**6. Changes in** *P. downsi* **behaviour since colonising the Galápagos Islands**

#### **6.1 Age of larval cohort in host nests**

There is evidence that the oviposition behaviour of female *P. downsi* has changed since its discovery on the Galápagos archipelago. *Philornis downsi* flies are now known to oviposit during any stage of the nesting cycle [45]. In the first decades following initial discovery of *P. downsi* in Darwin's finch nests, changes in the proportions of instar classes among *P. downsi* have been observed, with evidence that oviposition occurred earlier and more synchronously in the nesting phase in the later years of the study [54]. Synchronisation in oviposition date may lead to an increase in larval competition for host resources, and as a consequence result in increased virulence for nestlings that must contend with a greater number of large,

*Life Cycle and Development of Diptera*

**5.1 Mating behaviour**

the field [20, 72].

**5.2 Oviposition behaviour**

average tree height and/or other ecological variables.

A one-year study on Floreana Island found that male and female *P. downsi* display dimorphic flight patterns, with females more likely to be caught in high and low traps (2 m, most common at 6–7 m), and males more likely to be caught in traps of intermediate height (4–5 m) [58]. As the pattern of male and female abundance are quadratic opposites, this has tentatively been suggested to be an advantage for females to avoid male flies, as frequent mating in other Dipterans has been found to decrease female reproductive success and lifespan [60, 61]. This flight pattern may also explain why certain host species experience higher parasite intensities, such as the medium tree finch (*Camarhynchus pauper*) Ridgway (Passeriformes: Thraupidae) that has an average nest height of 6 m, thus making it more susceptible to being encountered by female *P. downsi* [58, 62, 63]. However, the factors that cause bird species to experience differing intensities of *P. downsi* are complicated and vary between years. Comparison of flight height in *P. downsi* on different islands is needed to test the generality of this pattern, which may be influenced by

The mating behaviour of *Philornis* in general is not well understood, though there are some insights into *P. downsi* mating patterns. While mating has not been observed at or inside the nest, multiple *P. downsi* flies have been video recorded to enter host nests concurrently [45, 64]. Analysis of offspring genetic relatedness has provided estimates of the re-mating frequency of *P. downsi* [65]. Evidence for multiple mating by females has been frequently detected, and each larval infrapopulation (i.e., within nests) is sired by 1–5 males (average ~1.9 males per female) [65]. How *P. downsi* adults find each other to initiate mating is unknown. Pheromones for attraction and aggregation in muscid flies have been identified and studied [66–68]. Cuticular compounds show promise for determining if *P. downsi* produces pheromones, as females and mature males showed distinct cuticular profiles and females respond to chemicals produced by males [69–71]. Cuticular profiles could be developed as an attractant to capture flies in

Studies into oviposition in the genus *Philornis* have revealed that species spanning diverse larval feeding habits are oviparous [1, 9, 31, 73, 74]. This current view has previously been hotly debated, in part because the majority of species remain unstudied. Laboratory rearing and field observation have confirmed that *P. downsi* is oviparous [45, 56, 57, 75]. *Philornis* flies enter and oviposit in nests regardless of nesting phase or nestling age but have not been observed to enter nests abandoned by the parent birds during the incubation phase [45, 47]. From in-nest video recordings, *P. downsi* flies have been observed entering nests throughout the day, but generally during dusk between 1500 and 1800, with visiting rates peaking around 1700 [45, 64]. Visit length averaged 1.3–1.5 min and occurred most commonly when the adult host is away from the nest and completed once the adult host returned [45, 64]. Eggs have been generally deposited on nesting material and the base of the nest [45, 57], however on one occasion, eggs have been also laid directly by the naris of a nestling [45]. A genetic study of *P. downsi* larvae estimated that 1–6 adult females (average ~3 females) oviposit within a single nest, supporting previous observations of different sized larval groups within nests and suggesting repeated nest infestations throughout the

**58**

nestling period [7, 65].

mature larvae at a younger age [16]. The fitness consequences of female oviposition behaviour are further supported by observations in other *Philornis* systems. Host nests that are infested later in the nesting cycle are more likely to have higher fledging success than nests parasitized early in the nesting cycle [50, 78].

#### **6.2 Larval feeding on adult birds**

*Philornis* larvae are generally exclusive parasites of developing nestlings, whether they be subcutaneous or free-living semi-hematophagous species. Infestation of host nests can happen quickly and is often observed within 24 h of the first nestling hatching [41, 43, 50, 79]. Many studies on *Philornis* species in their native range found no evidence of larvae present during incubation [47, 48, 80, 81]. There have been a few cases of larvae feeding on adults in subcutaneous species [82–84], however these reports are rare, with generally only a few larvae per adult. For this reason, larval feeding on adults is generally regarded as opportunistic [2]. More data are needed to examine the oviposition behaviour of *Philornis* species to determine whether larvae are present during the incubation phase.

