BIOLOGICAL CONSERVATION 141 (2008) 601– 609

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Short communication Effects of the introduced parasite downsi on nestling growth and mortality in the medium ground finch ( fortis)

Sarah K. Huber*

Graduate Program in Organismic and Evolutionary Biology, University of Massachusetts, 319 Morrill Science Center, Amherst, MA 01003, USA

ARTICLE INFO ABSTRACT

Article history: Invasive have the potential to detrimentally affect native ecosystems by out com- Received 27 May 2007 peting or directly preying upon native organisms, and have been implicated in the extinc- Received in revised form tion of endemic populations. One potentially devastating introduced species in the 13 November 2007 Gala´pagos Islands is the parasitic fly . As larvae, P. downsi parasitize nestling Accepted 25 November 2007 and have been associated with high nestling mortality and reduced growth rates. Available online 31 January 2008 Here I document nestling growth and mortality in a bimodal population of the medium ground finch, Geospiza fortis. Observations were conducted over three years, and under var- Keywords: iable ecological conditions. Annual parasite prevalence in nests ranged from 64% to 98%, Darwin’s finches and nestling mortality in nests with parasites ranged from 16% to 37%. Parasite load and parasite load per nestling follow a skewed distribution with many nests having relatively Ectoparasite few parasites, and few nest having many. Parasite load, however, was not correlated with Nestling mortality onset of breeding, clutch size, the number of nestlings, nestling survival or fledgling suc- Growth rate cess. Parasite load per nestling, on the other hand, was correlated with clutch initiation Gala´pagos Islands date and the proportion of nestlings that died in parasitized nests. Neither nestling size nor growth rate differed between parasitized and unparasitized nests. In addition, male and female morphology was not correlated with parasite load, breeding variables or nestling survival. Thus, while overall mortality due to is high, ecological condi- tions and possible host defenses may potentially counter some of the detrimental affects of P. downsi on nestling size and growth. These results taken together suggest that parasitism of P. downsi larvae on nestling G. fortis has the potential to lead to large population declines. Ó 2007 Elsevier Ltd. All rights reserved.

1. Introduction Cleland, 2001; Snyder and Evans, 2006; Strayer et al., 2006; Weinig et al., 2007). Invasive species often out-compete or The ecological and evolutionary impacts of invasive species prey on native species, and have been implicated in extinc- on native organisms has been a major emphasis of recent re- tions of native organisms (Warner, 1968; Van Riper et al., search in conservation biology (Sakai et al., 2001; Mooney and 1986, 2002; Vitousek et al., 1987; Mack et al., 2000). Island

* Present address: Department of Biology, University of Utah, 257 South 1400 East Salt Lake City, UT 84112, USA. Tel.: +1 413 695 7069. E-mail address: [email protected]. 0006-3207/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2007.11.012 602 BIOLOGICAL CONSERVATION 141 (2008) 601– 609

