranges (Table 3); the core ranges of cats C2417 and C2419 also overlapped.
Overlap between cat ranges and selected species of interest
Red-footed boobies and other waterbirds
The recorded red-footed booby nesting area was not within any of the home ranges of the cats. GPS fixes of only one cat, C2418, came within 500 m of the nesting area and only one of its fixes was somewhat close (101 m away). The salt lake—which attracted multiple waterbird species—did sit within the home range of cat C2417 and partially overlapped with its core range, but the GPS fixes for that cat did not appear to cluster around its edge in comparison with other areas. The home range of the second nearest cat, C2419, was 110 m from the lake at its closest point. It did pass by the lake on one occasion and was recorded at 6 m from it, but did not stay in the area for long, being recorded in a different location 30 min later.
Galápagos marine iguanas
The home and core ranges of all three cats overlapped to some extent with one or both iguana colonies (Fig. 2). The average overlap of the home and core ranges with the iguana ranges was 11.9% and 6.2% respectively (Table 4). The home range of C2418 had one outlying area, which overlapped precisely with a recorded iguana nesting area (Fig. 2). This cat made five separate visits to this area on 10, 14 and 20 May and 2nd and 12 June. All visits were between 01:30 and 09:00.
Although the exact date of iguana hatching in this area was unknown, frequent surveys allowed the estimation of a date (1 June) which was likely within a 3-day window of the first hatchings. When comparing home and core ranges created using only GPS fixes from before, or from after, the 1 June, for each cat, we found that there was very little change in the overlap of the before and after ranges with the iguana areas (Table 4).
Of the three cats studied, C2417 had a home range that overlapped significantly with the recorded green turtle (Chelonia mydas) nesting areas, with 23% of GPS fixes within the nesting zone. Our observations of these nests indicated high hatching activity in the early evening in keeping with published data for this species (Salmon and Reising 2014). When the movements of C2417 during the early evening (defined as 18:00–22:00) were considered in isolation, the proportion of GPS fixes within the nesting area rose to 59%. Twenty-eight percent of the core range created only from the GPS fixes within the early evening time frame overlapped with the turtle nesting area, compared with 18% of the original core range encompassing all the fixes for this cat (Fig. 3).