On the Galápagos Islands between 1998 and 2005, there have been no reported cases of *P. downsi* larvae present in host nests with eggs that would suggest that larvae also feed on incubating females. Two studies during this time period specifically stated that no *P. downsi* larvae have been found during host incubation (**Table 2**) [21, 85]. On Santa Cruz Island during 1998–2010, published studies report findings for 38 nests with eggs that have been inspected for the presence of *P. downsi* and found no larvae (**Table 2**) [21, 85]. In 2012, Cimadom and colleagues first observed *P. downsi* larvae in host nests during incubation where larvae have been found present in 17 of the 26 nests inspected [85]. Since this initial observation, the prevalence of *P. downsi* in host nests with eggs has increased to 80% in some species and years on Santa Cruz Island, with larvae and puparia found in 70 of 177 nests inspected with eggs [86]. Concurrently across this time period, brooding Darwin's finch females have *P. downsi* antibodies that are associated with decreased *P. downsi* intensity, but not increased fledging success [87, 88]. This suggests that *P. downsi* larvae on the Galápagos Islands may have switched to feed on adult finches at some stage [87]. On Floreana Island, inspection of nests that failed during incubation during 2006 and 2016 found *P. downsi* larvae in 4 of 72 (5.6%) nests with host eggs (**Table 2**). In 2006, three medium ground finch (*G. fortis*) nests with eggs in the arid lowlands have *P. downsi* larvae and puparia, and in 2010 one highland small tree finch (*C. parvulus*) has *P. downsi* larvae during the egg stage. During a period of intense drought from 2003 until 2006 with less than 300 mm of rain per year in the lowlands, there were very few active host nests available for oviposition, which may be an explanation for a shift in *P. downsi* female oviposition and larval feeding on incubating females at the end of the drought during 2006. Notably, smaller larvae and eggs are not easily visible in nests and it is possible that *P. downsi* is present, but not detected during incubation in the early years of study.

In laboratory trials, *P. downsi* hatching success is found to be the same in nests with host eggs and nests with finch hatchlings (*Lonchura striata domestica*) Linnaeus (Passeriformes: Estrildidae) [89]. In these trials, there is even a fitness benefit for *P. downsi* that hatched during incubation and hence earlier during the host cycle, as they survived for longer [89]. Other than *P. downsi*, there is one report of an unidentified *Philornis* species parasitising adults in the pearly-eyed thrasher (*Margarops fuscatus*) Vieillot (Passeriformes: Mimidae) studied in Puerto Rico [49]. About 46% of incubating and brooding females and 13% of attending adult males sustained subcutaneous *Philornis* [49]. It has been suggested that this *Philornis* species may have invaded Puerto Rico, as the patterns of prevalence and host

**61**

*Taxonomic Shifts in* Philornis *Larval Behaviour and Rapid Changes in* Philornis downsi*…*

**Island Host species Total no. of nests** 

[85] 1998–2010 SC ST, WF na/21 No Larvae not found in 21 ST

SG 24/na

[91] 2000, 2004 SC SG, MG 27/na Larvae not found

249/na

515/na

43/na

2006 FL MT, SG, MG 129/27 Yes Larvae and puparia

2010 FL ST, MT, SG 153/38 Yes Larvae found in 1 ST

MG Brooding female MG

SC SG, MF, CF 63/na

SC MG 63/na

[45] 2008 FL ST, SG, MG 11/5 No Larvae not found in

[62] 2006, 2008 FL ST, MT 63/2 No Larvae not found in 2

FL SG 71/na

SC SG, MG, ST, LT, WP, WF

SC ST, LT, SG, WF, WP

MG, WF, WP, CF, SBA, YW, VF, DBC, GM

**examined/no. inspected during egg phase**

*P. downsi* **larvae during the egg phase**

105/17 No Larvae not found in 17

**Comments**

SG, ST, WF and WP nests that failed during incubation

and WF nests abandoned during incubation (reported as part of a study during 2012–2015 listed below [86])

in SG and MG nests depredated shortly after host hatch

had *P. downsi*-specific antibodies, suggesting nesting females are parasitised

4 SG and 1 ST nests abandoned with eggs

MT nests depredated during egg phase

found in 3 MGF nests abandoned with eggs in the arid lowlands

nest depredated with eggs in the highlands

*DOI: http://dx.doi.org/10.5772/intechopen.88854*

[21] 1998, 2000 SC ST, LT, SG,

**study**

[90] 2004 SC, FL,

2005

2001, 2002, 2004, 2005

2003, 2004, 2005

2006

2002, 2004

[87] 2008 SC,

[44] 2000, 2004,

[92] 1998, 2000,

[93] 1998, 2000,

[87] 2004, 2005,

[94] 2000, 2001,

[95] 2004, 2005,

Kleindorfer (unpubl. data)