biotas are thought to be particularly vulnerable to the nega- A 91° 90° tive impacts of invasive species, in part, because island habi- Pinta 50 km tats have lower species diversity, with fewer niches exploited Genovesa by native fauna, and they tend to support small population Marchena sizes of endemic organisms (Diamond, 1984; Dobson and 0° 0° McCallum, 1997; Dobson and Foufopoulos, 2001; Altizer Santiago et al., 2001; Vitousek, 2002). The Gala´pagos Islands of Ecuador have served as an excel- Santa lent model system for the study of ecology and Fernandina Cruz (Grant, 1999; Wikelski, 2005; Arbogast et al., 2006). However, an increasing number of invasive species are threatening na- Isabela tive biotas (Wikelski et al., 2004; Causton et al., 2006; Parker 1° San Cristobal 1° et al., 2006), with a recent count estimating the number of Floreana exotic species to be over 800 (Bensted-Smith et al., 2000). Espanola One potentially devastating invasive species is the fly Philornis 91° 90° downsi, which in the larval stage is a hematophagous parasite of nestling birds. In 1964 several specimens of P. downsi were B collected in the Gala´pagos Islands (Causton et al., 2006). Addi- tional collections were made in 1981; however, the presence of P. downsi went relatively unnoticed until 1997, when larvae were discovered in the nests of land birds (Fessl et al., 2001). To date, P. downsi has been found to parasitize at least 12 species in the Gala´pagos Islands (Fessl and Tebbich, 2002), El Garrapatero and has been recorded from 13 to 15 major islands (Wieden- feld et al., 2007). Philornis downsi has been implicated in the decline of populations of the mangrove finch, Cactospiza helio- Academy bates, and the warbler finch, Certhidea olivacea (Dvorak et al., Bay 2004; Grant et al., 2005). 10 km of the genus Philornis are common throughout the Neotropics (Dodge, 1955, 1963, 1968; Dodge and Aitkin, 1968; Fig. 1 – (A) Map of Gala´pagos Islands. Field work was Couri, 1984, 1986). Females presumably lay their eggs directly conducted at (B) El Garrapatero, Santa Cruz Island. on nestlings (Meinert, 1890; Dodge, 1971; Arendt, 1985), and larvae feed on the blood and fluid of nestlings. The early in- stars will often burrow in the nostrils of nestlings, causing two morphs on different food types, perhaps coupled with in- long term morphological damage (Fessl et al., 2006a). Later in- tra- or inter-specific . Indeed, intermediate birds stars live in the nesting material and surface at night to feed survive at lower rates between years in comparison to large on nestlings. Philornis downsi parasitism has been correlated and small morphs (A.P. Hendry et al., unpublished data). In with a reduction in nestling survival in several species of Dar- addition to determining the effects of P. downsi parasitism win0s finches, including the woodpecker finch (Cactospiza pall- on nestling growth and mortality, I will test whether beak size ida), small tree finch ( parvulus), warbler finch (C. confers a selective advantage with respect to parasitism. olivacea), small ground finch (Geospiza fuliginosa), and medium ground finch (Geospiza fortis)(Fessl and Tebbich, 2002; Fessl 2. Materials and methods et al., 2006b). Parasitism by P. downsi also has been found to impact negatively nestling growth rates in a combined analy- 2.1. Parasite prevalence and nestling mortality sis of small ground finches and medium ground finches (Fessl et al., 2006b), and result in reduced hemoglobin and immature Research was conducted at El Garrapatero, Isla Santa Cruz, red blood cell counts in the small ground finch (Dudaniec Gala´pagos, Ecuador (Fig. 1, GPS coordinates S 00°4002000– et al., 2006). 4102000;W90°1301000–1404000). Nests of G. fortis were observed This study documents the impacts of P. downsi parasitism from January to April 2004, January to May 2005, and January on nestling survival and growth in the medium ground finch, to March 2006. The average monthly rainfall during these G. fortis, at El Garrapatero, Santa Cruz Island, Gala´pagos periods was collected by weather stations at the Charles Dar- (Fig. 1). Interestingly, the population of G. fortis at El Garrapa- win Research Station in Academy Bay (Fig. 1). tero is bimodal with respect to beak morphology, with birds In 2004, the fate of each nest was recorded as ‘‘successful’’ falling mainly into large and small beak size classes with rel- (nestlings fledged) or ‘‘failed’’ (nestlings were predated or atively few intermediates (Hendry et al., 2006; Huber and Po- found dead in the nest). No data was collected on rates of pre- dos, 2006). As in other populations of G. fortis (Grant and dation or parasitism; however, dead nestlings were examined Grant, 2006; Grant, 1999; Price, 1987; Keller et al., 2001), beak for P. downsi larvae and the presence of lesions associated size variation likely has a strong additive genetic basis and re- with larval feeding. flects selection imposed by variation in the size and hardness In 2005, nests were located, and checked every 3–4 days of seeds. The bimodality is likely due to specialization by the during the breeding season. During each nest check the BIOLOGICAL CONSERVATION 141 (2008) 601– 609 603