Kleindorfer (unpubl. data)

2006

IS

13 islands incl. SC and FL

DMj

[96] 2008 SC MG 48/na

**Ref # Year (s) of** 


*Taxonomic Shifts in* Philornis *Larval Behaviour and Rapid Changes in* Philornis downsi*… DOI: http://dx.doi.org/10.5772/intechopen.88854*

*Life Cycle and Development of Diptera*

**6.2 Larval feeding on adult birds**

mature larvae at a younger age [16]. The fitness consequences of female oviposition behaviour are further supported by observations in other *Philornis* systems. Host nests that are infested later in the nesting cycle are more likely to have higher fledg-

*Philornis* larvae are generally exclusive parasites of developing nestlings, whether they be subcutaneous or free-living semi-hematophagous species. Infestation of host nests can happen quickly and is often observed within 24 h of the first nestling hatching [41, 43, 50, 79]. Many studies on *Philornis* species in their native range found no evidence of larvae present during incubation [47, 48, 80, 81]. There have been a few cases of larvae feeding on adults in subcutaneous species [82–84], however these reports are rare, with generally only a few larvae per adult. For this reason, larval feeding on adults is generally regarded as opportunistic [2]. More data are needed to examine the oviposition behaviour of *Philornis* species to

On the Galápagos Islands between 1998 and 2005, there have been no reported cases of *P. downsi* larvae present in host nests with eggs that would suggest that larvae also feed on incubating females. Two studies during this time period specifically stated that no *P. downsi* larvae have been found during host incubation (**Table 2**) [21, 85]. On Santa Cruz Island during 1998–2010, published studies report findings for 38 nests with eggs that have been inspected for the presence of *P. downsi* and found no larvae (**Table 2**) [21, 85]. In 2012, Cimadom and colleagues first observed *P. downsi* larvae in host nests during incubation where larvae have been found present in 17 of the 26 nests inspected [85]. Since this initial observation, the prevalence of *P. downsi* in host nests with eggs has increased to 80% in some species and years on Santa Cruz Island, with larvae and puparia found in 70 of 177 nests inspected with eggs [86]. Concurrently across this time period, brooding Darwin's finch females have *P. downsi* antibodies that are associated with decreased *P. downsi* intensity, but not increased fledging success [87, 88]. This suggests that *P. downsi* larvae on the Galápagos Islands may have switched to feed on adult finches at some stage [87]. On Floreana Island, inspection of nests that failed during incubation during 2006 and 2016 found *P. downsi* larvae in 4 of 72 (5.6%) nests with host eggs (**Table 2**). In 2006, three medium ground finch (*G. fortis*) nests with eggs in the arid lowlands have *P. downsi* larvae and puparia, and in 2010 one highland small tree finch (*C. parvulus*) has *P. downsi* larvae during the egg stage. During a period of intense drought from 2003 until 2006 with less than 300 mm of rain per year in the lowlands, there were very few active host nests available for oviposition, which may be an explanation for a shift in *P. downsi* female oviposition and larval feeding on incubating females at the end of the drought during 2006. Notably, smaller larvae and eggs are not easily visible in nests and it is possible that *P. downsi* is present, but

ing success than nests parasitized early in the nesting cycle [50, 78].

determine whether larvae are present during the incubation phase.

not detected during incubation in the early years of study.

In laboratory trials, *P. downsi* hatching success is found to be the same in nests with host eggs and nests with finch hatchlings (*Lonchura striata domestica*) Linnaeus (Passeriformes: Estrildidae) [89]. In these trials, there is even a fitness benefit for *P. downsi* that hatched during incubation and hence earlier during the host cycle, as they survived for longer [89]. Other than *P. downsi*, there is one report of an unidentified *Philornis* species parasitising adults in the pearly-eyed thrasher (*Margarops fuscatus*) Vieillot (Passeriformes: Mimidae) studied in Puerto Rico [49]. About 46% of incubating and brooding females and 13% of attending adult males sustained subcutaneous *Philornis* [49]. It has been suggested that this *Philornis* species may have invaded Puerto Rico, as the patterns of prevalence and host