number of eggs or nestlings was recorded, and nestlings were growth trajectories of nestlings from parasitized to unparasit- inspected for external signs of parasites (e.g., lesions on the ized nests, I used reduced major axis regression (RMA). A RMA body). A decrease in the number of eggs or nestlings from that results in a regression line with a slope of one would indi- one visit to the next was presumed to be the result of preda- cate that size at a given age is not statistically different be- tion. Predators of Darwin0s finches include mockingbirds, tween parasitized and unparasitized nestlings. owls, and in some cases rats (Grant, 1999). There is no evi- Growth rates were determined for individual nestlings by dence to suggest that eggs are ejected by Darwin0s finch calculating the average change in a variable per day. Growth adults or nestlings, and visual inspection of the ground under rates were then averaged across all nestlings in a nest. Due predated nests in this study failed to recover any eggs. When to high nestling mortality, growth rates could be calculated nests were no longer active (e.g., due to predation, the death for two unparasitized nests and seven parasitized nests. A of nestlings, or fledging), nests were collected and the number Kruskal–Wallis test was used to compare median growth of P. downsi larvae and pupae per nest were recorded. Because rates between parasitized and unparasitized nests for each larvae feed at night, it was not possible to determine the variable. number of parasites infecting a given nestling within a nest. Therefore, to determine parasite load per nestling, I divided 2.3. Adult beak morphology the total number of parasites by the number of nestlings in the nest. In summary, I recorded for each nest: clutch initia- Adult birds were captured in mist nets and banded with un- tion date, clutch size, the number of eggs predated, the num- ique combinations of one metal and three color bands. The ber of nestlings predated, the number of nestlings dead in the following measurements were taken from each bird, as in nest, the number of nestlings fledged, the parasite load, and Grant et al. (1985): beak length, beak depth, and beak width. the parasite load per nestling. The first axis of a principle component analysis (PC1) that in- In 2006, nests were checked every other day and the num- cluded beak length, beak depth, and beak width was used in ber of eggs and nestlings recorded. Birds were again examined future analyses of adult beak morphology. Birds were classi- for the presence of lesions associated with P. downsi larval fied into three beak sizes classes (small, intermediate, and feeding. I recorded for each nest: clutch initiation date, clutch large) based on an hierarchical cluster analysis of PC1 (for size, the number of eggs predated, the number of nestlings methods see Huber et al. (2007)). A v-square test was used predated, the number of nestlings dead in the nest, the num- to determine if the beak size of breeding birds was represen- ber of nestlings fledged, and whether parasites were present tative of the population as a whole. or absent. The number of nests with P. downsi larvae was re- To test for the influence of parental beak morphology on corded, but the number of larvae per nest (parasite load) nestling mortality and parasite loads I correlated male and fe- was not determined. male beak PC1 scores from 2005 with clutch initiation date, Observations of nestling survival were terminated prior to clutch size, number of nestlings, number of dead nestlings the end of the breeding season in 2005 and 2006. At the end of in nests with parasites, proportion of dead nestlings in nests the observation period in 2005, the fate of 12 nestlings from with parasites, number of nestlings fledged, proportion of four nests was unknown. At the end of the observation period nestlings fledged, parasite load, and parasite load per nest- in 2006, the fate of 10 nestlings from five nests was unknown. ling. Male and female beak PC1 scores from 2006 were corre- These 22 nestlings were removed from further analyses. lated with clutch initiation date, clutch size, and number of nestlings. Sample sizes were too low in 2006 to correlate beak 2.2. Nestling size and growth rate morphology with the number of dead nestlings in nests with parasites (n = 3 nests), the proportion of dead nestlings in Data on nestling size and growth rate was collected in 2006 nests with parasites (n = 3 nests), the number of nestlings only. Individual nestlings within a nest were marked with a fledged (n = 1 nest), or the proportion of nestlings fledged unique colored plastic leg band, or by coloring a toenail with (n = 1 nest). In 2006, parasite load was not determined. Rather, non-toxic paint. Nestlings were measured every other day, the presence or absence of parasites was assessed. A logistic and measurements included: mass, tarsus, gape, beak length, regression was used to determine if beak morphology was a beak width, and beak depth. predictor of parasite presence or absence. For measures of both size and growth rate the mean value The collection of data in this study was done in concor- of nestlings within a nest was used for statistical analysis. dance with use protocols approved by the University This is because siblings are genetically similar, and thus sta- of Massachusetts Amherst. tistically non-independent. The mean size of nestlings within a nest was determines for each day post hatching (1 day old, 3. Results 2 days old, etc.). Nestlings hatch asynchronously, and so mea- surements were collected at days one, three, five, etc., for 3.1. Parasitism and nestling mortality some nestlings within a nest, and days two, four, six, etc., for other nestlings within the same nest. Because nestling High nestling mortality occurred in all three years of this mortality was high, in some nests only a single nestling was study. In 2004, 100% of nests failed: all nestlings were pre- measured at given age. dated or were found dead in the nest. Dead nestlings had le- The size of parasitized nestlings at each age was then plot- sions indicative of larval feeding by P. downsi, and larvae or ted against the size of unparasitized nestlings of the same age pupae were found in the nesting material of all nests with for all morphological variables. To determine the size and dead nestlings. In 2005, nestling mortality associated with 604 BIOLOGICAL CONSERVATION 141 (2008) 601– 609

parasitism was high, though not as high as predation rates, 3.2. Nestling size and growth rate and larvae and pupae of P. downsi were found in all nests that contained dead nestlings (Table 1). Larvae were frequently Nestlings from parasitized and unparasitized nests were found feeding inside the head and body cavity of dead nes- similar in size throughout the nestling period (Fig. 3). The re- tlings. In 2006, fewer nestlings died in nests with parasites sults of a reduced major axis regression reveal no difference (Table 1). However, many nestlings that were ultimately pre- in size between parasitized and unparasitized nestlings in dated also showed signs of P. downsi parasitism (e.g., lesions mass, tarsus, beak gape, beak length, or beak width (Table on body and/or larvae and pupae in empty nest). 3). In all cases, except beak depth, the 95% confidence inter- Parasite load per nest follows a skewed distribution, com- vals of the slopes include one. Beak depth is the only vari- mon in many parasite systems (Fig. 2a), even when standard- able for which the 95% confidence intervals do not ized for the number of nestlings per nest (Fig. 2b). However, encompass a slope of one. Young birds have similar beak parasite load was not a significant predictor of any breeding depths; however, as nestlings get older the beak depth of variables, nestling mortality, or fledging (Table 2). Parasite unparasitized birds tends to be larger than that of unparasit- load per nestling was significantly correlated with clutch ini- ized birds (Fig. 3). tiation date (Table 2). Nests earlier in the breeding season had Mean growth rates of nestlings in parasitized and unpara- significantly more parasites per nestling than did nests later sitized nests were not significantly different for any of the six in the breeding season. This is likely because clutch initiation variables measured (Fig. 4; mass: v2 = 0.86, P = 0.77; tarsus: date has a significant, positive relationship with clutch size v2 = 0.54, P = 0.46; gape: v2 = 0.78, P = 0.38; beak length: (F = 39.66, R = 0.76, p < 0.001); clutch sizes were higher later v2 = 0.09, P = 0.77; beak depth: v2 = 0.78, P = 0.38; and bead in the breeding season. Parasite load per nestling also was sig- width: v2 = 0.10, P = 0.38). nificantly correlated with the proportion of nestlings in a nest that died; nests with a higher number of parasites per nest- 3.3. Ecological conditions and nestling mortality ling had a larger proportion of nestlings die (Table 2). In 2004, 100% of observed nestlings died, and this was one of the driest breeding seasons on record (total rain January– May = 51.1 mm). The early part of the breeding season in Table 1 – Summary of parasite prevalence and nestling 2005 was also extremely dry. In February 0.2 mm of rain fell, mortality in G. fortis and only six pairs bred. Of those, four out of six nests pro- duced nestlings. Heavy rains fell from March 9 to 15 2005 2006 (109.2 mm), with little to no rain falling after March 15. Very Number of nests with nestlings 47 16 little rain fell in April (0.8 mm). After the rains, breeding Number of nests with parasites 44 of 45 (98%) 9 of 14 (64%) was synchronous with two periods in which nestlings were Number of nestlings 129 32 present in nests: March 27–April 16 and April 28–May 10. Number nestlings predated 71 (55%) 25 (78%) In 2005, mortality in nests with parasites was highest in Number dead nestlings in 48 (37%) 5 (16%) February (86%: six out of seven nestlings), followed by the sec- nests with parasites Number fledged 10 (8%) 2 (6%) ond cohort after the rain, April 28–May 10 (46%: 19 out of 41 nestlings). The first cohort of nestlings to hatch after the rain,

12 12

10 10

8 8

6 6

Number of nests 4 4

2 2

0 0 0 20 40 60 80 100 120 140 160 180 200 0102030405060 Parasite load Parasite load per nestling