**60**


*The islands are abbreviated as Santa Cruz (SC), Floreana (FL), Isabela (IS), Daphne Major (DMj). The 'total number of nests examined' refers to all active nests monitored over the course of the study and 'number inspected during egg phase' is the sample size for the sub-set of nests examined during host incubation (usually following abandonment or predation) where 'na' denotes that nests have been not sampled during the egg phase. The column 'P. downsi larvae during the egg phase' states 'yes/no' referring only to nest inspections that occurred during the egg phase. Host species are abbreviated as small tree finch (ST), large tree finch (Camarhynchus psittacula) (LT), small ground finch (SG), medium ground finch (MG), woodpecker finch (Cactospiza pallida) (WP), warbler finch (Certhidea olivacea) (WF), cactus finch (Geospiza scandens) (CF), Galápagos mockingbird (GM), smooth billed ani (Crotophaga ani) (SBA), yellow warbler (Dendroica petechia) (YW), dark billed cuckoo (Coccyzus melacoryphus) (DBC), vermillion flycatcher (Pyrocephalus rubinus) (VF), vegetarian finch (Platyspiza crassirostris) (VGF), and Galápagos flycatcher (Myiarchus magnirostris) (GF).*

#### **Table 2.**

*Evidence of Philornis downsi larvae present in nests during incubation and before nestling hatching in studies on the Galápagos Islands.*

**63**

a novel environment.

**Acknowledgements**

*Taxonomic Shifts in* Philornis *Larval Behaviour and Rapid Changes in* Philornis downsi*…*

mortality mirror that of the *P. downsi* invasion in the Galápagos Islands [6, 48, 49]. *Philornis* consumption of attending adult hosts may be an oviposition tactic that is more prevalent under conditions of resource limitation. Resource limitation could be influenced by resource termination such as early host death, resource availability when there is a limited supply of host nests (e.g., during drought), and resource accessibility, for example when competition within and between fly cohorts

As one of three avian nest parasitic genera in Diptera, the genus *Philornis* provides a useful system to explore shifts in larval feeding behaviour in native and invasive species. *Philornis downsi* has been accidentally introduced to the Galápagos Islands and first observed in the nests of Galápagos land birds in 1997. In this chapter, we explored similarities and differences between *P. downsi* larval development and behaviour with what is known from the other 52 *Philornis* species. More basal *Philornis* (*aitkeni*-group) species have free-living coprophagous larvae and more recently evolved *Philornis* (*angustifrons*-group) tend to have subcutaneous hematophagous larvae with the exception of *P. downsi* that has free-living semi-hematophagous larvae. Since its introduction to the Galápagos Islands, there have been documented changes in the behaviour of *P. downsi*. During the early years after initial discovery of *P. downsi* on the Galápagos Islands, oviposition behaviour was asynchronous across the nesting cycle and larvae appeared to have fed exclusively on developing nestlings until 2005. In later years, *P. downsi* oviposition behaviour was earlier in the nesting cycle and more synchronous, and since 2006, larvae have also been recorded to feed on incubating females. The first records of *P. downsi* larvae in host nests with eggs rather than hatchlings occurred at the end of a four-year drought on the Galápagos in 2006. Since 2012, up to 80% of host nests with eggs may contain *P. downsi* larvae on Santa Cruz Island. Larval feeding by *P. downsi* on adult birds has been observed in laboratory finches and in one *Philornis* system (species unknown) in Puerto Rico. In light of changes in *P. downsi* larval feeding behaviour, we provided a description and photos of the larval instars for use in field identification. We compiled the observations to date of *Philornis* behaviour and ontogeny within a broad taxonomic framework and summarised patterns of change in the oviposition behaviour of *P. downsi* in its (presumably) novel habitat on the Galápagos Islands. By examining *P. downsi* in relation to other *Philornis* species, we provided a broad phylogenetic context for the potential behavioural repertoire of an invasive species under conditions of intense natural selection in

We thank the Galápagos National Park authority for research permits and the opportunity to work on the Galápagos, and the Charles Darwin Research Station for logistical support. We thank Charlotte Causton, Paola Lahuatte, Birgit Fessl, George Heimpel and Arno Cimadom for their useful comments on the manuscript. We thank Bradley Sinclair for advice on larval instar morphology. We thank Justin Holder, Grant Gully and Ben Parslow for their assistance with the photographs and guidance on using the Visionary System. This publication is contribution number

2277 of the Charles Darwin Foundation for the Galápagos Islands.

*DOI: http://dx.doi.org/10.5772/intechopen.88854*

changes [54].

**7. Conclusions**

*Taxonomic Shifts in* Philornis *Larval Behaviour and Rapid Changes in* Philornis downsi*… DOI: http://dx.doi.org/10.5772/intechopen.88854*

mortality mirror that of the *P. downsi* invasion in the Galápagos Islands [6, 48, 49]. *Philornis* consumption of attending adult hosts may be an oviposition tactic that is more prevalent under conditions of resource limitation. Resource limitation could be influenced by resource termination such as early host death, resource availability when there is a limited supply of host nests (e.g., during drought), and resource accessibility, for example when competition within and between fly cohorts changes [54].