Fig. 2 – Frequency distribution of parasite loads in 2005. (A) Parasite load was determined for individuals nests. (B) Parasite load was then standardized to the number of nestlings per nest. BIOLOGICAL CONSERVATION 141 (2008) 601– 609 605

Table 2 – Results of a Pearson correlation of parasite load and parasite load per nestling with breeding variables, nestling mortality, and fledging success (N = number of nests)

Parasite load Parasite load per nestling NR p R p

Clutch initiation date 32 0.28 0.12 0.37 0.04 Clutch size 32 0.11 0.55 0.34 0.06 Number of nestlings 32 0.03 0.88 0.33 0.06 Number of dead nestlings in nests with parasites 19 0.39 0.10 0.06 0.81 Proportion of dead nestlings in nests with parasites 19 0.36 0.13 0.56 0.01 Number of nestlings fledged 5 0.31 0.61 0.58 0.30 Proportion of nestlings fledged 5 0.31 0.61 0.58 0.30 Values in bold are statistically significant.

16 20 14 18 12 16 10 14 8 12 Mass (g) 6 Tarsus (mm) 4 10 2 8 0 6 0246810121416 6 8 10 12 14 16 18 20 Mass (g) Tarsus (mm) 11 7

10 6

9 5

8 4 Gape (mm) 7 3 Beak length (mm) PARASITIZED NESTS 6 2 67891011 234567 Gape (mm) Beak length (mm) 7 8

7 6 6 5 5 4 Beak depth (mm) Beak width (mm) 4

3 3 34567 345678 Beak depth (mm) Beak width (mm) UNPARASITIZED NESTS

Fig. 3 – Size of parasitized and unparasitized nestlings at days 1–8 and day 11. The dashed line has an intercept through the origin and a slope of one. Points falling on this line indicate that the size of parasitized and unparasitized nestlings is equal. from March 27 to April 16, had the lowest nestling mortality ever, when observations came to an end in May, 12 nestlings (29%: 23 out of 81 nestlings). Fledglings were recorded only were still alive. The fate of these nestling is unknown, from this cohort (12%: 10 out of 81 nestlings fledged). How- although all had visual signs of parasites. 606 BIOLOGICAL CONSERVATION 141 (2008) 601– 609

lated mortality, coupled with mortality due to predation, Table 3 – Results of a reduced major axis regression of size variables for parasitized and unparasitized nestlings resulted in only 12 fledglings out of 159 nestlings over a three year period. While combined mortality was high in all years, 2 Intercept Slope R predation of nestlings was much higher in 2006 than in Mass 1.12 (3.45, 1.20) 1.32 (1.00, 1.64) 0.93 2005, and nestling mortality in parasitized nests was lower Tarsus 0.76 (3.52, 1.99) 1.12 (0.91, 1.33) 0.95 in 2006 than in 2005 (Table 1). However, many of the nestlings Gape 0.52 (2.98, 1.93) 1.05 (0.79, 1.31) 0.92 that were predated in 2006 were also parasitized by P. downsi. Beak length 0.16 (1.05, 0.73) 1.07 (0.88, 1.27) 0.96 Had predation been lower, many of those nestlings would Beak depth 0.92 ( 1.82, 0.01) 1.26 (1.06, 1.46) 0.97 likely have died due to parasite related causes. Beak width 0.38 (2.17, 1.41) 1.12 (0.79, 1.44) 0.89 Parasite load was variable across nests with many nests The 95% confidence intervals are reported in parentheses. having relatively few parasites and a few nests having many parasites (Fig. 2). This distribution is a common feature of 1.8 many host-parasite systems (Anderson and Gordon, 1982). No parasites (n = 2 nests) Parasite load was not correlated with any aspects of host 1.6 Parasites (n = 7 nests) breeding behavior or nestling mortality in this system (Table 2). However, when parasite load was calculated as a function 1.4 of the number of nestlings in the nest, it was correlated with clutch initiation date (Table 2). This result suggests that birds 1.2 which breed first might be more vulnerable to parasitism than later nesting birds, and stands in contrast to previous studies 1.0 of hematophagous parasites in other species of birds which 0.8 found no relationship between parasite load and date of clutch initiation (Hurtrez-Bousses et al., 1999). Parasite load 0.6 per nestling was also correlated with the proportion of nes- tlings that died in a nest. These results together suggest that 0.4 selection should favor birds with larger clutch sizes, which Growth Rate (change per day) would effectively reduce the number of parasites per nestling 0.2 within the nest. 0.0 Nestling size and growth were not found to differ between Mass (g) Tarsus Gape Beak Beak Beak parasitized and unparasitized nests, a finding that stands in (mm) (mm) length depth width contrast to previous studies of P. downsi parasitism of Dar- (mm) (mm) (mm) win0s finches (Fessl et al., 2006b). The failure of this study to detect differences between parasitized and unparasitized Fig. 4 – Mean nestling growth rate (calculated as the mean nestlings may be due to small samples sizes, as few nests change in a variable per day) between parasitized and were parasite free. However, variation in ecological conditions unparasitized G. fortis nests. Differences are not statistically might also explain these results. During the Fessl et al. (2006b) significant. study in 2004, very little rain fell, and Santa Cruz experienced one of the most severe droughts in a 40 year period (Grant and 3.4. Beak morphology and nestling mortality Grant, 2006). Starvation and malnourishment potentially con- tributed to decreased nestling growth rates in parasitized Beak morphology was determined for males and females of nests. By contrast, late 2005 and 2006 were very wet years, 38 pairs in 2005 and 15 pairs in 2006. The distribution of beak and food resources were abundant. The increased availability morphology of breeding adults was representative of the pop- and quality of food during these years might have off-set the ulation as a whole (2005: v2 = 5.08, p = 0.08; 2006: v2 = 3.00, detrimental effects of parasitism to nestling growth. The 2005 p = 0.22). Beak morphology of males and females was not cor- breeding season provides some evidence that rainfall might related with any breeding factors in 2005 and 2006 (Table 4), or influence nestling condition. During the wettest part of 2005 with nestling mortality and parasite load in 2005 (Table 4). In (late-March to early-April) nestling mortality due to parasites 2006, only three nests contained dead nestlings, precluding a was the lowest (29%), and this was the only period of time in rigorous statistical test of the relationship between beak mor- which nestlings fledged. phology and nestling mortality. Parasite prevalence, but not The potential evolution of host defenses, such as in- load, was determined for nests in 2006. A logistic regression creased adult provisioning, also may help to explain why found that beak morphology was not a significant predictor growth rates in this study differed from those of Fessl et al. of parasite prevalence (male beak PC1: N = 13 nests; (2006b). Several studies documenting the impacts of hema- W = 1.42, p = 0.23 and female beak PC1: N = 13 nests; tophagous ectoparasites on nestling growth in other avian- W = 0.34, p = 0.56). parasite systems have found no difference in size or growth rates between parasitized and unparasitized nestlings (John- 4. Discussion son and Albrecht, 1993; Merino and Potti, 1995; Hurtrez-Bous- se`s et al., 1997; Miller and Fair, 1997). One hypothesis for this Nestling mortality in nests with P. downsi larvae was extre- result is that adults adjust feeding rates when nestlings are mely high in G. fortis at El Garrapatero (Table 1). Parasite re- parasitized (Mason, 1944; Johnson and Albrecht, 1993). In BIOLOGICAL CONSERVATION 141 (2008) 601– 609 607

Table 4 – Pearson correlation coefficients for beak morphology and breeding variables, nestling mortality, and fledging success for 2005 and 2006 (N = number of nests)

2005 2006 N Male beak PC1 Female beak PC1 N Male beak PC1 Female beak PC1

Clutch initiation date 28 0.01 0.06 23 0.18 0.21 Clutch size 28 0.15 0.05 21 0.06 0.09 Number of nestlings 26 0.30 0.22 15 0.02 0.18 Number of dead nestlings in nests with parasites 19 0.29 0.13 – – – Proportion of dead nestlings in nests with parasites 19 0.36 0.03 – – – Number of nestlings fledged 5 0.68 0.59 – – – Proportion of nestlings fledged 5 0.68 0.59 – – – Parasite load 26 0.002 0.17 – – – Parasite load per nestling 26 0.21 0.34 – – – No correlations were statistically significant (p > 0.05). support of this hypothesis, adult provisioning has been found 2006). Assuming that P. downsi was present at El Garrapatero to increase in nests of blue tits with hematophagous fly larvae during this period, Darwin0s finches may have begun to (Christe et al., 1996; Triplet and Richner, 1997; Hurtrez-Bous- evolve defenses against parasites in the 40 years ensuing se`s et al., 1998; Banbura et al., 2004). There are currently no the introduction of P. downsi. Possible host defenses that data on feeding rates of adult Darwin0s finches with respect have not been tested in any population of Darwin0s finches to parasite load or ecological conditions. to date include increases in adult provisioning, nestling Previous studies of beak morphology in this population behavior, and the evolution of a Philornis-specific immune suggest that large and small morphs may have a fitness response. advantage in comparison to birds with intermediate sized No long term data exist that document interactions be- (Hendry et al. 2006; Huber et al. 2007; A.P. Hendry tween Darwin0s finches and P. downsi in a single population, et al., unpublished data). However, beak morphology was and in general little is known about the natural history of P. unrelated to parasite load. Thus, it appears as though bill downsi (Dudaniec and Kleindorfer, 2006). Parasitism of P. morphology does not confer a fitness advantage with respect downsi larvae on nestling G. fortis has the potential to lead to parasitism. Interestingly, parasitized nestlings had smaller to dramatic population declines of Darwin0s finches, not just beak depths than unparasitized nestlings (Table 3, Fig. 3). in this population, but through out the archipelago as a whole Selection acts on beak size and shape in response to food (Wiedenfeld et al. 2007; Wikelski et al., 2004). Detailed, long- availability and inter- and intra-specific competition (Boag term research into the dynamics of host-parasite adaptation and Grant, 1981; Schluter et al., 1985; Grant and Grant, and coevolution in Darwin0s finches is urgently needed. 2006), and beak morphology, in turn, correlates with bite force (Herrel et al., 2005a,b) and acoustic features of songs (Podos, Acknowledgements 2001; Huber and Podos, 2006). Thus, parasitized offspring might have reduced fitness after reaching adulthood. In addi- I thank the Gala´pagos National Park and the tion, parasitized birds have been found to have deformed Research Station for providing support for this research. Jef- beaks as a result of larval feeding in the nasal cavity (Fessl frey Podos, Andrew Hendry, Anthony Herrel, Luis Fernando et al., 2006a). These results have interesting implications for de Leon, Bieke Vanhooydonk, Ana Gabela, Eric Hilton, Haldre how heritability is measured in populations. Heritability esti- Rogers, Dan Buresh, and Steve Johnson provided assistance in mates are calculated by comparing traits of adults to their off- the field. Funding was provided by National Science Founda- spring. Charmantier et al. (2004) found that nests containing tion Grant IBN 0347291 to Jeffrey Podos, National Science hematophagous fly larvae had decreased heritability esti- Foundation Grant DDIG 0508730 to Jeffrey Podos and S.K.H., mates of morphological traits in comparison to nests without an American Ornithologists0 Union Student Research Grant, larvae. Thus, knowledge about parasite prevalence is vital to an Explorer0s Club grant, a Sigma Xi Grant-in-Aid of Research, determining how trait variation is measured across genera- an Animal Behaviour Society Student Research grant, and a tions in natural populations. The degree to which parasitized University of Massachusetts Woods Hole Fellowship. Jeffrey nestlings differ from their parents with respect to beak mor- Podos, Andrew Hendry, Ethan Clotfelter, Elizabeth Jakob, Ben- phology has implications for the stability of the bimodality jamin Normark, Dale Clayton and three anonymous review- at El Garrapatero. ers provided comments on earlier drafts of this manuscript. In conclusion, the relationship between Darwin0s finches, P. downsi, and finch predators is dynamic. While overall mor- tality due to parasitism is high, ecological conditions and REFERENCES possible host defenses might have the potential to counter some of the detrimental affects of P. downsi on nestling size and growth. The earliest record of P. downsi on the Gala´pagos Altizer, S., Foufopoulos, J., Gager, A., 2001. Conservation and Islands is 1964, and collections made by Peck et al. on Santa diseases. In: Levin, S. (Ed.), Encyclopedia of Biodiversity. 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